Growing onion sets from seeds

1.1. Savings and Independence

Purchasing quality sets every season is a significant expense. By growing your own sets, you obtain planting material at the cost of seeds, which is substantially cheaper. Moreover, you are no longer dependent on store selections and can be certain that you have exactly the variety you want to grow (Brewster, 2008).

1.2. Quality Control and Plant Health

When buying sets, you cannot be fully certain of their health and varietal purity. They may be infected with diseases (such as downy mildew, white rot) or pests (onion fly, stem nematode), which can then spread to your garden beds. Growing your own sets from seeds allows you to control the entire process—from seed treatment to harvest—and guarantees that you are planting healthy material (Nonnecke, 1989).

1.3. Adaptation to Your Region

Sets grown on your own plot are better adapted to local conditions: day length, soil composition, and temperature fluctuations. Such bulbs establish better, are less prone to disease, and produce higher yields than imported material from a different climatic zone.

1.4. Producing Sets of the Desired Size

The size of sets is crucial. Bulbs smaller than 1 cm ("oat-sized") and larger than 2.5 cm are generally unsuitable for planting for bulb production. Small sets often produce large bulbs but may bolt if stored improperly, while overly large sets tend to prematurely produce flower stalks (Rubatzky & Yamaguchi, 1997). By growing your own sets, you can regulate their size through sowing density and harvest timing (Kasynkina, 2018).

Brief Summary: Growing your own onion sets from seeds ("nigella") is an investment in the health of your future harvest and a saving for your budget. It allows you to be the master of your garden rather than being dependent on market whims.

2.1. Goal: Small Bulbs, Not Large Ones

The primary objective when growing commercial onions is to create all conditions for the plant to produce the largest possible bulb. For this, you sow them sparsely, provide plenty of nutrients and moisture, allowing them to vegetate long and vigorously.

When growing sets, however, the main goal is to make the plant stop growing as early as possible and form a small, mature, and firm bulb that will be ideal planting material for the following year (Nonnecke, 1989). This is achieved by creating competition between plants and limiting nutrition.

2.2. Density: The Denser, The Better

While the optimal density for commercial onions is 40–80 plants per 1 m² (Rubatzky & Yamaguchi, 1997), for growing sets this figure increases 5–10 times. This is so-called dense sowing.

How it works: In dense stands, plants begin to compete with each other for light, water, and nutrients. Under crowded conditions, as soon as the day length reaches the critical value for bulb initiation, the plant switches to bulb formation. It cannot develop a large leaf apparatus, so the bulb remains small. This is a kind of "forced early start" to the formation of the storage organ, which usually works even for varieties intended for producing large bulbs (Kasynkina, 2018).

2.3. Nutrition: Hungry Sets, Not Well-Fed Onions

The next important difference is the fertilization regime. Commercial onions require abundant, balanced nutrition, especially nitrogen, to develop strong foliage. Sets, on the other hand, are grown on a "lean diet." Excess nitrogen, especially in the second half of the growing season, stimulates leaf growth and delays bulb formation, which may become too large and fail to mature (Nonnecke, 1989).

The recommended approach is to apply the main dose of fertilizers before sowing, with an emphasis on phosphorus and potassium, and to exclude nitrogen top-dressing during the active growth period. Ideally, plants should have enough strength to start, but by the time bulb initiation begins, nutrition should be at a minimum.

2.4. Harvest Timing: Don't Wait for Mass Lodging

Harvesting sets also occurs earlier than for commercial onions. They are not left in the ground until full maturity and lodging of tops, as this can cause the bulbs to overgrow and become unsuitable for planting.

Why: To obtain sets of the 1–2.2 cm fraction, you need to wait until the bulbs have formed but not allow them to increase further. The signal for harvesting is the beginning of yellowing and lodging of individual leaves, before the mass lodging of tops (Autko et al., 2012). This way, we get small but fully mature bulbs ready for storage.

Brief Summary: Growing sets is an art of "restrictions." We create conditions of heightened competition, limit nutrition, and harvest at optimal, earlier dates. All this is done to obtain bulbs of the ideal size for planting.

3.1. Why Are Dates So Critical?

Onion is a long-day plant. This means bulb initiation starts when the day length exceeds a certain critical threshold (Brewster, 2008). This threshold varies among different varieties, but in any case, to obtain sets, we need the bulb to start forming as early as possible, before the plants have had time to build up a large leaf mass.

Key principle: The earlier we sow the seeds, the more time the plants will have for vegetative growth before the critical day length arrives. The larger the plant at the onset of bulb initiation, the larger the bulb will be. To obtain small sets, we need the plants to be still small when the day becomes sufficiently long (Rubatzky & Yamaguchi, 1997).

3.2. Universal Rule: Focus on Photoperiod

Although exact dates depend on the region, there is a simple universal rule: sow nigella as early as possible, but after the threat of severe spring frosts has passed.

Why exactly this: Early sowing allows seedlings to get a certain "head start" in growth. But if you sow too early, cold weather will slow development, and when warmth arrives and the day becomes long, the plants will still be too small to form a full bulb and will bolt.

At the same time, if you delay sowing, by the time the plants begin to form bulbs, they will be too large, and the sets will be overgrown—more than 2.5 cm in diameter (Autko et al., 2012).

3.3. Practical Guidelines by Region

3.4. Important Warning

Remember the main rule of growing sets: we are fighting against overgrowth. Therefore, when determining sowing dates, it is better to lean toward earlier dates in your region. If in doubt, choose the earlier date—this is always better than a later one. Sets from early sowing will be smaller, but that's exactly what we need!

3.5. Influence of Variety

Be sure to consider varietal characteristics. For obtaining sets, pungent-semi-pungent varieties (e.g., Bessonovsky, Stuttgarter Riesen, Strigunovsky) are most suitable. They are generally more adaptable and produce stable results when grown for sets under different conditions.

Brief Summary: Sowing dates for nigella are determined by the need to create conditions where plants have time to grow a little before the critical day length arrives. Focus on early dates in your region and consider varietal characteristics. Remember: it's better to get slightly smaller sets than slightly larger ones.

4.1. Why Does Density Work?

It's simple: the denser the plants grow, the less light, moisture, and nutrition each receives. Under conditions of fierce competition, leaf growth slows, and as soon as the day length reaches the critical threshold for bulb initiation, the plant cannot grow a large bulb—it simply doesn't have time. Instead, it forms a small but fully-formed bulb, which is ideal for planting the next season (Rubatzky & Yamaguchi, 1997).

This is a kind of "forced" programming of the plant to complete its vegetation early. That is why sets use 5–10 times higher sowing rates than commercial onions.

4.2. Optimal Plant Density

To obtain quality sets, there should be 1000–1300 plants per 1 m² (Nonnecke, 1989). For comparison, the norm for commercial bulbs is 40–80 plants per 1 m². Notice the difference?

Rough calculation: If you sow in rows with 10 cm spacing, each linear meter of the row should contain about 100–130 seeds (assuming 100% germination). Since germination is always below 100%, in practice you sow even more.

4.3. Sowing Patterns for Sets

For small gardens and home plots, it is most convenient to use multi-row strips. In professional farming, strips with 5–17 rows are used, where the distance between rows is 7.5–15 cm, and between strips is 50–70 cm (Nonnecke, 1989). In the garden, you can adapt this scheme as follows:

• Make a bed 1–1.2 m wide.
• Cut 5–7 furrows at a distance of 10–12 cm from each other.
• Sow seeds in these furrows at a rate of 7–10 g/m².
• Leave a passage 40–50 cm wide between such beds for ease of care.

Important: The seeding depth should be no more than 2–3 cm. At greater depths, onion seeds germinate worse and slower (Kasynkina, 2018).

4.4. What if You Can't Sow in Rows?

If you don't have a seeder or are sowing by hand, you can use broadcast sowing within a narrow bed. Level the surface, scatter the seeds over the bed as evenly as possible, then lightly cover with soil (no more than 2 cm) and firm down. This method is less precise, but it works on small areas. The main thing is not to skimp on seeds: the seeding rate must be high to ensure dense seedlings.

4.5. What Does Such Density Provide?

1. Small bulb size. In dense plantings, bulbs rarely exceed 1.5–2.2 cm, which is ideal for sets.

2. Uniformity. With high density, plants develop more or less synchronously, and by harvest you get uniform-sized material.

3. Space saving. On a small bed, you can grow enough sets to cover a significant planting area the following year.

4.6. Important Warning

Do not overdo the density if you are using varieties prone to forming multiple growing points (multi-budded). In very dense plantings, they can form not one but several small bulbs in a nest, which reduces the quality of the sets. For growing sets, it is better to choose varieties with few growing points (Autko et al., 2012).

Brief Summary: To obtain sets, high density is needed—about 1000 plants per 1 m². Use multi-row strips or broadcast sowing, but be sure to ensure dense seedlings. The denser, the smaller and more uniform the sets will be—and that is our goal.

5.1. Why Is Excess Nutrition Harmful for Sets?

Nitrogen is the main stimulator of green mass growth. The more nitrogen in the soil, the more actively the foliage grows and the longer the plant "delays" transitioning to bulb formation. On nitrogen-rich soil, the plant enters the bulb formation phase later, has time to develop a powerful leaf apparatus, and ultimately produces a large bulb—exactly what we don't need for sets (Rubatzky & Yamaguchi, 1997).

Furthermore, excess nitrogen delays bulb maturation. Such plants have thick necks, dry poorly, and store less well. Sets grown on an excessive nitrogen background often have thick necks and large mass but store poorly and are more susceptible to rot (Nonnecke, 1989).

Golden rule: Sets should be "modest"—grown under limited nutrition, especially nitrogen.

5.2. What You Need to Know About Fertilizer Application for Sets

5.3. Practical Recommendations for the Garden

What to do:

1. Prepare the soil in autumn with phosphorus-potassium fertilizers (superphosphate and potassium salt) at roughly 40–50 g/m² of a mixture (where N:P:K ratio is about 1:2:2).

2. In spring, before sowing, apply a small amount of nitrogen fertilizer (e.g., 10–15 g/m² of ammonium nitrate) and incorporate it during bed preparation.

3. After emergence—no top-dressing! All further care should be directed at keeping the soil moist but not waterlogged and controlling weeds.

What not to do:

• Do not apply fresh manure or poultry droppings in spring under sets.
• Do not feed sets with nitrogen fertilizers during the growing season.
• Do not use complex fertilizers with high nitrogen content (e.g., 20-20-20) for sets.

5.4. Effect on Storage Quality

Sets grown under limited nutrition have denser outer scales, a thin neck, and store better. They suffer less from storage diseases such as neck rot (Botrytis allii), which develops especially actively on "overfed," immature bulbs (Nonnecke, 1989). Remember: sets should not be "fat."

Brief Summary: Sets need a lean nitrogen "diet." The main emphasis is on phosphorus and potassium. A starter dose of nitrogen is only for the start. After emergence—only watering and weeding, no top-dressing. This is the key to obtaining dense, mature, and well-storing bulbs of ideal size.

6.1. What Size Do We Need?

Before discussing control methods, let's define the target parameters. The ideal sets for planting in the next season have a diameter of 1.0 to 2.2 cm (Rubatzky & Yamaguchi, 1997; Kasynkina, 2018). This material produces the best results.

Why this range?

• Less than 1 cm ("oat-sized"). Such small bulbs often produce a good crop, but they are less resistant to bolting if stored improperly. Additionally, very small sets produce smaller bulbs, though with high uniformity.
• 1.0–2.2 cm (ideal sets). Optimal size. Such bulbs produce strong, uniform shoots, are less prone to disease, and hardly bolt when stored correctly.
• More than 2.5 cm ("oversized"). Large bulbs more often bolt, especially if storage conditions are not ideal. Furthermore, such sets produce multi-budded, loose bulbs that store poorly (Nonnecke, 1989).

Our task is to obtain sets from the middle fraction, avoiding both overgrowth and excessive smallness.

6.2. Three Main Control Levers

Set bulb size is regulated by three main factors, which we have already discussed in previous sections:

6.3. How to Assess Set Readiness in the Field?

There is a simple technique to understand that the sets are ready for harvest:

1. Select several representative plants in different parts of the bed.

2. Carefully dig them up and examine the bulbs.

3. Assess the diameter. If it has reached 1–2.2 cm and the outer scales have begun to acquire the characteristic color for the variety and dry out, it's time to prepare for harvest.

Don't be afraid to check. It's better to harvest a few days earlier than a few days later. Overgrown sets lose their value as planting material.

6.4. Influence of Variety

Remember that varieties with different numbers of growing points react differently to density. Varieties with few growing points (1–2) produce more uniform and well-aligned sets. Multi-budded varieties (3–4) in dense plantings can form nests of several small bulbs, reducing the quality of the material. If possible, choose varieties with few growing points for growing sets (Nonnecke, 1989).

6.5. Practical Advice

If you plan to plant sets on a large area the following season, lay down a little more material than you need when growing. Some sets will inevitably be rejected during grading. Having a surplus, you can always select only the best, graded bulbs.

Brief Summary: Set size is controlled by sowing density, nutrition level, and harvest timing. Use these three levers to obtain bulbs 1–2.2 cm in diameter. Ideal sets are the result of conscious growth management, not chance.

7.1. Why Is It Important Not to Miss the Moment?

Sets are bulbs in the active growth stage. If not harvested on time, they will continue to increase in size. In warm, wet weather, this happens very quickly: within 5–7 days, bulbs can overgrow from the ideal fraction of 1.5–2 cm to "oversized" specimens 2.5–3 cm in diameter, which are no longer suitable for planting (Nonnecke, 1989).

Moreover, over-stayed sets mature worse: their necks remain thick, outer scales do not have time to fully form, and they become more susceptible to diseases during storage.

Golden rule: It's better to harvest sets 3–5 days earlier than 3–5 days later.

7.2. Signs of Readiness for Harvest

Unlike commercial onions, for harvesting sets we do not wait for 100% leaf lodging. We rely on other, earlier indicators.

When to start harvesting:

1. Leaf lodging. When 60–80% of the plants in the bed have lodged and started to yellow, this is a signal to start harvesting (Kasynkina, 2018). Some plants may still have individual green leaves, but the majority have already begun to wilt.

2. Drying of the neck. At the base of the bulb, where it transitions into the leaf apparatus, the neck becomes thin and begins to dry. This indicates that the outflow of nutrients from leaves to the bulb is complete.

3. Color of outer scales. The outer scales of the bulb begin to acquire the characteristic color of the variety (golden, brownish) and dry out.

What this looks like in practice: Go to the bed and assess the general condition. If you see that leaves have yellowed and lodged in about two-thirds of the plants, and the remaining third is still green—it's time to harvest. Don't wait for all of them to lodge.

7.3. Harvesting Technique

7.4. Harvesting Features in Different Conditions

7.5. What to Do After Harvesting?

Immediately after collecting the sets, send them for drying. This is a critically important stage, which we will discuss in detail in the next section. The sooner you start drying the sets, the better they will keep.

Brief Summary: Harvest sets when 60–80% of leaves have lodged, in dry weather. Do not leave them in the ground too long—this leads to overgrowth and reduced storage ability. Carefully dig them out without damaging the bulbs, and immediately send them for drying.

8.1. Why Is Drying So Important?

Imagine a freshly dug bulb. It is a living organism in which metabolic processes continue. If left damp and without air access, in warmth, fungal and bacterial rots will quickly develop.

Key tasks of drying:

1. Removal of moisture. The moisture content of freshly harvested sets can reach 80–85%. During drying, moisture is reduced to 14–16% (Nonnecke, 1989). Only at this moisture level can bulbs be stored for a long time without loss of quality.

2. Formation of outer scales. As they dry, the outer fleshy scales turn into dense, dry, papery "tunics." They protect the inner parts of the bulb from moisture evaporation and pathogen entry.

3. Healing of micro-wounds. During harvesting, minor damage is inevitable. In warmth and with air access, they dry out and "seal over," becoming inaccessible to infection.

4. Ripening. During drying, the outflow of nutrients from leaves into the bulb continues. Sets that have been dried for 7–10 days are more mature and store better than those immediately put into storage (Autko et al., 2012).

8.2. How to Dry Sets Properly?

There are two main methods: field drying (on the bed) and drying under a canopy or in a room with forced ventilation.

8.3. How to Tell If Sets Are Dry?

There are several reliable signs:

1. Leaves completely dry. They easily separate from the bulb with light pressure.

2. Outer scales. They have become dense, papery, acquired the characteristic color of the variety (golden, brown, straw-yellow) and rustle when poured.

3. Neck. The junction of the bulb with the leaves should be thin and dry.

4. Sound. If you take a handful of dry sets and shake them, they produce a characteristic rustling sound.

Important: Over-drying is also harmful. With excessive drying, the outer scales become brittle and can crack, exposing the inner tissues.

8.4. What to Do After Drying?

After the sets are dry, it is necessary to remove the remnants of dry leaves and roots. This can be done manually or with special machines (for large volumes). Then the sets are sent for grading and storage, which we will discuss in the following sections.

Brief Summary: Drying is a mandatory and extremely important stage. It removes moisture, forms protective scales, and heals micro-wounds. Use field drying (in good weather) or drying under a canopy (in bad weather). Take your time, let the sets dry completely. This is the key to their long storage.

9.1. Why Is Grading So Important?

If you plant bulbs of different sizes in one bed, they will develop unevenly. Large bulbs will produce strong, fast shoots and will inhibit the small ones. As a result, you will get a non-uniform harvest: some bulbs will overgrow and bolt, while others will remain small and immature (Nonnecke, 1989).

Key principle: Uniform planting material gives uniform shoots, harmonious development, and a high marketable yield.

9.2. Standard Set Fractions

In professional seed production, sets are divided into three main fractions (Nonnecke, 1989; Kasynkina, 2018):

Important: It is the 15–22 mm fraction that is the "gold standard" for planting for bulbs. This size of sets gives the best results (Rubatzky & Yamaguchi, 1997).

9.3. How to Grade Sets at Home?

For small volumes (up to 50–100 kg), you can use simple improvised tools.

9.4. What to Do with Different Fractions?

Ideal sets (15–22 mm). Send for storage for planting in the next season. This is our main planting material.

Large oversized (more than 22 mm). Such sets can be used for planting for greens (early forcing) or for obtaining early production, but you need to be prepared that some plants may bolt. Store them at a lower temperature (0–2 °C) to minimize the risk of bolting (Nonnecke, 1989).

Small sets (less than 15 mm). Used for obtaining greens or for planting for bulbs, but with a lower seeding rate (so that plants are not too crowded). Such sets usually do not bolt.

9.5. Grading Is Not Only About Size

Although the main criterion is diameter, when grading it is also important to pay attention to:

• Density. Bulbs should be firm to the touch. Soft, flabby specimens are rejected.
• Health. Discard bulbs with signs of disease (mold, rot, spots) or damage.
• Color. Scales should have the characteristic color of the variety and be intact.

Golden rule: It's better to have 5 kg of quality, graded sets than 10 kg of substandard material. Don't spare time on grading—it will pay off handsomely.

9.6. Storage of Different Fractions

Different fractions require different storage conditions (more details in the next section). Ideally, store different fractions separately. This will allow you to create an optimal temperature regime for each and avoid confusion during planting.

Brief Summary: Grading is a mandatory stage in preparing sets for storage. It allows you to divide bulbs by size, reject substandard material, and ensure uniform planting material for the future harvest. The ideal set size for planting for bulbs is 15–22 mm in diameter.

10.1. Why Do Sets Require Special Storage?

Unlike commercial onions that we eat, sets are intended for planting. Their main enemies during storage are not only rot but also premature sprouting and bolting (formation of flower stalks) after planting.

Two main risks:

1. Sprouting. If sets begin to sprout in storage, they will use up reserve nutrients and produce weak shoots after planting.

2. Bolting. This is when after planting, the plant produces a flower stalk instead of forming a bulb. This happens if the sets were stored at temperatures that stimulate the formation of flower buds during a certain period (Nonnecke, 1989).

Golden rule: Sets should be stored so that they "sleep" until planting but do not pass through the temperature range dangerous for bolting.

10.2. Optimal Storage Conditions

There are two main approaches to storing sets, each with its own advantages.

10.3. How to Store Sets at Home?

Here are some practical options for gardeners and small-scale farmers.

10.4. What Not to Do When Storing?

1. Do not store sets in heat and humidity simultaneously. If the temperature is above 15 °C and humidity is above 80%, the sets will begin to sprout and rot.

2. Do not allow temperature fluctuations. Sharp temperature changes stimulate respiration and consumption of reserves.

3. Do not store sets together with other vegetables. Onions emit ethylene, which can accelerate the sprouting of potatoes and other vegetables.

4. Do not store sets in a damp cellar. Dampness is the main enemy of onions.

10.5. How to Check Set Condition in Winter?

Once a month, check the storage and inspect the sets. Select a few bulbs and examine them. If they have started to sprout, the temperature is too high or humidity too high. If they have become soft, a rotting process may have begun.

If you find rotten or moldy bulbs, remove them immediately to prevent the infection from spreading to healthy ones.

10.6. Preparation for Planting

2–3 weeks before planting, the sets need to be "awakened"—moved to a warm, bright room (18–20 °C) for 2–3 days. This activates metabolic processes and ensures uniform shoots after planting.

If you stored the sets using the cold method (in the refrigerator or cellar), be sure to let them warm up. Cold sets planted in warm soil may bolt.

Brief Summary: Sets can be stored using the cold method (0–5 °C) or the warm method (18–25 °C). The main thing is to avoid the 5–15 °C range, in which flower buds are initiated. Ensure dryness, ventilation, and regularly check the condition of the bulbs. And remember: properly stored sets are the key to a future harvest.