Physiological disorders

What Are Physiological Disorders?

Simply put, they are “breakdowns” in the normal growth and development cycle of a plant, caused by adverse environmental factors. They are not an infection that can be cured with a fungicide, nor an attack by a pest that can be eliminated with an insecticide. The cause is stress that the plant experiences due to unsuitable growing conditions.

Such stress factors include:

• Extreme temperatures (intense heat or unexpected frosts).
• Lack or excess of moisture in the soil.
• Improper nutrition (e.g., too much nitrogen or a boron deficiency).
• Damage to the root system during transplanting.

How to Distinguish from Diseases?

This is a key point for the gardener. The main difference is the absence of a pathogen. In physiological disorders, there are no fungal spores, bacteria, or viruses on the plants. Symptoms do not spread from a diseased plant to a healthy one (Teverson, 2020). If several plants are affected while others remain healthy, you are likely dealing with an infection. If the problem is widespread and symmetric (e.g., lower leaves turning yellow on all plants at the same time), the cause is probably in the growing conditions.

Example for comparison:

• Disease: Yellow V‑shaped spots with black veins appear on the leaves and gradually enlarge. This is a classic sign of bacterial black rot – caused by Xanthomonas campestris.
• Physiological disorder: The edges of leaves inside the head or on young leaves begin to die and turn brown. This is a typical symptom of tipburn, caused by a calcium deficiency that cannot reach the rapidly growing tissues due to a lack of moisture (Wien and Stützel, 2020).

How to Distinguish from Nutrient Deficiency?

Physiological disorders are often caused by nutrient deficiencies, but that is only one possible reason. The concepts of “deficiency” and “disorder” overlap but are not identical.

• Nutrient deficiency – when the soil simply lacks an available form of an element. For example, acidic soil may have little molybdenum. This is a “pure” deficiency.
• Physiological disorder – when the element is present in the soil but the plant cannot absorb it. For instance, during drought, roots cannot take up calcium from the soil even if it is abundant. As a result, tipburn occurs (Wien and Stützel, 2020).

Thus, a physiological disorder is a broader concept that includes “shortage” due to stress, not just because the soil is depleted.

The Impact of Stress

The broccoli plant is a complex organism in which all processes are balanced. Any stress disrupts this balance. In response to unfavourable conditions, the plant switches from growth and yield formation to survival mechanisms. This is what leads to physiological disorders.

For example, at high temperatures, the plant spends energy on cooling (transpiration) rather than on forming a dense head. Under moisture deficit, it sheds some buds to conserve water for the roots. Understanding these mechanisms is the key to a quality harvest. In the following chapters, we will examine each disorder in detail, learn how to diagnose them and, most importantly, how to prevent them.

Now we have the title and introduction. We are ready to move on. Next comes Chapter 2 – “Small Central Head (Buttoning)”. This is the most frustrating disorder, when instead of a large harvest, the gardener gets several tiny “buttons”.

How to Recognise?

Outwardly, the plant may look normal, but instead of growing leaves and preparing to form a large inflorescence, it “switches” to the reproductive phase. As a result, a small head appears in the centre of the rosette, which grows very slowly or not at all. Such broccoli cannot be “saved” – it will never produce a normal yield.

Why Does It Happen?

Buttoning is a classic example of a stress response. The plant, sensing a threat to its life (or to reproduction), rushes to produce seeds in a panic. This is a protective mechanism triggered by several factors:

1. Overgrown seedlings. This is perhaps the most common cause, especially among novice gardeners. If seedlings are kept warm and fed for too long, they outgrow. Such plants are already on the verge of flowering. When transplanted into open ground, especially in cool weather, they receive a strong stress that triggers premature head formation (Wien and Stützel, 2020). “A large, overgrown seedling with a thick stem is most likely a future ‘button’,” say plant physiologists (Wien and Stützel, 2020).

2. Cold after transplanting. After transplanting into cold, unsettled weather, vegetative growth (leaves) of broccoli slows down significantly. However, if the plant has already reached a certain size and received enough low temperatures to initiate flowering, it may initiate head formation without having built sufficient leaf mass (Старцев, 2021). This is one reason why it is important to observe optimal transplanting dates when the soil and air are warm enough.

3. Nitrogen deficiency and water stress. Nitrogen is the main “building material” for leaves. If it is lacking, leaf growth stops. Moisture deficit in the soil also stops plant growth. In both cases, even if the plant is genetically capable of forming a large head, it cannot “feed” it due to a lack of resources, and the head remains small (Wien and Stützel, 2020).

4. Varietal characteristics. Some varieties are more prone to buttoning at the slightest stress. These are usually early‑maturing varieties or those intended for specific climatic zones. When choosing seeds for your region, pay attention to resistance to this disorder.

How to Prevent:

• Grow quality seedlings: Do not allow them to overgrow. The optimal age for transplanting is 4‑5 weeks with 4‑5 true leaves. Harden off the plants before transplanting.
• Observe planting dates: Transplant when the threat of severe return frosts has passed and the soil has warmed to at least 8‑10 °C.
• Provide good nutrition: Apply a balanced fertiliser before planting. For active growth at the beginning of the season, broccoli needs a good dose of nitrogen (Basics of Broccoli Production, 2024).
• Water regularly: Broccoli is demanding of moisture, especially during active leaf growth and head formation. Do not allow the soil to dry out.
• Choose resistant varieties: Purchase seeds of hybrids that are bred for your region and have better stress tolerance.

Understanding the causes of buttoning is the first step to preventing this nuisance. The next disorder, loose curd, is also often a consequence of stress, but of a different kind.

How to Recognise?

Instead of a tight, compact head with small, tightly packed buds, the inflorescence looks sparse. Individual buds and the stems on which they are attached are visible to the naked eye, with noticeable gaps between them (Wien and Stützel, 2020; Basics of Broccoli Production, 2024). The head may feel light and lack the “firmness” characteristic of quality broccoli.

Why Does It Happen?

Loose curd is the result of uneven or disrupted development of reproductive organs (flower buds). The main cause is a “conflict” between vegetative and reproductive growth caused by adverse conditions. Let us break down the main factors:

1. High temperature (heat stress). This is one of the main causes. Broccoli is a cool‑season crop. To form a dense head, it needs moderate temperatures: ideally 15–18 °C. When the temperature rises above 25 °C, bud development speeds up while stem and leaf growth slows down. As a result, buds and flower stalks begin to elongate rapidly without accumulating dense mass, leading to looseness (Wien and Stützel, 2020; Basics of Broccoli Production, 2024). Look at Figures 2 and 3 in our article – the difference between a normal head and one affected by heat is obvious.

2. Uneven growth. Any factor that disrupts uniform growth can trigger looseness. For example, alternating drought and heavy watering, or sharp fluctuations between night and day temperatures, make the plant “nervous” and activate survival mechanisms, which affects the structure of the inflorescence. It seems to “stretch out” in an attempt to complete its life cycle faster and produce seeds.

3. Excess nitrogen. Oddly enough, an overabundance of nitrogen fertilisers can also be harmful. Nitrogen stimulates rapid growth of green mass. If there is too much, the plant channels all its energy into leaf growth, while the formation of a dense, high‑quality head takes a back seat. The head may grow, but it will be more loose and watery, with elongated flower stalks (Старцев, 2021). In addition, lush foliage creates shading and poor ventilation, which also indirectly affects head quality.

4. Varietal characteristics. Genetic predisposition plays a big role. Some varieties, especially early‑maturing ones, are naturally more prone to loose curd than late‑maturing ones. When choosing seeds, always check the variety description – it often indicates how dense the head is.

How to Prevent:

• Choose the right variety: Select varieties and hybrids that are recommended for your region and known for their heat tolerance and ability to form dense heads.
• Observe planting dates: Try to ensure that the head formation and growth phase falls during a cooler period. Transplant so that harvest does not coincide with the peak of summer heat.
• Balanced nutrition: Do not over‑feed broccoli with nitrogen. It is important that the soil has adequate phosphorus and potassium, which are responsible for the formation and quality of floral organs.
• Regular and moderate watering: Keep the soil moist. Avoid both drying out and waterlogging so that plant growth is as uniform as possible. Remember that loose curd and premature flowering (which we will discuss in the next chapter) are very similar symptoms, often caused by the same factors.

We have understood the causes of loose heads. Often this phenomenon is a harbinger of an even more serious problem – premature flowering. That will be the subject of the next chapter.

How to Recognise?

The buds on the inflorescence open. First, individual yellow flowers appear (often at the edges of the head), then the process quickly spreads over the entire surface. The head loses its compactness, becomes “fuzzy”, yellow, and sometimes bitter to the taste (Teverson, 2020).

Why Does It Happen?

Premature flowering (or “bolting”) is a classic plant response to critical stress. Broccoli, like all living organisms, under extreme conditions strives to leave offspring as quickly as possible. The plant “realises” that conditions are becoming unfavourable (too hot, dry, or the life cycle is ending) and triggers the programme for flowering and seed production. The main triggers of this process:

1. High temperature (heat stress). This is the main enemy of a dense head. Heat (temperatures above 25–28 °C) acts as a trigger. It sharply accelerates the development of flower buds, causing them to “shoot” into flowers, bypassing the stage of forming a quality inflorescence (Wien and Stützel, 2020; Basics of Broccoli Production, 2024). Research shows that plants are particularly sensitive to high temperature during the period when the head is just beginning to form (Wien and Stützel, 2020).

2. Stress and senescence. As we have already discussed, any strong stress (drought, waterlogging, nutrient deficiency) can trigger flowering. In addition, broccoli is mostly an annual crop. If the head is not cut in time, the plant perceives this as a signal that its life cycle is ending and begins to flower. Physiologically, this is a natural ageing process (senescence). First, yellow spots appear on the buds, then they open into full flowers (Teverson, 2020).

3. Varietal characteristics. Some broccoli varieties and hybrids are much more prone to premature flowering than others. Early‑maturing varieties generally transition to flowering more quickly, especially if they are not intended for hot climates. Conversely, modern hybrids for summer and autumn production often have genetic resistance to this disorder (Wien and Stützel, 2020). When choosing seeds for your region and specific season (spring/autumn), be sure to pay attention to this characteristic.

4. Late harvest. This is the simplest and most common cause for gardeners. If the head is not cut in time, it will inevitably begin to flower. Technical maturity occurs when the buds are tightly closed but not yet open. This period can last only a few days in different varieties, especially in hot weather. The warmer it is, the faster the head “over‑ripes” and transitions to flowering.

How to Prevent:

• Timely harvesting: Inspect the planting regularly. Cut the head as soon as it reaches the characteristic size and density for the variety, without waiting for the buds to start “lifting” and separating. In hot weather, check the planting every day.
• Choose the right variety: For spring and autumn planting, choose varieties and hybrids labelled “heat tolerant” and “slow bolting”.
• Regular watering: In dry weather, water broccoli regularly. Drought stress greatly accelerates ageing and flowering. Try to keep the soil moderately moist.
• Protection from heat: In regions with hot climates, shade the planting during the hottest part of the day with shade nets, or plant broccoli so that the head formation period falls during cool weather (early spring or autumn).

So, we see that many disorders are related to extreme temperatures and stress. However, there are also problems caused by nutritional imbalance and rapid growth, such as hollow stem. That will be the topic of the next chapter.

How to Recognise?

Externally, before cutting, it is almost impossible to detect a hollow stem. There are no external signs on the leaves or head. The disorder is diagnosed only at harvest – when cutting the stem or cutting it lengthwise. You will see a void that can extend from the base of the head 5‑15 cm down the stem (Старцев, 2021).

Why Does It Happen?

Hollow stem is not rot or an infection. It is the result of mechanical tearing of the internal stem tissues. The cause is too rapid and uneven growth of the plant, when the inner part of the stem (the pith) simply cannot keep up with the growth of the outer layers. The tissues seem to “tear” from the inside, forming a cavity. The main factors provoking this disorder:

1. Rapid growth and excess nitrogen. This is one of the main causes. When plants receive too much nitrogen fertiliser, especially in combination with abundant watering and warm weather, they begin to grow “broad” very quickly. The stem rapidly elongates and thickens, while the internal tissues do not have time to form and strengthen properly. As a result, tears and cavities appear (Wien and Stützel, 2020; Rana, 2018). Research shows that the higher the nitrogen dose, the greater the likelihood of hollow stem formation (Basics of Broccoli Production, 2024).

2. Boron deficiency (controversial factor). For a long time, the main cause of hollow stem was thought to be boron deficiency. However, modern research shows that the relationship is not so clear‑cut. In some experiments, boron addition did not reduce the number of hollow stems, while in plants grown on “fast” nitrogen nutrition, this disorder appeared regardless of boron levels (Wien and Stützel, 2020). Nevertheless, boron plays an important role in cell wall formation, and its deficiency can exacerbate the problem. It can be said that rapid growth is the “trigger” and boron deficiency is the “fuel” for the fire. Broccoli is particularly sensitive to boron deficiency (Basics of Broccoli Production, 2024).

3. Uneven moisture and weather. Sharp fluctuations in soil moisture and temperature also contribute to uneven growth and can consequently provoke cavity formation. For example, after a dry period, heavy watering or rain can cause a sudden growth spurt that leads to internal tissue tearing.

4. Varietal characteristics. Like many other physiological disorders, the tendency to hollow stem varies from variety to variety. Some hybrids are genetically more resistant to this disorder and tolerate high nitrogen loads better.

How to Prevent:

• Balanced nitrogen nutrition: Do not over‑feed broccoli with nitrogen. It is important to apply it in split doses, especially on fertile soils. Follow the recommended fertiliser rates for your region. Excess nitrogen, especially during active stem growth, is highly undesirable.
• Supply plants with boron: Although not a panacea, preventive application of boron (e.g., as boric acid or borax) reduces the risk of hollow stem, especially on light sandy soils. It is best applied as a foliar spray during head formation.
• Maintain optimal plant density: Too sparse planting encourages individual plants to grow very quickly and “vigorously”, increasing the risk of cavity formation. Optimal density promotes more balanced development (Wien and Stützel, 2020). Research shows that denser plantings suffer less from this disorder (Wien and Stützel, 2020).
• Even watering: Try to water broccoli regularly, avoiding large fluctuations in soil moisture.

So, we have established that hollow stem is primarily a consequence of imbalances in nutrition and water regime. The next disorder, bead browning, is also linked to physiological stresses, but of a slightly different kind.

How to Recognise?

Brown, grey‑brown or black spots appear on the surface of the inflorescence. They are most often located on the top or edges of the head. Externally, it may resemble a burn or rot, but the affected areas are dry, without signs of mould or wet rot. When cut open, it is clear that the discolouration has affected only the superficial buds, while the inner part of the inflorescence remains healthy and green (Teverson, 2020; Wien and Stützel, 2020).

Why Does It Happen?

Bead browning is the result of oxidative stress caused by a combination of unfavourable factors. Essentially, it is a reaction of bud cells to damage when they cannot cope with the load and begin to die. The main causes:

1. Heat stress (high temperature). This is the most common cause. When hot weather sets in during head formation (especially combined with high humidity), the delicate bud tissues are damaged. Sun rays, especially direct ones, cause overheating and burning of the head surface (Basics of Broccoli Production, 2024). Research shows that plants are most sensitive to this disorder at the early head formation stage (Wien and Stützel, 2020).

2. Calcium deficiency. This is another important factor. Calcium is responsible for the strength of cell walls. When it is lacking, tissues become weak and easily damaged under even slight stress. In rapidly growing head tissues, calcium may not be supplied in sufficient quantities, especially in hot weather when transpiration (water evaporation) slows down. This leads to the death of individual bud areas (Wien and Stützel, 2020; Teverson, 2020). This is the same mechanism as tipburn in cabbage or lettuce leaves.

3. Water regime disturbances. As we have already discussed, any sharp fluctuations in moisture (drought → heavy watering) create stress for the plant. During drought, roots cannot absorb calcium, and then, with sudden wetting, rapid growth begins that “consumes” this calcium. As a result, calcium deficiency occurs in head tissues, and buds begin to die (Wien and Stützel, 2020).

4. Insect or hail damage. Any mechanical damage to the head surface creates “entry points” for stress. At the site of damage, cells die and browning may develop around them, especially if the weather is hot or humid.

5. Varietal characteristics. Some broccoli varieties are more prone to bead browning than others. Hybrids with a dense, compact head and good heat tolerance usually suffer less from this disorder.

How to Prevent:

• Regular watering: Keep the soil evenly moist. This is especially important during head formation to allow calcium to move freely into the plant.
• Protection from heat: In hot weather, use shade nets. They reduce temperature and protect delicate buds from direct sunlight.
• Foliar calcium application: Although the effectiveness of this method is not always high, in stress situations (e.g., in heat), spraying leaves with a solution of calcium nitrate (1‑2%) can help reduce the risk of browning.
• Choose resistant varieties: When purchasing seeds, pay attention to varieties and hybrids that are noted as resistant to bead browning and heat stress.
• Timely harvesting: Cut the head at technical maturity, do not allow it to overgrow. The older the head becomes, the greater the risk of browning.

So, bead browning is a vivid example of how calcium deficiency and heat stress can spoil the appearance of the harvest. The next disorder, yellowing of the inflorescence, is also related to heat and light stress, but manifests somewhat differently.

How to Recognise?

The green buds on the head surface start to lighten, losing their characteristic pigment, and become yellow or yellowish‑white. The process usually begins at the edges of the head or on individual areas most exposed to the sun, and then spreads over the entire surface. The tissue becomes softer and sometimes even watery. In advanced cases, the head becomes completely yellow and unfit for consumption (Teverson, 2020).

Why Does It Happen?

Yellowing is a consequence of the breakdown of chlorophyll – the green pigment essential for photosynthesis. This is a natural ageing process (senescence) of plant tissues, accelerated by external factors. The main causes:

1. Exposure to sunlight (light stress). This is one of the main causes of yellowing in hot weather. Direct sunlight literally “burns out” chlorophyll in the surface tissues of the head (Basics of Broccoli Production, 2024). This happens especially quickly if heat is combined with moisture deficiency. Ultraviolet radiation destroys pigments, and green turns to yellow (Старцев, 2021).

2. Ageing (late harvest). If the head is not cut in time, the plant moves to the next development phase. Chlorophyll begins to break down to free up resources for seed formation. This is a natural process that accelerates with head age. Even without direct sunlight, a few days after technical maturity the head will begin to yellow (Teverson, 2020).

3. High temperature. Heat during head formation and growth accelerates ageing and chlorophyll breakdown (Wien and Stützel, 2020; Basics of Broccoli Production, 2024). The optimum temperature for maintaining green colour is cool, around 15–18 °C. At temperatures above 25 °C, the yellowing process accelerates significantly.

4. Ethylene exposure. Ethylene is a gas emitted by many fruits and vegetables during ripening (e.g., apples, bananas, tomatoes, melons). It acts as an ageing hormone and accelerates chlorophyll breakdown. If broccoli is stored near these products, it will quickly turn yellow (Teverson, 2020). This is one reason why broccoli should not be stored with apples.

How to Prevent:

• Timely harvesting: Cut the head as soon as it reaches the characteristic size and density for the variety. Do not wait until it becomes “really big” – in hot weather, you risk getting a yellow overgrown head.
• Protection from sun: On hot, sunny days, you can shade the heads by tying the upper leaves over them. This simple technique creates natural shade and protects from UV rays.
• Proper watering: Keep the soil moist, especially in dry and hot weather. This will help the plant tolerate stress better and slow down ageing.
• Proper storage: After cutting, the broccoli head should be cooled as quickly as possible. Store it in the refrigerator at about 0 °C and high humidity, and try not to keep it near fruits that emit ethylene (apples, bananas, pears). Ideally, wrap it in a slightly damp towel or place it in a perforated bag.

As we can see, yellowing is essentially a signal that the product is overripe or under stress conditions. The next problem, leafy head, is related to a disruption in the formation of the inflorescence itself, when vegetative processes take precedence over reproductive ones.

How to Recognise?

Small, underdeveloped leaves appear on the surface or inside the inflorescence. They may be green, and sometimes even white or pale green (depending on the variety and conditions). The head loses its typical “bumpy” structure and becomes looser, and the leaves make it unattractive. Often this disorder is accompanied by other symptoms, such as looseness or yellowing (Wien and Stützel, 2020; Старцев, 2021).

Why Does It Happen?

Leafy head is a result of disruption of the normal process of inflorescence formation. Instead of completely switching to the production of flower buds, part of the meristem (growing point) “reverts” to vegetative development and begins to form leaves. This phenomenon is called reversion. The main causes:

1. High temperature (heat stress). This is the main cause of leaf formation inside the head. When hot weather (above 25 °C) sets in during inflorescence formation, the bud formation process is disrupted. The plant seems to “get confused” – it does not know whether to flower or continue growing. As a result, leaves begin to form in the head (Wien and Stützel, 2020). Research shows that this is especially typical for varieties that are not adapted to heat. Heat‑tolerant hybrids suffer from this disorder much less frequently (Basics of Broccoli Production, 2024).

2. Excess nitrogen. As with loose curd, overfeeding with nitrogen stimulates vigorous vegetative growth. This can trigger a “return” to leaf formation even at the head‑formation stage, especially when combined with heat stress (Старцев, 2021).

3. Disruption of inflorescence formation by cold. Interestingly, excessively low temperatures, especially at early development stages, can also provoke leaf formation in the head. This more often occurs if plants are exposed to prolonged cold immediately after transplanting, followed by a sharp warming. The temperature contrast “confuses” the plant’s development programme (Wien and Stützel, 2020; Rana, 2018).

4. Varietal characteristics. Genetic predisposition plays a huge role. Some varieties (often old open‑pollinated or early‑maturing ones) are much more prone to leaf formation in the head than modern hybrids. When choosing a variety for your region and season, be sure to pay attention to this characteristic (Rana, 2018).

How to Prevent:

• Choose resistant varieties: This is the most reliable method. Purchase modern hybrids that have performed well in your region. Look for varieties labelled “heat tolerant” and “good head uniformity”.
• Observe planting dates: Try to ensure that the head formation period does not coincide with the peak of summer heat. Transplant so that harvest falls during cool weather (early spring or autumn).
• Balanced nutrition: Do not over‑feed plants with nitrogen. It is important that the soil has adequate phosphorus, potassium and trace elements. This will help balance vegetative and reproductive growth.
• Protection from heat: In regions with hot climates, use shade nets during head formation.
• Regular watering: Keep the soil moist to prevent drought stress, which can worsen the problem.

We have considered disorders related to the formation of the main head. But broccoli has another feature – after cutting the central inflorescence, it can produce side shoots. Sometimes this does not happen, and that is also considered a disorder. That will be the topic of the next chapter.

How to Recognise?

After cutting the central head, the plant begins to die back or simply stands still with no signs of new growth. Side shoots either do not appear at all, or appear in very small numbers – 1‑2 weak shoots instead of 5‑10 or more that a healthy bush could produce. The leaves begin to yellow and dry, and the plant seems to simply “complete” its life cycle.

Why Does It Happen?

Side shoots are a back‑up reproduction mechanism. The plant “knows” that if the main head is damaged or eaten, it still has a chance to produce seeds through side shoots. But this mechanism is activated only under certain conditions. If conditions are unfavourable or the plant is under severe stress, it may “decide” that it is better to channel all remaining resources into maturing seeds in the remaining part of the inflorescence (if not cut) rather than forming new shoots. The main reasons for the lack of side shoots:

1. Varietal characteristics. This is the most important factor. Some broccoli varieties and hybrids have been bred specifically for a single, very large central inflorescence (so‑called “single‑crop” broccoli). Such varieties have a genetically weak ability to produce side shoots. Other varieties are specifically bred for abundant side‑shoot production (“multiple‑crop” broccoli). When choosing seeds, always pay attention to this characteristic. If the variety description says “produces good side shoots” or “suitable for multiple harvests”, you will get a second crop. If the variety is positioned as “for large heads”, side shoots will be few (Basics of Broccoli Production, 2024).

2. Too late harvesting of the central head. If you cut the central inflorescence when it has already started to loosen or even yellow, the plant has already expended significant resources on its development. By that time, the axillary buds may be weakened or completely lose their ability to grow actively. In addition, if individual flowers have already appeared on the head, the plant has received a signal to complete its life cycle, and side‑shoot formation slows down greatly or stops altogether (Wien and Stützel, 2020). The head should be cut strictly at technical maturity, when the buds are tightly closed but not yet opening.

3. Deep cutting. If when cutting the central head you leave too short a stump and cut off part of the stem that carries axillary buds, you simply remove the “primordia” of future side shoots. Leave a stem length of at least 10‑15 cm (4‑6 inches) to retain enough living buds. The longer the stem, the more potential growth points for side shoots (Wien and Stützel, 2020).

4. Nutrient deficiency and water stress. The formation of side shoots requires significant resources – water and nutrients. If after cutting the main head the plant is “starving” or suffering from drought, it simply will not have the strength to produce a new crop. Nitrogen and potassium are especially important for resuming growth (Basics of Broccoli Production, 2024).

5. Stress after the first harvest. Cutting the central head is an injury to the plant. If at this time the weather is very hot or, conversely, has suddenly cooled, the plant may not cope with the stress and “freeze” in development, never producing side shoots.

How to Prevent:

• Choose the right variety: If you want to get a second crop, choose varieties specially designed for this. The description should include words like “multiple‑harvest”, “regrows well after cutting”, “for repeated harvesting”.
• Timely and correct cutting: Cut the central head at technical maturity, do not allow it to overgrow. Leave a stem length of 10‑15 cm to preserve as many axillary buds as possible.
• Fertilise after harvest: Immediately after cutting the central head, feed the plant. Apply a complex fertiliser (e.g., nitrophoska) or a solution of cow manure to stimulate side‑shoot growth.
• Regular watering: After cutting the central head, provide plants with regular watering. This is especially important in dry and hot weather. Without sufficient moisture, side shoots will not form.
• Remove old leaves: After cutting the head, remove the oldest, yellowed or damaged leaves. This will improve ventilation and light penetration to the lower part of the plant and direct nutrients to shoot development.

We have discussed why broccoli sometimes refuses to produce a second crop. This disorder is the result of a combination of varietal characteristics and agronomic mistakes. Finally, the last chapter of our article will address the broadest range of problems – damage from heat and cold. These are universal stresses that underlie many other disorders.

Optimal Temperature Regime

For successful broccoli cultivation, it is important to know its temperature preferences:

• Optimal temperature for growth and development: 15‑20°C (Basics of Broccoli Production, 2024). In this range, the plant actively builds leaf mass and forms a dense, high‑quality head.
• Optimal temperature for head formation: 15‑18°C (Wien and Stützel, 2020). During this period, broccoli is most sensitive to temperature fluctuations.
• Critical heat threshold: Above 25°C, growth slows, and at temperatures above 28‑30°C, serious disorders in inflorescence formation begin (Wien and Stützel, 2020; Basics of Broccoli Production, 2024).
• Critical cold threshold: Broccoli can tolerate short frosts down to -5°C, especially if plants are hardened. However, prolonged exposure to temperatures below 10°C slows growth and can trigger premature buttoning (Wien and Stützel, 2020; Старцев, 2021).

Heat Damage (Heat Stress)

Heat is the main enemy of broccoli. As we have seen in previous chapters, heat stress manifests as a cascade of disorders:

• Loose curd and premature flowering (bolting): High temperature accelerates the development of flower buds, causing them to open into flowers, bypassing the dense head formation stage (Wien and Stützel, 2020).
• Yellowing and browning of buds (bead browning): Direct sunlight and overheating damage the delicate bud tissues, causing them to die and change colour (Teverson, 2020).
• Leaf formation inside the head (leafy head): Heat disrupts the inflorescence development programme, provoking a “return” to vegetative growth and leaf formation inside the head (Wien and Stützel, 2020).
• Hollow stem: In heat, the plant grows very quickly, especially with excess nitrogen and moisture. The internal tissues of the stem cannot keep up with the outer layers, leading to tearing and cavity formation (Wien and Stützel, 2020).

How to Protect Broccoli from Heat:

1. Choose heat‑tolerant varieties. This is the most effective method. Modern hybrids (e.g., labelled “heat tolerant”) have genetic resistance to heat stress and form heads better in hot weather (Basics of Broccoli Production, 2024).

2. Observe planting dates. Try to ensure that the head formation period falls during the cooler part of the year – early spring or autumn. In regions with hot summers, broccoli is best planted in winter or very early spring to harvest before the heat arrives (Wien and Stützel, 2020).

3. Use shading. During the hottest part of the day (12 to 16 hours), cover the planting with shade netting (30‑50% shade). This will reduce the temperature near the ground and protect the heads from direct sunlight.

4. Provide regular watering. Keep the soil moist. This helps the plant cool through evaporation and tolerate heat better. Water preferably in the morning or evening to prevent immediate evaporation.

5. Mulch the soil. A layer of mulch (straw, grass, compost) retains moisture in the soil and protects the roots from overheating.

Cold Damage (Cold Stress)

Although broccoli is considered a cold‑hardy crop, sharp temperature fluctuations and prolonged cold can cause harm:

• Frosts. Young seedlings and tender heads are more sensitive to frost. Although broccoli can withstand short cold spells down to -5°C, prolonged frosts below -10°C can damage or even kill plants (Basics of Broccoli Production, 2024).
• Cold stress (temperatures below +10 °C but above 0 °C). Prolonged exposure to low positive temperatures (especially combined with waterlogging) slows plant growth. This can lead to buttoning – the formation of small heads, especially if the seedlings were overgrown (Wien and Stützel, 2020). Cold can also provoke leaf formation inside the head (leafy head) when a sharp warming follows prolonged cold (Rana, 2018).
• Damage to the growing point (blindness). Severe cold or frost can damage the apical meristem, causing the plant to stop growing upward and fail to form a head (Wien and Stützel, 2020).

How to Protect Broccoli from Cold:

1. Proper hardening of seedlings. Before transplanting, gradually accustom the plants to lower temperatures by taking them outdoors for several hours. This will increase their resistance to possible frosts.

2. Use covers. When frost threatens, cover the planting with non‑woven fabric (spunbond, agrotex) or film on arches. This is especially important for young plants and during head formation.

3. Choose cold‑tolerant varieties. For early spring and late autumn plantings, choose varieties that tolerate lower temperatures well and transition to flowering more slowly under stress.

4. Do not plant too early. Do not rush to transplant seedlings into open ground until the soil has warmed to at least 8‑10°C. Cold soil slows growth and can trigger buttoning.

5. Provide good nutrition and drainage. Nutrient deficiency and waterlogging make plants more vulnerable to cold stress.

Now you know that many problems when growing broccoli are not related to diseases but to physiological disorders caused by stress conditions. Understanding these mechanisms is your main tool for obtaining a healthy and abundant harvest.

Brief Summary for the Gardener:

• The main enemies of broccoli: heat (above +25 °C), drought, and sharp temperature fluctuations.
• Keys to success: moderate climate, regular watering, balanced nutrition, and choosing the right varieties.
• Do not panic at the first signs of disorders; instead, analyse the growing conditions and adjust your care.
• Remember: preventing a problem is always easier than fighting it. Good agronomic practices are the best prevention of all physiological disorders.

We wish you bountiful harvests of firm and tasty broccoli!