Watering

1. Water Consumption by Growth Phase

Understanding why and when to water onion begins with recognizing that the plant's water needs change throughout its life. It's not just "I'm thirsty" — it's a strict alignment with physiological processes. The onion root system is poorly developed and shallow (Balashev and Zeman, 1981; Brewster, 2008). It does not penetrate deep into the soil; the bulk of the roots (up to 80%) are in the 20–30 cm layer, with only a few roots reaching 90–100 cm deep (Balashev and Zeman, 1981). This means onions cannot "mine" water from deep horizons and are entirely dependent on the moisture in the topsoil. Consequently, they respond well to irrigation but are also extremely sensitive to both deficit and excess (Brewster, 2008).

Watering onions can be compared to fine-tuning: how correctly you supply water determines whether the plant will develop foliage or form a large bulb. Let's look at the main phases.

• Seed germination and emergence: During this period, water is critical for seed swelling and initiating growth processes. Onion seeds germinate slowly, and if the topsoil lacks sufficient moisture, seedlings will be uneven or may not appear at all (Balashev and Zeman, 1981). This stage sets the pace for the entire future harvest.
• Leaf growth (vegetative phase): After emergence, the plant's main task is to build a strong leaf apparatus. The more leaves, the more photosynthesis and the more nutrients will be stored in the future bulb. This requires a lot of water. Moisture at this stage promotes rapid and healthy growth of green mass (Autko et al., 2012).
• Bulb formation and filling: This is the most critical and decisive period in terms of water consumption. It is now that nutrients are redistributed from the leaves to the bulb, and it begins to actively increase in size. Moisture at this stage is needed to keep this process continuous and at full strength. This is the key to achieving a high yield (Brewster, 2008).
• Bulb maturation: As harvest approaches, the need for moisture drops sharply. During this period, it is important to allow the bulb to "ripen" so that the outer scales dry out and the neck becomes thin and dry. This is the main condition for good storage and long shelf life (Rubatzky & Yamaguchi, 1997; Brewster, 2008). Excess moisture at this time will only do harm.

General rule: The optimal soil moisture for onions in the first two stages (germination and leaf growth) and at the beginning of bulb formation is 75–80% of Field Capacity (FC). This is a state where the soil contains both water and air, essential for root respiration. By maintaining this moisture level, we create ideal conditions for growth (Autko et al., 2012). At the end of the growing season and during maturation, watering is reduced to a minimum or stopped altogether.

2. After Planting

Proper watering right after planting is the foundation upon which the entire future harvest is built. How uniformly and quickly seedlings emerge or bulbs root determines whether the plant will have enough time to build sufficient leaf mass before bulb formation begins. It is at this stage that the yield potential is set.

3. Leaf Growth

Once onions have rooted and the first true leaves begin to grow, the active vegetative phase begins. This is the period when the plant works as a "green factory": it absorbs sunlight, water, and nutrients to create biomass. The gardener's main task at this time is to provide this "factory" with enough moisture for uninterrupted operation.

Why it matters

Each new leaf is an investment in the future harvest. The number and size of leaves directly determine the size of the bulb. Research shows that to form a large bulb, the plant needs to develop a certain leaf area (Brewster, 2008). Moisture is a key resource for this growth, as it participates in photosynthesis and the transport of nutrients from roots to leaves.

If onions experience thirst during this period, leaf growth slows down. The plant will not have time to build the required mass by the time bulb formation begins. As a result, the bulb will be small, even if you water it abundantly later — the lost time cannot be made up.

How to water

In this phase, onions consume the greatest amount of water throughout the growing season. Optimal soil moisture should be maintained at 75–80% of field capacity (FC) (Autko et al., 2012). In practice, this means that the soil in the root zone (15–30 cm) should be constantly moist, but not wet.

Frequency and amount:

• On average, in a temperate climate without extreme heat, watering is done about once every 5–7 days.
• In hot and dry weather, intervals are shortened to 3–4 days.
• In cool or rainy weather, watering can be skipped or done less frequently.
• The amount of water per application should be sufficient to wet the root zone to a depth of at least 20–30 cm. Roughly, this is 10–15 liters per square meter (corresponding to 100–150 m³/ha for industrial scales) (Autko et al., 2012).

Watering methods

The best method for this phase is drip irrigation. It delivers water directly to the plant roots, evenly moistening the soil, without wetting the leaves and without promoting fungal diseases (Brewster, 2008). Moreover, drip irrigation prevents soil erosion and crust formation.

If drip irrigation is not available, you can use sprinkling or furrow irrigation. However, these methods have drawbacks:

• Sprinkling can promote downy mildew (peronosporosis), especially in cool and humid weather. Therefore, it is better to water at the base or along furrows.
• Furrow irrigation provides abundant moisture but can lead to overwatering and nutrient leaching. With this method, it is important to ensure that water does not stagnate around the roots.

Important nuances

1. Do not allow drying out. Even short-term drying of the topsoil can stress the plant and stop leaf growth. Growth will resume, but the lost time and developmental delay will affect the yield.

2. Avoid waterlogging. Excess moisture is no less dangerous than deficiency. When overwatered, roots experience oxygen starvation, which slows their function and can lead to basal rot (Brewster, 2008).

3. Consider soil type. On light sandy soils, water drains and evaporates faster, so watering should be more frequent but with smaller amounts. On heavy clay soils, water stays longer, so watering can be less frequent but more abundant.

4. Observe the plants. Onions themselves signal moisture deficiency: leaves lose turgor, become limp, and tips turn yellow. This is a signal that it is time to water. However, it is better not to let it get to that point.

Practical tip: Rely on weather and soil moisture. Take a handful of soil from a depth of 10–15 cm and squeeze it in your fist. If the soil is moist and does not crumble — it is too early to water. If the soil is dry and crumbles — it is time to water. This simple test will help you avoid both overdrying and overwatering. Remember: your goal is to maintain stable moisture, not to swing from one extreme to the other.

4. Bulb Formation

This stage is the culmination of all your efforts. The plant switches from building green mass to storing nutrients in the bulb. It is now that the foundation of the future harvest is laid: its size, quality, and storage ability. Watering during this period requires special attention and understanding of physiological processes.

Why it matters

During bulb formation, resources are redistributed. The plant stops actively producing new leaves and directs all its energy to "filling" the bulb. This is the most critical moment in terms of water consumption (Brewster, 2008). If moisture is insufficient, the filling process slows or stops, and the bulb remains small. Excess moisture, especially combined with high temperatures, can cause bulb cracking, premature sprouting, and disease development (Rubatzky & Yamaguchi, 1997; Brewster, 2008).

The key principle of this period is abundant but not excessive watering. As soon as the bulb begins to increase in diameter, the plant's water demand reaches its peak.

How to water

In this phase, soil moisture should be maintained at the same high level — 75–80% of FC (Autko et al., 2012). However, the frequency and volume of watering may be adjusted depending on the weather.

Frequency and amount:

• Water generously, but with the same frequency as in the leaf growth phase: roughly once every 5–7 days in normal weather.
• In hot and dry weather, reduce intervals to 3–4 days.
• The amount should be sufficient to wet the soil to a depth of at least 30 cm. Approximate rate — 15–20 liters per square meter. This ensures moisture availability to the entire root system (Brewster, 2008).

Important nuance: At the onset of active bulb formation, many gardeners make the mistake of watering even more frequently and abundantly, thinking it will speed up growth. However, this is not the case. Excessive moisture does not accelerate filling; on the contrary, it can trigger diseases and reduce quality. The plant cannot absorb more water than it needs. The surplus either evaporates or stagnates, creating problems.

Practical recommendations for this period:

1. Monitor soil and plant condition. Continue using the handful test at a depth of 10–15 cm. If the soil is moist and does not crumble — postpone watering. If dry — water.

2. Consider soil type. On light sandy soils, water drains faster, so water more frequently but in smaller volumes. On heavy clay soils, water less often but more abundantly. On loamy and sandy loam soils, onions perform best (Balashev and Zeman, 1981; Autko et al., 2012).

3. Avoid watering during hot daytime hours. If using sprinkling, do it early in the morning or in the evening so that water can soak into the soil rather than evaporate, and so that water droplets on leaves do not cause sunburn.

4. Pay special attention to water quality. Onions are very sensitive to soil salinity. Using water with high salt concentration can cause root burn and slow growth. Whenever possible, use soft water or water that has been settled and warmed in the sun (Brewster, 2008).

5. Monitor the weather. In rainy weather, reduce or skip watering altogether to avoid waterlogging. Water stagnation around roots is one of the main causes of fungal diseases, especially fusarium and gray rot.

Hint: In this period, drip irrigation is especially effective. It provides precise water dosing, does not over-moisten the soil, and does not create favorable conditions for diseases. If you have the opportunity to set it up, it is the best choice for obtaining a high-quality harvest of large bulbs (Brewster, 2008).

The main goal of watering during bulb formation is to ensure continuous, smooth filling. As soon as you notice that bulbs have reached the desired size and leaves begin to yellow and lodge, it is time for the next, final stage — preparation for harvest. We will discuss how to properly finish the irrigation season in the next chapter.

5. Maturation

The final stage of onion cultivation is a time when your task as a gardener changes dramatically. Throughout the previous period, you tried to supply the plants with moisture so they could grow actively and accumulate mass. Now comes the moment when watering should not just be reduced but practically stopped. This is one of the most important and, unfortunately, often underestimated stages, which directly determines how long and well your harvest will store.

Why it matters

When the bulb reaches the desired size, the plant begins to prepare for dormancy. Leaves turn yellow, lose turgor, and lodge. All nutrients that were in the leaves are transferred to the bulb. At this time, the outer scales of the bulb begin to dry, becoming dense and leathery. The neck of the bulb (the junction of leaves and bulb) should become thin, dry, and tightly closed. This provides protection against pathogen entry during storage (Rubatzky & Yamaguchi, 1997; Brewster, 2008).

If you continue to water onions as abundantly as during growth or filling, the maturation process is disrupted. The bulb will continue to absorb moisture, the neck will remain thick and juicy, and the outer scales will be thin and insufficiently dried. Such onions store poorly: they are prone to rot, especially neck rot, and sprout quickly (Autko et al., 2012). Therefore, reducing watering is not a whim but a necessary agronomic measure.

When to start reducing watering

Two signs serve as guides:

1. Bulbs have reached the size typical for the variety (you can assess visually or by digging up a few control bulbs).

2. About 50–80% of plants have begun to lodge, i.e., leaves have bent toward the ground and started yellowing (Rubatzky & Yamaguchi, 1997; Autko et al., 2012).

From this moment on, gradually reduce watering. Depending on weather and soil type, the process of stopping watering may take 2–3 weeks.

How to reduce watering

1. Gradually reduce frequency and volume. Do not stop watering abruptly if the soil is dry. Increase intervals between waterings (e.g., from 5–7 days to 10–12 days) and reduce the amount of water per application.

2. Complete cessation of watering. Approximately 2–3 weeks before the expected harvest, watering should be stopped completely (Autko et al., 2012). At this time, the bulb should ripen in dry soil. If it rains, try to cover the bed or ensure drainage to prevent soil from becoming waterlogged.

3. If there is extreme heat. In very hot and dry weather, when the soil cracks, you can give one light watering 10–12 days before harvest to prevent overheating and cracking of bulbs, but generally this is not recommended. It is better to rely on plant condition and soil moisture.

What happens in practice

When watering stops, the soil begins to dry. This stimulates further bulb maturation. The outer scales become dry and dense, the neck shrinks and closes. The onion essentially "preserves" itself under natural conditions.

Signs that onions are ripe and ready for harvest:

• Leaves are completely yellow and lodged.
• The neck of the bulb is thin and dry.
• Outer scales have acquired the characteristic color for the variety and become firm, papery.
• The bulb easily separates from the soil (Rubatzky & Yamaguchi, 1997).

What not to do:

• Water onions 2–3 weeks before harvest unless absolutely necessary (e.g., severe drought that could cause cracking). Even one watering during this period can reduce storage ability.
• Leave onions in wet soil if it rains. Try to protect the bed from excess moisture.
• Hasten harvest while leaves are still green and upright. Allow onions to fully mature, otherwise they will store poorly.

Important tip: If you notice that some plants are starting to bolt (flower stalks), be sure to break them off. Bolting diverts energy from the plant and reduces bulb quality. This is especially important for varieties prone to bolting under adverse conditions (e.g., sudden temperature changes) (Balashev and Zeman, 1981).

The maturation phase is a kind of "hardening" of onions before a long winter. Correctly executed reduction of watering ensures that your bulbs will be not only large and tasty but will also last until the next harvest without losses.

6. When to Stop Watering

This is perhaps the most important question gardeners ask themselves: when exactly should you finally stop watering onions to get a harvest that will store long and well? The answer is simple but requires attentiveness and observation.

The main rule: Stop watering 2–3 weeks before the expected harvest, when a significant portion of the plants (50–80%) have begun to yellow and lodge (Rubatzky & Yamaguchi, 1997; Autko et al., 2012).

This rule is the gold standard that works in most climatic zones. However, like any rule, it requires adjustment depending on weather conditions, soil type, and plant condition.

How to determine the exact moment

Rely not on the calendar but on the condition of the plants. Here are specific signs that tell you it's time to stop watering:

1. Leaf lodging. This is the most reliable sign. When leaves lose turgor, bend toward the ground, and begin to yellow, the plant signals: "I have finished vegetation, preparing for rest." At this point, bulb filling is practically complete.

2. Leaf yellowing. Yellow and drying leaf tips are a natural aging process. If not only the tip but the entire leaf blade is yellowing, the onion is fully maturing.

3. Softening of the neck. The neck of the bulb (the junction of leaves and bulb) becomes soft and thin. This indicates that the neck is beginning to dry and close, protecting the bulb from infections.

4. Bulb size. Make sure bulbs have reached the size characteristic of your variety. If you see that they have stopped increasing in volume and leaves are wilting, it is time to stop watering.

Why this works

When we stop watering, the plant is placed in conditions close to natural for the end of the growing season. It receives a signal: "Moisture is leaving, time to finish." In response, the following happens:

• Nutrient outflow from leaves to the bulb accelerates.
• Outer scales dry and become dense, papery. This is a natural "packaging" that protects the bulb from moisture loss and pathogen entry.
• The neck shrinks and closes tightly. This is the main barrier against neck rot (Rubatzky & Yamaguchi, 1997).

What if it rains?

This is the most difficult case. If prolonged rains occur during bulb maturation, your task is to protect the bed from excess moisture. Here is what you can do:

• Set up a temporary cover. Stretch film or any waterproof material over the bed to prevent rainwater from reaching the plants.
• Ensure drainage. If the bed is in a low area, dig drainage channels so that water drains away rather than stagnates.
• Do not water, even if the soil seems dry. During rains, soil moisture is already high. Better to wait until dry weather sets in.

What if there is extreme heat and drought?

In very hot weather, when the soil cracks, you may face another problem: bulbs may start to "bake" in the dry, hot soil, leading to cracking and loss of marketable appearance. In this case, it is permissible to give one light watering 10–12 days before harvest, but this is more an exception than a rule. Usually, onions tolerate dryness well at the end of the growing season.

A simple readiness test

If you are unsure whether it is time to stop watering, conduct a simple test:

1. Dig up a few bulbs from different parts of the bed.

2. Assess the neck condition: it should be soft and thin.

3. Look at the outer scales: they should be dry and dense.

4. If bulbs separate easily from the soil and leaves have yellowed, you can safely stop watering and prepare for harvest.

Conclusion:

Stopping watering is not just "stop pouring water." It is a deliberate agronomic technique that triggers the natural maturation mechanism of the bulb. How correctly you do this determines whether your onions will last all winter or start rotting within a couple of months. And remember: it is better to under-water than to over-water during this period.

In the next chapter, we will cover the most common mistakes in watering onions so you can avoid them and achieve maximum yield.

Brief summary on timing for stopping watering:

7. Mistakes

Even experienced gardeners sometimes make mistakes when watering onions. But if you understand plant physiology, you can easily avoid them. In this chapter, we will analyze the most common miscalculations so that you can recognize them in time and correct your actions.

References

1. Brewster, J.L. (2008). ‘Agronomy and crop production.’, in Onions and other vegetable alliums. Wallingford: CABI, 251-307.
2. Hochmuth, G.J., Sideman, R.G. (2023). ‘Field Planting’, in Knott's Handbook for Vegetable Growers. : John Wiley & Sons, pp. 155-198.
3. Rubatzky, V.E., Yamaguchi, M. (1997). ‘Alliums’, in World Vegetables. Boston, MA: Springer US, 279-332.
4. Аутко, А.А. (2012). ‘Современные технологии выращивания овощных культур [Modern technologies for growing vegetable crops]’, in Современные технологии в овощеводстве [Modern technologies in vegetable growing]. Минск, Белоруссия: Беларус. навука, pp. 93-347.
5. Балашев, Н.Н., Земан, Г.О. (1981). ‘Луковые овощи [Onion vegetables]’, in Зуев, В.И. (ed.) Овощеводство [Vegetable growing]. Ташкент, Навои, Узбекистан: Укитувчи, pp. 266-286.