Plum

From Wild Fruit to Cultivated Tree

The history of the plum spans at least 6,000 years. Archaeological finds of plum stones in Ukraine and Germany date back to 4000–6000 BCE (Westwood, 1993; Gull et al., 2023). This makes the plum one of the oldest fruit crops that humans began cultivating as early as the Neolithic era.

Where did it originate?

The modern cultivated plum (Prunus domestica) is not a wild plant but a natural hybrid resulting from the crossing of two wild species:

• Cherry plum (Prunus cerasifera) — a diploid species (2n=16)
• Blackthorn (Prunus spinosa) — a tetraploid species (2n=32)

Nature did this work for humans: through spontaneous chromosome doubling in a sterile hybrid, a fertile hexaploid form (2n=48) emerged — precisely the domestic plum we know today (Westwood, 1993; Kuchay et al., 2023).

Where to look for the birthplace?

The centre of origin of the domestic plum lies in the region of the Caucasus Mountains and the Caspian Sea (Faust & Surányi, 1999; Gull et al., 2023). Right here, in the mountains of the Caucasus and Transcaucasia, one can still find wild forms of cherry plum and blackthorn, as well as their natural hybrids.

However, the world of plums is far more diverse: different species arose independently in three global centres:

• Western Asia and the Caucasus — home of the European plum (Prunus domestica) and its wild ancestors
• China — home of the Japanese plum (Prunus salicina), which was cultivated there more than 2,000 years ago
• North America — home of American plum species (Prunus americana, P. angustifolia, P. nigra, and others)

The Path into Cultivation: How the Plum Conquered the World

The Ancient World. Plums were known and valued long before our era. The Greek poet Archilochus (680–645 BCE) mentioned plums in his works, and the philosopher Theophrastus (370–287 BCE) already described three cultivated plum varieties (Gull et al., 2023). The Romans played a key role in spreading plums across Europe — they brought them from Syria after conquering the region in 65 BCE (Westwood, 1993).

The Middle Ages. In Europe, plums were grown in monastic gardens and royal estates. In the document "Capitulare de Villis" (circa 800 CE), Emperor Charlemagne ordered that plums be cultivated on all imperial lands, along with apples, pears, cherries, and peaches (Westwood, 1993).

The Modern Era. The great geographical discoveries led to an exchange of varieties between continents. A key figure in plum history was the American breeder Luther Burbank (1849–1926). In the 1870s, he began working with Japanese plums brought to California from Japan. Thanks to his work, legendary varieties appeared — Santa Rosa, Formosa, Beauty, Wickson (Martin, 2019; Kuchay et al., 2023). The Santa Rosa variety, bred in 1907, remains a benchmark for many breeders to this day.

Plums Today: Scale and Geography

The plum ranks fifth among stone fruit crops worldwide in production volume — about 9.8 million tonnes per year (Rieger, 2010). For comparison:

Where are they grown most?

• Asia — 58% of world production (leader China with 6.8 million t)
• Europe — 29% (Romania, Serbia, Germany, Bulgaria)
• America — 10% (USA, Chile, Argentina)
• Africa and Oceania — 3%

(FAOSTAT, 2021; Gull et al., 2023)

Which species is preferred where? This is important to know when choosing a variety for your region:

• In Europe, the European plum (Prunus domestica) predominates — about 90% of all plantings
• In Asia, the Japanese plum (Prunus salicina) dominates — about 82%
• In North America, the picture is mixed: 53% European, 38% Japanese, the rest American species

Why does this matter to the gardener? The European plum is better adapted to cool climates, flowers later (less risk of spring frosts), and has higher winter hardiness. The Japanese plum is more heat‑loving, flowers earlier, but produces larger and sweeter fruits — these varieties are more often chosen for southern regions.

From Wild Form to Table Variety: Evolution at the Service of Humankind

Over millennia of cultivation, humans tirelessly selected the best forms:

1. Selection for taste and size — from small sour fruits of wild cherry plum to large sweet varieties of modern breeding

2. Selection for ripening time — varieties appeared that extend the consumption season from July to October

3. Selection for keeping quality — some varieties are only suitable for immediate consumption, others can be stored for weeks

4. Selection for resistance — frost‑resistant forms for cold regions, drought‑resistant for southern areas

By the beginning of the 21st century, there are more than 6,000 varieties and 20 species of plum worldwide, suitable for cultivation in different climatic zones — from subtropics to regions with harsh winters (Kuchay et al., 2023; Mandal et al., 2021).

Plum in the Plant Kingdom

To successfully grow plums, it is useful for the gardener to understand its "family tree". Taxonomy is not just dry classification — it is the key to understanding which varieties cross with which, which rootstocks are compatible, and why some plums grow in Siberia while others only grow in the subtropics.

Botanical passport of the plum:

(Westwood, 1993; Rieger, 2010; Potapov et al., 2000)

What the gardener needs to know: all plums, apricots, peaches, cherries, and almonds are close relatives from the same family. This explains why they can be grafted onto each other (though not always with success). The genus Prunus comprises about 200 species, distributed mainly in the temperate zones of the Northern Hemisphere (Westwood, 1993).

Main Cultivated Plum Species

For the amateur gardener, there are essentially only three key species — they underlie all the varietal diversity.

Secondary Species and Their Importance

Although these species are rarely grown commercially, they are important for understanding the diversity of plums and their uses.

(Westwood, 1993; Kurennoi, 1985; Kuchay et al., 2023)

Key difference: Prunus domestica is hexaploid, while Prunus salicina is diploid. This means that direct hybrids between these two species are very difficult to obtain (different ploidy). However, breeders bypass this limitation by creating complex hybrids involving diploid and tetraploid species (Kuchay et al., 2023).

Intraspecific Diversity: Variety Groups

In the domestic plum (P. domestica), four historically established groups have formed, which are important to know when choosing a variety:

1. Prunes (Hungarian plums) — commercially the most important group. Fruits oval, dark blue or purple, with high sugar content (up to 30° Brix), free stone. Good for drying (prunes). Varieties: Common Hungarian, Italian Hungarian (Fellenberg), Agen, Stanley (Westwood, 1993; Mandal et al., 2021).

2. Greengages (Reine Claude, Green Gage) — round fruits, green, yellow or purple. Flesh very sweet, aromatic, high quality. Varieties: Green Gage, Althann's Greengage, Bave's Greengage. Demanding of warmth (Westwood, 1993).

3. Yellow Egg plums — large, oval, yellow fruits. Used for canning. Varieties: Yellow Egg, Red Magnum Bonum (Westwood, 1993).

4. Lombard plums — large, oval, red or pink fruits, somewhat inferior in quality to the first two groups. Used fresh. Varieties: Victoria, Lombard, Bradshaw (Westwood, 1993).

In the Japanese plum (P. salicina), internal division is less formalised, but varieties are distinguished by flesh colour (yellow, red) and fruit shape (round, heart‑shaped).

Subgenera and Sections: How Not to Get Confused

In the genus Prunus, several subgenera are distinguished. For the gardener, it is important to understand that:

(Westwood, 1993)

Practical Significance of Taxonomy for the Gardener

1. Choice of rootstock

• For European and Japanese plums, P. cerasifera (cherry plum) is most often used — it gives vigorous trees, drought‑tolerant
• For dwarf orchards, clonal rootstocks are used: P. insititia (St. Julien, Pixy), P. cerasifera × P. munsoniana (Marianna), P. persica (peach — in warm regions) (Hartmann et al., 2011; Kuchay et al., 2023)
• American plum species (P. americana, P. besseyi) serve as rootstocks for frost‑hardy varieties in Canada and the northern US states (Westwood, 1993)

2. Graft compatibility

• Plums of the same subgenus graft onto each other easily
• Grafting peach onto plum requires caution — not all combinations are successful (Westwood, 1993)
• P. domestica and P. insititia are interchangeable as rootstocks

3. Pollination

• Most varieties of European plum are self‑fertile (Italian Hungarian, Stanley) or partially self‑fertile
• Japanese plum is mostly self‑sterile — requires pollinator varieties
• American species are also self‑sterile (Westwood, 1993)

4. Climate adaptation

• P. domestica — most winter‑hardy, withstands frosts down to ‑25…‑30°C
• P. salicina — heat‑loving, recommended only for southern regions with mild winters and sufficient heat
• American species — extremely frost‑hardy (down to ‑35°C), but their fruits are smaller and often more acidic

Summary for the gardener:

If you live in a region with cold winters and don't want to take risks — choose Prunus domestica. If you have a warm climate and want especially sweet, large fruits — consider Prunus salicina. For the harshest conditions, look at American species or their hybrids.

General Appearance: Tree or Shrub?

The European plum (Prunus domestica) in the garden is usually formed as a tree 4–8 m tall, but under favourable conditions individual specimens reach 10–12 m (Westwood, 1993; Kurennoi, 1985). On dwarf rootstocks, the height can be limited to 2–3 m — convenient for harvesting and maintenance.

Depending on the variety and rootstock, plums may have:

• Pyramidal (narrow) crown — compact, branches directed upward
• Spreading (broad) crown — branches diverge at obtuse angles, crown spreading (Westwood, 1993)

For the gardener, it is important: spreading varieties take up more space but are better lit; pyramidal varieties are more compact but require more careful formative pruning.

The life form of the plum is a tree (unlike the bush‑like cherry). However, some varieties and on dwarf rootstocks can form a multi‑stemmed bush, especially when the central leader is damaged by frost (Kurennoi, 1985; Potapov et al., 2000).

Root System: Hidden Support and Nourisher

The root system of the plum has a number of features that directly affect the choice of site, watering, and care.

Structure:

• The root system is predominantly surface — the bulk of the roots (70–80%) lie at a depth of 20–60 cm (Kurennoi, 1985; Potapov et al., 2000)
• Horizontal roots spread far beyond the crown projection — often up to 1.5–2 times the crown diameter
• Vertical roots go down to a depth of 2–3 m, but their development depends on the soil and groundwater level

Types of roots:

• Skeletal — thick, perennial roots (up to 10 cm in diameter), serve for support and transport (Kurennoi, 1985)
• Feeder (absorbing) — thin, active roots that absorb water and minerals. They live for several weeks or months, then die and are replaced by new ones

Why this is important for the gardener:

1. Watering — because of the shallow root system, plum is sensitive to drying of the topsoil. Regular watering is necessary, especially during fruit formation. At the same time, waterlogging is fatal — the roots suffocate.

2. Soil cultivation — do not dig deeply in the root zone to avoid damaging surface roots. Recommended cultivation depth — no more than 8–10 cm.

3. Fertilising — nutrients should be applied exactly in the zone of active roots (at a depth of 20–40 cm in the crown projection), not at the trunk.

4. Groundwater level — plum does not tolerate a high water table (less than 1.5–2 m from the surface). Roots rot, the tree becomes diseased.

5. Root suckers — many plum varieties (especially own‑rooted) produce abundant suckers from the roots. This complicates care; to reduce it, it is better to use grafted saplings on rootstocks that do not produce suckers (e.g., on cherry plum).

Trunk and Crown: The Tree's Skeleton

The trunk of the plum, like that of any tree, consists of:

• Root collar — the transition point from root to trunk. Important for the gardener: when planting, do not bury the root collar — this can cause bark necrosis and death of the tree.
• Stem (trunk) — the lower part of the trunk from the root collar to the first scaffold branch. The stem height depends on the training system:
• For vigorous orchards — 60–80 cm
• For semi‑dwarf rootstocks — 40–60 cm
• For creeping forms — 20–40 cm
• Central leader — the main vertical axis from which scaffold branches arise.
• Scaffold branches (first order) — the main branches forming the framework of the crown. Plums usually have 4–5 of them.
• Second‑ and third‑order branches — arise from scaffolds, bearing fruiting twigs.

(Kurennoi, 1985; Potapov et al., 2000; Tarasov, 1981)

For the gardener: when forming the crown, it is important that scaffold branches diverge from the trunk at an angle of at least 45° (preferably 60–70°). Acute angles (less than 45°) are weak points — under the weight of the crop, branches may break off.

Shoots: Annual Growth

Shoots are the current year's growths on which leaves, buds, and sometimes flowers develop.

Types of shoots in plum:

1. Vegetative (growth) shoots — long shoots (40–100 cm), bearing only vegetative (growth) buds. They serve to increase the crown.

2. Mixed shoots — medium‑length shoots (15–40 cm), which may have both vegetative and flower buds. These shoots bear the basis of next year's crop.

3. Short fruiting structures:

• Spurs — short (1–3 cm) twigs with several flower buds at the tip and one vegetative (growth) bud at the end. Lifespan — 2–4 years (Tarasov, 1981).
• Bouquet branches (up to 5–10 cm) — in Japanese plum and some varieties of European plum. They bear groups of buds (flower and vegetative).

For the gardener: the plum crop is set on last year's growth — both on long mixed shoots and on short spurs (Westwood, 1993; Tarasov, 1981). If shoots are too long (more than 50 cm), flower buds are weakly set. If too short (less than 15 cm), the branches quickly become bare. The optimal growth length for good fruiting is 25–40 cm.

Buds: The Future Harvest

Buds in plum are rudimentary shoots protected by scales. Two main types are distinguished (and this is important to know when pruning):

In plum, flower buds are simple — that is, only flowers develop from them, not shoots with flowers (unlike apple, where buds are mixed). Moreover, in most varieties, 1–2 flowers develop from one bud, rarely 3 (Westwood, 1993; Tarasov, 1981).

Bud arrangement:

• Flower buds in plum are located laterally (in leaf axils), not terminally (as in apple).
• In one leaf axil, several buds often form — two flower and one vegetative (Westwood, 1993; Tarasov, 1981; Kurennoi, 1985).

Why this matters:

• When pruning, it is important to distinguish bud types so as not to remove flower buds and lose the crop.
• Well‑developed flower buds indicate future harvest. Their number on shoots depends on:
• shoot length: optimal 15–40 cm
• shoot age: on young shoots (1–2 years) flower buds are set more actively
• conditions of the year: sunny, warm summer — more flower buds

Leaves: The Harvest Factory

Plum leaves are simple, oval or ovate, with finely serrated margins. There are differences between species:

Leaves play the main role in photosynthesis. A healthy leaf apparatus is the basis of a high yield. Leaf diseases (e.g., shot hole, or Coryneum blight) lead to weakening of the tree and reduced yield. Therefore, leaf protection is one of the key tasks.

Flowers: From Bud to Fruit

Plum flowers are bisexual (contain both stamens and pistil). Colour — white or pink, size — about 2–2.5 cm in diameter.

Flowering features:

• In most varieties, flowering is early — often before the leaves appear (Westwood, 1993; Tarasov, 1981).
• Flowers are arranged singly or in inflorescences (simple umbels) of 2–3 on short pedicels.
• Flowering time varies among varieties:
• Early‑flowering — in late April – early May (for the middle belt)
• Mid‑season — 5–7 days later
• Late‑flowering — even later (these are the most valuable varieties for regions with late frosts)

For the gardener: early flowering is the main vulnerability of plum. In regions with late frosts, it is necessary to choose late‑flowering varieties or provide flower protection. Temperatures of ‑2…‑3°C during flowering can ruin the crop. Critical temperatures for flower buds (Potapov et al., 2000; Trunov & Samoshchenkov, 2012):

• Bud stage: ‑3…‑5°C
• Flowering stage: ‑1…‑2°C
• For young fruit set: ‑1…‑1.5°C

Fruit: A Drupe with Secrets

The plum fruit is a drupe: a fleshy fruit with a hard stone inside. It has three layers:

1. Exocarp — thin skin (colour varies from green, yellow to red, purple, dark blue)

2. Mesocarp — fleshy pulp (edible part)

3. Endocarp — hard stone (seed inside)

Species differences in fruits:

• European plum: fruits oval, often with a "waxy" bloom; flesh yellow‑green or amber; stone separates well in most varieties (freestone)
• Japanese plum: fruits round or heart‑shaped, flesh yellow or red, stone often adhering (clingstone)
• Cherry plum and blackthorn: fruits small, often astringent; stone may be free or adhering

Why it is important to know these differences:

• Freestone varieties are good for fresh consumption and drying (prunes). Clingstone varieties are valued for very juicy flesh, but are more difficult to process.
• In European plum, fruits have high sugar content (up to 15–20%) and pectin substances, making it ideal for jams, jellies, prunes. Japanese plums have more watery fruits, but sweeter and larger (Mandal et al., 2021; Rieger, 2010).

Fruit sizes vary widely among varieties:

• Small (20–30 g): cherry plum, blackthorn, some old varieties
• Medium (30–50 g): most European varieties
• Large (50–80 g and more): Japanese varieties, some modern hybrids
• Record holders — up to 100–120 g (Mandal et al., 2021)

Lifespan and Age Periods

Knowing the age stages helps plan care and assess the tree's prospects.

(Shitt, cited in Tarasov, 1981; Potapov et al., 2000; Mandal et al., 2021)

For the gardener: plum on seedling rootstocks can live 50–60 years or more (Westwood, 1993), but peak yield occurs at 8–20 years. On dwarf rootstocks, the productive period is shorter (12–20 years), but yield per unit area is higher.

Bud, Shoot, and Fruiting Peculiarities — What Matters for Care

Let us summarise the most important practical conclusions from botany:

1. Flower buds in plum are lateral, simple (from each — only a flower). This means that when pruning, it is important not to remove shoots with a large number of flower buds. Shortening should be done carefully, leaving enough generative buds.

2. Fruiting on last year's growth. The crop is formed on shoots that grew in the previous season. Therefore, pruning should stimulate sufficient annual growth. If there are few shoots or they are short (less than 15 cm), yield declines.

3. Lifespan of fruiting structures. Spurs and bouquet branches live 2–4 years, then die. Therefore, it is important to constantly renew them by stimulating new growth. Without pruning, the tree quickly becomes bare — fruiting shifts to the periphery of the crown, yield drops.

4. Crown density. In varieties with good shoot‑forming ability, the crown thickens — this leads to reduced light penetration, decreased flower bud set, and poorer fruit quality. Thinning is mandatory.

Summary for the gardener:

Plum is a tree with a shallow root system that forms flower buds on last year's growth. Its fruits can be freestone (for drying and jam) or clingstone (for fresh consumption). Knowing these features, you will be able to choose the right variety, plant the tree, and provide care that yields maximum returns.

4.1. Temperature Requirements: Cold Winter Rest and Warm Summer

4.2. Light: Sunlight Minimum

Plum is a light‑loving crop. Like all stone fruits, it reacts negatively to shading.

Light requirements:

• For normal growth and fruiting, plum should receive direct sunlight for at least 6–8 hours a day (preferably in the morning)
• In shade, few flower buds are set, fruits become smaller, lose colour and sugar content, and disease incidence increases (Westwood, 1993; Potapov et al., 2000)

Practical conclusions:

• Do not plant plum near tall trees that will shade it.
• Spacing between trees should ensure good light penetration throughout the crown during the day.
• When pruning, create an open crown so that sunlight penetrates inside (this stimulates flower bud set and improves fruit quality).
• South‑facing slopes are preferable to north‑facing ones — they receive more heat and light.

4.3. Moisture: Between Drought and Waterlogging

Plum is a crop that requires good but not excessive moisture.

Total water consumption:

• Annual water use of a bearing plum orchard is 5000–8000 m³/ha at a yield of 20–30 t/ha (Trunov & Samoshchenkov, 2012)
• Transpiration coefficient (water needed to produce unit dry matter) — about 300–450, comparable to apple and higher than apricot or peach (Kurennoi, 1985; Trunov & Samoshchenkov, 2012)

Critical moisture periods:

1. Period of intensive shoot growth and flowering (April–May) — moisture deficit at this time reduces flower bud set for the next year.

2. Fruit set and growth (May–July) — lack of moisture leads to small fruits, reduced sugar content, and fruit drop.

3. Post‑harvest period (August–September) — watering promotes accumulation of reserves, improving winter hardiness.

Irrigation:

• To maintain optimal soil moisture in the root zone (40–60 cm), it is recommended to keep moisture at 70–80% of field capacity (Trunov & Samoshchenkov, 2012)
• Drip irrigation or furrow irrigation is preferable; sprinkling during flowering should be avoided (it hinders pollination)

Drought tolerance:

• Plum is moderately drought‑tolerant. It is less drought‑tolerant than apricot and peach, but more tolerant than apple under some conditions (Kurennoi, 1985).
• Drought tolerance varies among species: P. domestica — moderate, P. cerasifera (cherry plum) — higher, P. salicina — more demanding of moisture (Agustí, 2010; Westwood, 1993).

Waterlogging:

• Plum does not tolerate stagnant water around the root system. Groundwater must be at least 1.5–2 m deep.
• Under waterlogging, roots suffocate (oxygen deficiency), are affected by root rots (Phytophthora), the tree weakens and may die (Westwood, 1993; Potapov et al., 2000).

For the gardener:

• If your region has dry summers, be sure to provide irrigation — especially in June (active fruit growth). The best method is drip irrigation, which saves water and ensures even moistening.
• If the plot has a high water table (less than 1.5 m), do not plant plum — either install drainage or plant on raised beds.
• Mulching the root zone (straw, bark, mown grass) helps retain moisture and reduce watering frequency.

Relative air humidity: plum performs best at moderate humidity (60–75%). Too high humidity favours fungal diseases (shot hole, brown rot). In dry air, fruits may suffer from sunburn.

4.4. Soils: Foundation of Health

Plum is not overly demanding, but has clear preferences.

Mechanical composition:

• Best soils: loam and sandy loam — well drained, sufficiently moisture‑retentive and aerated (Westwood, 1993; Kurennoi, 1985; Potapov et al., 2000)
• Heavy clay: poor air permeability, retain moisture, promote root rots. Can be improved by adding sand and organic matter.
• Sandy: dry out quickly, nutrient‑poor, require frequent irrigation and fertilisation.

Depth of the rootable layer:

• For plum, it is desirable that the active soil layer (for roots) be at least 0.8–1.2 m (according to Potapov et al., 2000, for stone fruits — 120–160 cm). This means that the soil should be cultivated deeply enough, without compacted horizons (plough pan) that hinder root development.

Acidity (pH):

• Optimal range: pH 6.0–7.5 (slightly acidic to slightly alkaline) (Westwood, 1993; Potapov et al., 2000; Mandal et al., 2021)
• Plum tolerates slight acidification, but on acidic soils (pH < 5.5), nutrient uptake, especially phosphorus and calcium, is impaired, and the risk of chlorosis increases.
• Alkaline soils (pH > 8.0) may cause chlorosis due to iron deficiency (on carbonate soils).

Fertility:

• Plum has moderate requirements for fertility, but on poor soils it gives small fruits and weak growth.
• Organic matter (humus, compost) significantly improves soil structure and water regime.

For the gardener:

1. Check pH — dip litmus paper into a water extract from the soil. If pH is below 5.5, apply liming (250–500 g of dolomite flour per 1 m²) 1–2 years before planting.

2. Digging depth — before planting, it is advisable to dig the site deeply (spade depth) and incorporate organic matter (5–10 kg/m² of well‑rotted manure or compost).

3. Drainage — if the site is wet, create drainage ditches or plant on raised beds.

4. Soil type — on heavy clays, improve structure by adding sand and organic matter; on sands, increase the clay fraction (imported loam) and use mulch.

4.5. Summary Table of Ecological Requirements

4.6. Practical Summary: The Ideal Plum Site

So, if you can choose a site for plum — look for:

• South or south‑west slope — more heat and light, fewer frosts
• Sheltered from strong winds — wind dries flowers, hinders pollinating bees
• Low groundwater level (deeper than 1.5 m) — roots will not rot
• Loamy or sandy‑loam soil, pH 6.0–7.5 — ideal water‑air balance

If your plot is not ideal, do not despair: you can improve the soil, install drainage, choose a suitable variety and rootstock, and provide irrigation. But remember: in a cold, wet, or heavily shaded spot, the plum will be diseased, grow poorly, and give meagre yields.

Summary for the gardener:

Plum is a temperate‑zone crop that requires a cold winter (for chill accumulation) and a warm, sunny summer for fruit ripening. It is sensitive to frost during flowering, likes moist but not waterlogged soil, and prefers loamy, neutral soils. The main enemies are water stagnation and lack of light. Proper site selection is the key to a long and productive life of the tree.

5.1. Annual Cycle of Development

Like all deciduous trees of the temperate zone, plum goes through alternating two main periods: vegetation (active growth) and dormancy (winter rest). For successful cultivation, it is important to understand what happens in each of these periods.

Dormancy period (autumn–winter–early spring). After leaf fall, the tree enters a state of dormancy. This is not just a cessation of growth, but a complex physiological process that prepares the plant for overwintering.

Why is this important? In deep dormancy, the tree is most frost‑hardy. However, the duration of deep dormancy in flower buds is shorter than in vegetative buds — that is why they are more often damaged in winter (Trunov & Samoshchenkov, 2012; Potapov et al., 2000). In warm winters or in regions with mild climates (southern Russia, Mediterranean, southern USA), dormancy may not be deep enough — buds "wake up" prematurely and are damaged by spring frosts (Agustí, 2010).

Vegetation period (spring–summer–autumn). It begins with sap flow (appearance of "bleeding" when branches are wounded) — a sign of root awakening and activation of nutrient transport. Then buds swell, leaves and flowers appear.

5.2. Dormancy and Chill Requirement

Plum has a clearly expressed need for winter chill for normal development (Agustí, 2010; Faust, 1989).

How it works:

• In autumn, with shortening days and falling temperatures, growth inhibitors accumulate in the buds — substances that prevent bud sprouting.
• Under the influence of low temperatures (0…+7°C), these inhibitors gradually break down. The duration of this process depends on the variety — from 200 to 1200 hours.
• When the chill requirement is satisfied, the buds emerge from deep dormancy (endol‑atency) and enter a state of imposed dormancy (ecodormancy) — they are only waiting for warmth to start growing.

What the gardener needs to know:

1. If the winter was warm, chilling hours are not accumulated — flower buds may not wake up at all or wake unevenly, with a great delay. Flowering will be weak, fruit set may drop (Agustí, 2010).

2. If the winter was cold and long, buds will wake up quickly and early — the risk of damage by spring frosts increases.

3. Choosing a variety by chill requirement is critically important:

• In regions with mild winters (Crimea, Black Sea coast, Mediterranean countries, southern USA), varieties with low chill requirement (200–500 h) are needed — Japanese varieties (Mandal et al., 2021).
• In regions with cold winters (central Russia, Northern Europe, northern USA), varieties with high chill requirement (700–1200 h) are needed — European, domestic (Westwood, 1993; Faust, 1989).

Practical conclusion: if you plant a southern (low‑chill) variety in a cold region, it may wake up too early, even in February, and suffer from frost. If you plant a northern (high‑chill) variety in a warm region, it will "sleep" too long, yield will decrease. Always compare the variety's chill requirement with the climate of your area.

5.3. Flower Bud Initiation: Where the Future Crop is Set

In plum, flower buds are initiated in the previous season, a year before fruiting. This is one of the most important physiological processes for the gardener.

When and how does initiation occur:

• In most European plum varieties, flower bud initiation begins in June–July, about 2–3 weeks after the end of active shoot growth (Agustí, 2010; Tarasov, 1981).
• In Japanese plum, this process may begin somewhat later — towards the end of July.
• Inside the bud, vegetative primordia (leaves) are formed first, then the growing point transforms into flower tubercles (Agustí, 2010).

Conditions favouring flower bud initiation:

1. Good light — light stimulates the formation of generative organs. Shaded parts inside the crown do not set flower buds (Agustí, 2010).

2. Balanced nutrition — especially phosphorus and potassium with sufficient but not excessive nitrogen. Excess nitrogen stimulates vegetative growth at the expense of generative growth (Westwood, 1993).

3. Moderate moisture deficit — mild water stress in late June – early July promotes switching growth from vegetative to generative (Agustí, 2010).

4. Sufficient shoot length — on too short shoots (less than 15 cm), flower buds are not set; on too long shoots (more than 50 cm), they are set weakly.

Timing of flower bud initiation in main species:

(Agustí, 2010; Tarasov, 1981)

Practical conclusion: if you want a good harvest, provide the tree with good crown light (pruning — thinning), balanced nutrition (do not overfeed with nitrogen), and sufficient moisture in the first half of summer. This is precisely when the future crop is set. Drought in June–July can sharply reduce flower bud set.

5.4. Flowering and Pollination

Flowering is the most critical stage for obtaining a crop. Plum blooms early in spring, often before leaves appear. In the middle belt, flowering falls in late April – the first decade of May (Westwood, 1993; Tarasov, 1981).

Physiology of the flower:

• The plum flower has one pistil and many stamens.
• Pollination is mainly by insects — bees, bumblebees (Westwood, 1993; Rieger, 2010).
• Pollen must land on the stigma of the pistil; then the pollen tube grows down the style to the ovary, where fertilization of the ovule occurs.
• From pollination to fertilization takes 1–3 days in warm weather and up to 5–7 days in cool weather (Rieger, 2010).

Self‑fertility — a key factor:

According to their ability to set fruit without a pollinator variety, plums are divided into three groups:

Sources: Westwood, 1993; Kurennoi, 1985; Tarasov, 1981; Mandal et al., 2021.

Peculiarities of plum pollination:

• Plum pollen is not wind‑borne — bees are needed. In cloudy, cold, or windy weather, bee flight is reduced, pollination decreases.
• Optimal temperature for bee activity — above +15°C.
• If weather during flowering is unfavourable, artificial pollination can be done (transferring pollen with a soft brush from flower to flower), though it is labour‑intensive.

Practical conclusion for the gardener:

1. If you are planting only one variety, choose a self‑fertile one (e.g., Italian Hungarian or Stanley). This guarantees a crop even without pollinators.

2. If you plant several varieties, choose them so that flowering times coincide — they will pollinate each other, and yield will be higher.

3. During flowering, try not to use insecticides — they kill bees.

4. In cold weather during flowering, you can spray flowers with a weak sugar solution (1 tbsp per 1 L of water) — this attracts bees.

5.5. Fruit Growth and Development

After fertilization, fruit growth begins. Physiologically, it proceeds in three stages (Westwood, 1993; Mandal et al., 2021):

Stage 1 (S1): active cell division (first 4–6 weeks after flowering)

• Fruits grow rapidly in length and width.
• The final fruit size is determined during this period.
• Critical period for irrigation — drought at this time leads to small fruits (Mandal et al., 2021).

Stage 2 (S2): stone formation (about 2–4 weeks)

• Fruit growth slows or almost stops.
• The endocarp lignifies — the hard stone forms.
• During this period, the so‑called "June drop" occurs — the tree sheds some fruit set that cannot be supported (natural load regulation). This is normal, but if drop is too severe (>30% of set) — possibly lack of moisture or nutrition (Westwood, 1993; Mandal et al., 2021).

Stage 3 (S3): fruit filling (from stone formation to ripening)

• Second intensive fruit growth due to cell enlargement.
• Accumulation of sugars, organic acids, anthocyanins (colour) (Mandal et al., 2021).
• In this period, fruits are most sensitive to moisture deficit — irrigation is critical.
• Duration of stage S3 varies greatly among varieties: from 3–4 weeks (early varieties) to 8–10 weeks (late).

Duration of fruit development in different varieties:

• Very early: 70–80 days from flowering to ripening
• Early: 80–100 days
• Mid‑season: 100–120 days
• Late: 120–140 days
• Very late: more than 140 days

(Westwood, 1993; Faust, 1989; Mandal et al., 2021)

Changes in fruits during ripening:

• Acidity decreases, sugar content increases.
• Skin colour changes (green → yellow, red, blue depending on variety).
• Some varieties develop a waxy bloom ("bloom").
• In freestone varieties, the stone begins to separate from the flesh.

5.6. Shedding of Flowers, Fruit Set, and Fruits

Plum, like most fruit trees, sheds part of its generative organs. This is a natural self‑regulation process, but sometimes crop loss is excessive.

Three periods of shedding:

1. Flower shedding (immediately after flowering). Causes:

• Insufficient pollination (no pollinators, cold weather) — unfertilised flowers drop.
• Weakness of the tree (nutrition deficit, disease).

2. Fruit set shedding (June drop). Causes:

• Competition for nutrients (the tree "decides" how many fruits it can support).
• Lack of moisture or nutrition.
• Frost damage (even if the set looks healthy externally, internally it may be damaged).

3. Pre‑harvest fruit drop. Causes:

• Over‑cropping (too many fruits).
• Drought.
• Diseases (brown rot) or pests (plum moth).
• Mechanical competition — larger fruits displace smaller ones.

What is normal: in large‑fruited varieties, only 4–6% of the original number of flowers remain on the tree (Westwood, 1993). For small‑fruited — up to 10–20%. This is not a problem but a normal physiological phenomenon.

For the gardener: if shedding is excessive (>70% of fruit set), check:

• Was there drought after flowering?
• Is the tree adequately nourished?
• Are pollinating insects present?
• Is the tree shaded?

If the problem recurs year after year — perhaps the variety is unsuitable for your climate (e.g., needs a pollinator that is absent).

5.7. Root Growth

Root growth in plum occurs in waves that are important to know for care (especially for watering and fertilising).

During the season, 2–3 waves of active root growth are observed (Kurennoi, 1985; Potapov et al., 2000; Tarasov, 1981):

1. Spring (late April – May) — coincides with the start of sap flow and bud break. Roots begin to grow before shoots, ensuring water and mineral supply for the start of growth.

2. Summer (June–July) — coincides with the period of active shoot and fruit growth. During this period, roots require a lot of moisture and nutrients.

3. Autumn (September–October) — after shoot and fruit growth has ceased. During this period, roots actively accumulate reserve nutrients for wintering.

Practical conclusion: fertiliser application and irrigation should be timed to these periods, especially to the summer wave, when the tree actively consumes water and nutrients.

5.8. Hormonal Regulation

Understanding hormonal processes helps to understand why some practices work and others do not.

Main hormones affecting plum:

Biennial bearing ("on‑year, off‑year"):

This is one of the main problems related to hormonal regulation. In a heavy crop year, seeds of developing fruits synthesise large amounts of gibberellins, which suppress flower bud set for the following year. As a result, in the next season few buds are set, and yield drops sharply (Agustí, 2010; Westwood, 1993).

How to counteract:

• Timely thinning of fruit set (reducing crop load) is one of the main ways to prevent biennial bearing.
• Regular pruning to stimulate new growths on which new flower buds are set.
• Balanced nutrition, especially potassium and phosphorus, which promote flower bud set.

5.9. Biennial Bearing

The phenomenon: one year abundant crop, the next very poor or none. This is typical for many plum varieties, especially large‑fruited ones (Westwood, 1993; Mandal et al., 2021; Faust, 1989).

Causes of biennial bearing:

• Hormonal: seeds in heavy crop year produce gibberellins that suppress flower bud set in that same year (Agustí, 2010; Faust, 1989).
• Physiological: the tree depletes carbohydrate reserves, insufficient for bud set in the crop year (Westwood, 1993).
• Correlative: competition between growth and fruiting — fruits "take" nutrients away from buds.

Which varieties are most prone: large‑fruited, highly productive varieties, especially those where fruits set predominantly on one‑year‑old shoots (Westwood, 1993).

Preventive measures:

• Thinning — removing part of the fruit set in years of heavy flowering (especially for large‑fruited varieties). It is recommended to leave fruits 10–15 cm apart.
• Pruning — stimulates growth of new shoots on which flower buds are set.
• Fertilisation — in years of heavy crop, it is especially important to provide the tree with nutrients (potassium, phosphorus, micronutrients).
• Regular pruning — maintains young fruiting shoots, reducing the load on older branches.

Summary for the gardener:

Plum physiology is about understanding four key processes:

1. Dormancy and winter — without sufficient chill (chilling hours), buds will not wake in spring. Choose a variety suited to your climate.

2. Flower bud initiation — occurs in June–July. Provide light, water, and nutrition at this time — and next year's crop will be set.

3. Flowering and pollination — plum needs bees. Self‑fertile varieties are more reliable, but with a pollinator the yield is always higher. Cold weather during flowering is a risk for the crop.

4. Biennial bearing — a problem for many large‑fruited varieties. It can be countered by thinning fruit set, pruning, and balanced nutrition.

6.1. What Lies Within the Fruit: Main Components

The plum fruit consists of skin (exocarp), fleshy pulp (mesocarp), and a hard stone with seed. For the gardener, the pulp and skin are of most interest — they determine nutritional value and taste.

Water and dry matter content:

• Water makes up 86–88% of the ripe fruit mass (Mandal et al., 2021).
• Dry matter — 12–14%, of which the main part is carbohydrates and fibre.

For the gardener: high water content makes plum juicy but leads to rapid spoilage. Varieties with higher dry matter content (>15%) store better and are suitable for drying.

Carbohydrates (sugars):

• Total sugar content varies from 6.7 to 9.9% (and up to 30% in prune varieties) (Mandal et al., 2021; Westwood, 1993).
• Main sugars: glucose, fructose, sucrose (their ratio depends on variety and ripeness).
• In unripe fruits, starch predominates, which hydrolyses to simple sugars during ripening, explaining the accumulation of sweetness.

Organic acids:

• Main acids — malic, quinic, citric (to a lesser extent) (Mandal et al., 2021).
• Titratable acidity averages 16–30 meq H⁺/100 g (Mandal et al., 2021).
• The sugar‑acid ratio determines taste: the higher the sugar‑to‑acid ratio, the sweeter the fruit is perceived.

For the gardener: in sour varieties, acidity is higher, and they are better suited for processing (jam, jellies) where sugar is added. Sweet varieties (high sugar‑acid ratio) are best eaten fresh, but they store less well and often require less added sugar in processing.

Fibre (dietary fibre):

• Fibre content — 1.3–2.4% (Mandal et al., 2021).
• Fibre is good for digestion, but in processed products (juices, purées) it is removed or reduced.

Pectic substances:

• Pectin content — 0.81–0.98% (Mandal et al., 2021).
• Pectin is a gelling agent that determines the ability of fruits to form jellies and jams. High pectin content makes plum ideal for jams (especially prunes and some European plum varieties).

Proteins and fats:

• Protein — 0.4–0.8% (Mandal et al., 2021).
• Fats — in trace amounts (less than 0.2%).
• Energy value — 125–187 kJ (about 30–45 kcal) per 100 g (Mandal et al., 2021).

6.2. Vitamins and Minerals

Plum is a valuable source of vitamins and minerals, especially when consumed fresh.

Vitamin C (ascorbic acid):

• Content varies from 4 to 11 mg per 100 g of flesh (Mandal et al., 2021).
• This is a moderate level, comparable to apples, but significantly lower than in citrus fruits or blackcurrants.

Carotenoids (provitamin A):

• Content — 0.26–0.78 mg per 100 g (Mandal et al., 2021).
• Carotenoids give yellow and orange colour to pulp and skin, especially in varieties with yellow flesh.

B‑group vitamins:

• Thiamine (B₁) — 0.02–0.05 mg/100 g (Mandal et al., 2021).
• Other B‑group vitamins are present in smaller amounts but play a role in metabolism.

Mineral elements:

• Potassium (K) — 120–190 mg/100 g (Mandal et al., 2021). Potassium is important for heart function and water‑salt balance.
• Calcium (Ca) — 6–8 mg/100 g (Mandal et al., 2021).
• Magnesium (Mg) — 4–7 mg/100 g (Mandal et al., 2021).
• Sodium (Na) — 0–3 mg/100 g (Mandal et al., 2021). This is very low, beneficial for people on salt‑free diets.
• Iron (Fe) — 0.1–0.4 mg/100 g (Mandal et al., 2021).
• Zinc (Zn) — about 0.1 mg/100 g (Mandal et al., 2021).

For the gardener: fresh plums are a good source of potassium and vitamin C. Especially valuable are dark‑coloured varieties (with anthocyanins), which provide more antioxidants. If you grow plums for your own consumption, keep in mind that vitamins are better preserved in fresh fruits, not in heat‑processed ones.

6.3. Phenolic Compounds: Colours and Health

Plum is rich in phenolic compounds, which give fruits astringency, colour the skin and pulp, and possess antioxidant activity.

Main groups of phenolic compounds (Mandal et al., 2021):

• Anthocyanins — pigments that give dark (purple, red, blue) colour to skin and pulp. Main anthocyanins in plum are cyanidin‑3‑rutinoside and peonidin‑3‑rutinoside (Mandal et al., 2021). Levels of anthocyanins in dark varieties can reach 926 mg per 100 g (Mandal et al., 2021).

• Flavonoids — 46.3–57 mg/100 g (Mandal et al., 2021). This group includes quercetin, kaempferol, and others. They have antioxidant, anti‑inflammatory properties.

• Phenolic acids — caffeic, chlorogenic, ferulic, coumaric (Mandal et al., 2021). They give fruits a slight astringency.

• Catechins and other polyphenols — also present, especially in the skin.

For the gardener: dark blue and purple varieties contain significantly more anthocyanins than yellow or green ones. If the goal is to obtain the most health‑beneficial product, preference should be given to such varieties. However, anthocyanins also participate in oxidative processes — fruits of dark varieties may darken faster during processing (especially during drying and storage in cut form).

Amygdalin (cyanogenic glycoside):

• In seeds (stones) of plum, amygdalin is present — a glycoside that can release hydrocyanic acid upon hydrolysis (Mandal et al., 2021).
• Important: plum stones (like apricot, peach, cherry) should not be eaten in large quantities. They may be toxic. However, during fruit processing (jam, juices), small amounts of stones are not dangerous if they are not crushed and not subjected to prolonged boiling.

6.4. Comparative Composition of Different Plum Species

Different plum species have significant differences in chemical composition, which directly affects their use.

Sources: Mandal et al., 2021; Westwood, 1993; Rieger, 2010.

Practical conclusions:

• For drying (prunes): choose varieties with high sugar content (>15%) and firm flesh (e.g., Italian Hungarian, Agen, Stanley). Dry matter content should be at least 20%. Such varieties yield quality prunes without fermentation.
• For jams and jellies: sugar and pectin are important. European plum with high pectin (0.8–1%) gives good jelly without added thickeners. Varieties with lower pectin require additional pectin or sugar.
• For fresh consumption: Japanese varieties with juicy, aromatic flesh and a balance of acids and sugars are ideal. But they usually store less well and are not suitable for drying due to lower dry matter and more watery texture.

6.5. Changes in Chemical Composition During Ripening

Understanding how fruit composition changes from set to full ripeness helps determine the timing of harvest and use.

Ripening stages (Mandal et al., 2021; Westwood, 1993):

1. Mature for harvest (for storage and transport):

• Fruits reach characteristic size and shape.
• Skin begins to colour (changes from green to yellow, red, or blue depending on variety).
• Flesh still firm, stone separates in freestone varieties.
• Sugar content reaches 50–70% of maximum, acidity still high.
• Fruits can be picked and ripened in storage (at 0–2°C).

2. Consumer ripeness (for immediate consumption):

• Fruits fully coloured, wax bloom appears in some varieties.
• Flesh becomes soft, juicy, aromatic.
• Sugars reach maximum, acids decrease to minimum.
• Sugar/acid ratio reaches optimal values.
• For Japanese varieties and many dessert varieties — at this moment the taste is most complete.

Changes during ripening (Mandal et al., 2021):

• Sugars: increase from 6–7% in unripe to 10–20% in ripe (up to 30% in prunes).
• Acids: total acidity decreases, but absolute malic acid content may remain stable or slightly decrease. Reduction in acidity is due to acid utilisation in respiration and conversion to sugars.
• Pectin: solubility changes — from insoluble protopectin (in unripe fruits) to soluble pectin (in ripe), leading to flesh softening.
• Anthocyanins: accumulate during ripening, especially in sunny weather and moderate temperatures.
• Aromatic compounds: synthesised in the final stages of ripening, giving the characteristic plum aroma.

For the gardener: do not harvest fruits for storage at full consumer ripeness — they will be overripe and spoil quickly. For long‑term storage (2–4 weeks at 0–2°C), pick fruits at harvest maturity, when they are already sufficiently sweet but flesh is still firm. For immediate consumption or processing (jam, juices), let fruits fully ripen on the tree.

6.6. Practical Application: How to Choose a Variety by Chemical Composition

When choosing a variety for your garden, you should consider not only winter hardiness and yield but also the chemical composition of the fruits — it determines for what purposes you will be able to use the crop.

Source: Mandal et al., 2021; Westwood, 1993.

6.7. Human Health: What Plum Provides

Plum and especially prunes are known for their health benefits, confirmed by scientific research.

Main beneficial effects:

1. Digestive regulation — due to fibre and sorbitol, plum has a mild laxative effect. Prunes are traditionally used for prevention and treatment of constipation (Mandal et al., 2021; Gull et al., 2023).

2. Antioxidant protection — polyphenols, anthocyanins, and vitamin C neutralise free radicals, reduce oxidative stress, and slow ageing (Mandal et al., 2021). Dark varieties are especially rich in antioxidants.

3. Bone health — prunes contain boron and other trace elements that help strengthen bone tissue. Studies show that regular consumption of prunes may reduce the risk of osteoporosis (Mandal et al., 2021; Gull et al., 2023).

4. Cardiovascular system — potassium helps regulate blood pressure, and antioxidants protect vessel walls (Mandal et al., 2021).

5. Vitamin K — plum contains vitamin K, which participates in blood clotting and may help reduce the risk of Alzheimer's disease (Mandal et al., 2021).

For the gardener: if you grow plums not only for pleasure but also for health, try to consume fruits fresh in season, and dry surplus (prunes) — in dried form, the concentration of beneficial substances is much higher. Especially useful are dark purple varieties with high anthocyanin content.

6.8. Practical Recommendations for Determining Ripeness

How to determine when plums are ready for harvest? Here are the main signs to help you decide:

1. Colour — each variety has a characteristic colour at full ripeness (golden‑yellow, red, dark blue, purple). Use the variety description as a guide.

2. Firmness — for harvest maturity, fruits should be firm but not rock‑hard. For consumer ripeness — soft but not too soft.

3. Taste — the most reliable method. Try one fruit — if it is sweet and aromatic, start harvesting.

4. Waxy bloom — many varieties (especially Hungarians) develop a bluish bloom ("bloom") at full ripeness.

5. Ease of detachment from the branch — in most varieties, a ripe fruit separates easily, while the pedicel remains on the branch (in some varieties, e.g., greengages, the pedicel may come off with the fruit).

6. Sugar content — with a refractometer, you can measure soluble solids (Brix). For dessert varieties, optimal is 12–14% and above. For prunes — more than 18–20%.

For the gardener: remember that plum ripening is not uniform — fruits on one tree ripen over 1–3 weeks. Therefore, harvest should be selective, in several rounds, as they ripen.

Summary for the gardener:

Chemical composition of plum is the key to understanding what a particular variety is suitable for. High sugar and dry matter make a variety ideal for drying (prunes). High pectin — for jam. Balance of sugar and acid — for fresh consumption. Dark colour — a sign of high anthocyanin and antioxidant content.

When choosing a variety, always specify for what purposes you will use it. If you plan to dry plums — choose Hungarians (sugary, firm). If to eat fresh — prefer dessert varieties of Japanese or European plum with good flavour. And remember that fruit composition changes during ripening, so keep track of harvest timing.

7.1. Classification by Biological Species

This is the most important classification for the gardener, as it determines climate adaptation, winter hardiness, pollination requirements, and of course fruit characteristics.

7.2. Classification by Ripening Time

This is an important criterion for planning the consumption season and choosing a variety for a specific climate.

(Westwood, 1993; Kurennoi, 1985; Mandal et al., 2021; Tarasov, 1981)

For the gardener: in regions with short summers and early autumn frosts, choose early and early‑mid varieties — they ripen before cold weather. In southern regions, late varieties can be grown, which give larger fruits and keep better.

7.3. Classification by Purpose (Use)

Depending on taste, sugar content, flesh firmness, and stone adherence, plum varieties are classified by purpose.

Dessert (table):

• Intended for fresh consumption.
• Large or medium fruits, excellent taste, juicy, often with pleasant aroma.
• Sugar content high, acidity moderate.
• Usually keep for a short time (3–7 days).
• Examples: Victoria, Anna Späth, Santa Rosa, Althann's Greengage.

Universal (dual purpose):

• Suitable both for fresh consumption and processing (jam, compotes).
• Good sugar and acid content, firm flesh.
• Examples: Common Hungarian, Green Gage.

Prune (for drying):

• Fruits with high dry matter (>20%) and sugar (up to 20–30%), firm flesh.
• Oval shape, stone easily separates.
• Main varieties: Italian Hungarian, Common Hungarian, Agen, Stanley.

For processing (jam, jellies, juices):

• Fruits with high pectin and acid content.
• May be small or medium, with firmer flesh.
• Examples: Greengages, Mirabelles, Damsons, many cherry plum varieties.

For technical processing:

• For winemaking, spirit liqueurs (e.g., slivovitz).
• Varieties with high sugar content and aroma, often small‑fruited.
• Examples: Požegača (variety for slivovitz in Serbia), some Hungarians.

(Westwood, 1993; Mandal et al., 2021)

7.4. Classification by Fruit Type (Freestone / Clingstone)

This is an important practical feature determining ease of processing.

For the gardener: if you plan to dry plums or make jam, choose freestone varieties — this saves a lot of time. If you prefer to eat plums fresh, clingstone varieties are often tastier and juicier.

7.5. Classification by Vigour and Crown Type

This is important for planning planting and choosing a training system.

(Westwood, 1993; Trunov & Samoshchenkov, 2012; Kuchay et al., 2023)

For the gardener: if you have a small plot, choose varieties on semi‑dwarf or dwarf rootstocks — they allow more trees on limited area, simplify harvest and care. However, dwarf trees are more demanding of care and watering (Trunov & Samoshchenkov, 2012).

7.6. Classification by Pollination Type

This is a critically important feature for choosing a variety, especially if you are planting only one or two trees.

(Westwood, 1993; Kurennoi, 1985; Tarasov, 1981; Mandal et al., 2021)

For the gardener: if you have a very small plot or want to plant only one tree — choose a self‑fertile variety. If you plant several trees, you can choose self‑sterile varieties, but be sure to select a partner with coinciding flowering times.

7.7. Classification by Rootstock (Practical Significance)

Although rootstock is technically not part of the variety, it determines vigour, winter hardiness, tree size, and yield. It is important to understand what rootstocks exist and what they give the gardener.

Sources: Hartmann et al., 2011; Kuchay et al., 2023; Trunov & Samoshchenkov, 2012; Westwood, 1993.

For the gardener: choosing a rootstock is a choice between longevity and early bearing. On seedling rootstocks, the tree will live longer, but fruits appear later. On dwarf rootstocks, you will get a crop in year 2–3, but the tree will serve less. For amateur gardens in the middle belt, semi‑dwarf rootstocks (St. Julien A, Pixy) are often recommended — they offer a good compromise.

7.8. Summary Table of Practical Classifications

For quick orientation in variety selection, use the following summary table.

Summary for the gardener:

Plum classification is multi‑layered, but in practice you need to remember only a few key points:

1. For cold climates — choose European plum (P. domestica), especially Hungarians, self‑fertile, freestone, with high winter hardiness.

2. For warm climates — you can choose Japanese varieties (P. salicina), they are larger and sweeter, but require pollinators and protection from frost.

3. For jam — mirabelle, Damson, Greengages, cherry plum — they give a lot of pectin, jam becomes jelly‑like.

4. For drying (prunes) — only Hungarians with high sugar content and freestone.

5. For small plots — choose semi‑dwarf or dwarf rootstocks with early and mid‑season varieties.

6. For large plots — vigorous rootstocks with late and very late varieties to extend the season.

In the next chapter, we will address the most practical question: how to choose a variety and cultivation method for your specific goals, considering all the classifications discussed.

8.1. Where to Start: Defining Your Objectives

Before browsing catalogues and reading variety descriptions, ask yourself five key questions. Their answers will determine all subsequent decisions.

8.2. Step 1: Assess Your Climatic Conditions

This is the most important step. If the variety does not match the climate, it will either not bear fruit or die.

What you need to know about your region:

1. Hardiness zone (USDA zone) — minimum winter temperatures.

• For central Russia (zones 4–5): requires winter hardiness down to –25…–30°C. Only European varieties (P. domestica) are suitable (Westwood, 1993; Faust, 1989).
• For southern Russia, Black Sea coast, Crimea (zones 6–7): Japanese varieties (P. salicina) can be grown; they are less hardy but give larger fruits (Westwood, 1993; Mandal et al., 2021).
• For northern regions (zones 3–4): only the hardiest varieties of European plum, as well as hybrids with American species.

2. Growing degree days (above +10°C) during the growing season.

• For early varieties, 1800–2200°C is enough.
• For mid‑season — 2200–2600°C.
• For late — 2600–3000°C (Agustí, 2010; Potapov et al., 2000).
• If the sum is less than 1800°C, even early varieties may not ripen.

3. Chill requirement (chilling hours) for your region.

• Find out how many hours in your region the temperature remains in the range 0…+7°C (usually November–February).
• If your region has mild winters (e.g., Crimea, Black Sea coast, Mediterranean), choose varieties with low chill requirement (200–500 h) — Japanese varieties (Mandal et al., 2021; Faust, 1989).
• If winters are cold and long (central Russia, Urals, Siberia) — choose varieties with high chill requirement (700–1200 h) — European varieties (Westwood, 1993; Faust, 1989).

4. Risk of late spring frosts during flowering.

• In regions with frequent May frosts, choose late‑flowering varieties (e.g., many Hungarians) (Westwood, 1993; Kurennoi, 1985).
• In warm regions, early‑flowering varieties can be planted.

Practical conclusion: first, determine which plum species can actually grow in your region. In central Russia — only European plum. In southern regions — both European and Japanese.

8.3. Step 2: Define Your Growing Objectives

Why do you want a plum? The answer determines which varieties to look for.

(Westwood, 1993; Mandal et al., 2021; Kurennoi, 1985)

For the gardener: if you can plant several trees, plant different varieties for different purposes — for example, one Hungarian for prunes, one Greengage for eating, one cherry plum for jam. This gives you maximum diversity.

8.4. Step 3: Assess Your Plot

The plot imposes constraints that need to be considered.

Light exposure:

• Plum requires full sun for at least 6 hours a day (Chapter 4). If the plot is shaded, yield will be weak, fruits small, diseases more severe.
• If the plot is sunny — great. If there is shade — either choose a different spot or accept lower yields.

Soil type:

• Loam or sandy loam — ideal (Westwood, 1993).
• Heavy clay — needs improvement (adding sand, organic matter) or choose varieties on more tolerant rootstocks (cherry plum) (Potapov et al., 2000).
• Sand — needs frequent irrigation and fertilisation; better to choose more drought‑tolerant species (cherry plum) (Kurennoi, 1985).
• Acidity (pH) — check. If pH <5.5, liming is needed (Chapter 4). Optimum pH 6.0–7.5 (Mandal et al., 2021; Westwood, 1993).

Groundwater:

• If water is closer than 1.5–2 m from the surface — plum cannot be planted (Westwood, 1993). Roots will rot. Solution: either drainage, planting on mounds, or abandon plum.

Area and location:

• For a vigorous tree, at least 25–30 m² (5–6 m diameter) is needed (Trunov & Samoshchenkov, 2012).
• For semi‑dwarf — 12–15 m² (4–5 m diameter).
• For dwarf — 6–10 m² (3–4 m diameter) (Trunov & Samoshchenkov, 2012).
• Consider that the crown will expand over time, do not plant too close to buildings or fences (minimum distance — 2–3 m for dwarf, 4–5 m for vigorous).

Availability of pollinators:

• If you have only one tree — choose a self‑fertile variety (Chapter 7). Otherwise, no crop.
• If you have 2–3 trees — you can choose self‑sterile ones, but with coinciding flowering times (Westwood, 1993).

8.5. Step 4: Choose a Rootstock

The rootstock is the "foundation" of your tree. It determines height, time to bearing, longevity, and care requirements.

Sources: Westwood, 1993; Hartmann et al., 2011; Kuchay et al., 2023; Trunov & Samoshchenkov, 2012.

For the gardener:

• If you have a large plot (from 600 m² and more) and you are willing to wait 5–7 years for a crop — choose seedling rootstocks. The tree will be powerful, long‑lived, less demanding.
• If the plot is medium (300–500 m²) and you want a crop sooner (in 4–5 years) — semi‑dwarf rootstocks are the best compromise.
• If the plot is small (up to 300 m²) or you want the earliest crop (in 2–3 years) — dwarf rootstocks. But be prepared for more frequent watering and fertilising, as well as installing support (Trunov & Samoshchenkov, 2012).

8.6. Step 5: Choose a Specific Variety

Now that you have determined species, objectives, plot conditions, and rootstock, you can proceed to selecting a specific variety.

Below are recommendations for different regions and purposes.

8.7. Step 6: Decide on Pollination

If you plant only one tree:

• Choose a self‑fertile variety (e.g., Stanley, Italian Hungarian, Victoria).
• Otherwise, there will be no crop.

If you plant 2–3 trees:

• You can choose both self‑fertile and self‑sterile varieties.
• For self‑sterile, be sure to select a pollinator variety with coinciding flowering time (Westwood, 1993).
• For example, for Santa Rosa (Japanese), Beauty, Shiro, or even some early European varieties flowering at the same time will work (Martin, 2019; Kuchay et al., 2023).

If you plant 4 or more trees:

• Usually enough diversity to ensure cross‑pollination.
• Can include both self‑fertile and self‑sterile varieties.

Universal pollinator: for Japanese varieties, Santa Rosa is often used as a universal pollinator (Martin, 2019; Kuchay et al., 2023). For European varieties, many Hungarians pollinate each other well.

8.8. Step 7: Choose a Cultivation Method

After selecting variety and rootstock, determine how you will grow the tree.

Planting layout:

• For vigorous trees: between rows 5–7 m, in row 4–5 m (Trunov & Samoshchenkov, 2012).
• For semi‑dwarf: 4–5 m between rows, 3–4 m in row.
• For dwarf: 3–4 m between rows, 2–3 m in row (can be denser for intensive orchards) (Trunov & Samoshchenkov, 2012).

Crown training:

• For vigorous and semi‑vigorous trees, sparse‑tiered or improved vase crown is often used (Westwood, 1993; Trunov & Samoshchenkov, 2012).
• For dwarf — spindle (slender spindle), bush, or columnar (for very dense planting) (Trunov & Samoshchenkov, 2012).
• Keep in mind that plum is more light‑demanding than apple, so the crown should be well thinned (Westwood, 1993).

Support:

• For dwarf rootstocks (especially M9 and similar), support is mandatory — trellis or stakes, because the root system is weak (Trunov & Samoshchenkov, 2012; Westwood, 1993).
• For semi‑dwarf, support is desirable but not mandatory (especially on well‑drained soils).
• For vigorous, support is not needed.

Care: For all types, mandatory are:

• Regular watering (especially during dry periods) (Chapter 4).
• Annual pruning (formative and sanitary) (Westwood, 1993; Trunov & Samoshchenkov, 2012).
• Protection from pests and diseases (especially brown rot and shot hole) (Chapters 4, 5).
• Fertiliser application (organic and mineral) (Westwood, 1993; Potapov et al., 2000).

8.9. Summary Table: How to Choose a Variety for Your Conditions

8.10. Final Advice

1. Start with proven varieties. If you are a beginner, do not chase exotic novelties. Choose one or two zoned, proven varieties (e.g., Stanley or Italian Hungarian for the middle belt). They will give you a guaranteed crop and allow you to gain experience.

2. Plant 2–3 trees of different varieties, even if they are self‑fertile. Cross‑pollination always increases yield and fruit quality (Westwood, 1993). Moreover, different ripening times extend the consumption season.

3. Consider ripening times. If you want to eat plums all summer and autumn, plant early, mid‑season, and late varieties. For example: Opal (early) + Stanley (mid‑season) + Italian Hungarian (mid‑late) or President (late).

4. Do not forget about the rootstock. It determines height and longevity. On a small plot, dwarf rootstock is a lifesaver. On a large plot, seedling or semi‑dwarf will give a more powerful tree.

5. Pay attention to disease resistance. Some varieties are more resistant to brown rot, shot hole, and other diseases. If these diseases are common in your region, choose resistant varieties or be prepared for regular treatments.

6. Buy saplings from reputable nurseries. Ensure that the variety and rootstock are correctly indicated. Better to take saplings with a closed root system (in containers) — they establish better.

7. Do not be afraid to experiment. Over time, you will understand which varieties best suit your garden. You can graft several varieties onto one tree (family tree) — this saves space and gives diversity (Hessayon, 1993).

Final checklist for variety selection:

• [ ] Determined climatic zone and winter hardiness.
• [ ] Found out growing degree days and chilling hours in my region.
• [ ] Decided for what purposes I will use plums (fresh, drying, jam).
• [ ] Assessed the plot: light, soil, groundwater level, area.
• [ ] Chose rootstock (vigorous, semi‑dwarf, dwarf) depending on area and desired bearing age.
• [ ] Chose species (European or Japanese) depending on climate.
• [ ] Decided on self‑fertility: if one tree — self‑fertile variety.
• [ ] Selected 1–3 specific varieties from those recommended for your conditions.
• [ ] Ensured I have or will have a pollinator for self‑sterile varieties.
• [ ] Checked availability of saplings in nurseries.