Watering tomatoes in cool weather
1. Why Tomato Wilts in Moist Soil
Imagine the situation: the soil under your tomatoes is moist, even wet—you recently gave them a generous watering—but by evening, the leaves have lost their turgor and drooped. Sound familiar? At such a moment, a gardener often reaches for the watering can to "top up" the water. This is the most common and dangerous mistake.
The key idea to grasp is: wet soil ≠ the plant is supplied with water.
Tomato wilting in cold, damp weather is a classic example of physiological drought. The plant cannot access water, even when it's literally "squelching" around its roots. Let's understand why this happens, so you never make this mistake again.
Roots are pumps, and cold stops them.
Water uptake by roots is not passive absorption like a sponge. It's an active physiological process that requires energy and depends on the function of root hair cell membranes (Heuvelink, 2018). The speed and efficiency of this process are directly dependent on soil temperature.
For normal root system function, tomatoes need an optimal soil temperature in the range of 20–25 °C (Jones, 2008). At this temperature, water is easily absorbed and transported through the vessels to the leaves, maintaining their turgor and supporting photosynthesis.
Problems arise when soil temperature drops below 14–16 °C. In this case:
1. Metabolism slows down: The rate of all biochemical reactions in root cells drops sharply. The activity of membrane pumps (special proteins) that "pump" mineral ions into the cell decreases (Jones, 2008). Without these ions, water cannot enter the root.
2. Membrane permeability decreases: Cell membranes become less elastic and permeable. Water has difficulty passing through them into the stem's conducting system (Kotov and Adrițkaya, 2016).
3. Root hair activity decreases: This root zone is responsible for the primary water absorption. Cold suppresses its work.
The atmosphere "pulls" water harder.
While the roots have "fallen asleep" from the cold, the above-ground part of the plant may continue to actively evaporate moisture. This is especially dangerous when a cold night is followed by a clear, windy day. The plant opens its stomata to cool down and absorb carbon dioxide, but because the roots can't keep up, it loses more water than it receives. An imbalance occurs, and cells lose turgor. Leaves wilt, even though water surrounds the roots.
This stress mechanism leads to a cascade of negative consequences: shoot growth ceases, flowers and fruit sets drop, and the plant becomes more susceptible to root rots (Akhatov, 2010).
Therefore, wilting is not a signal to "give water," but a signal to "warm the roots" and "stop pouring water." Adding even more water to cold soil only worsens the situation: you displace the remaining oxygen from the soil, which roots need for respiration, and create ideal conditions for the development of pathogenic fungi.
2. What Temperature Is Dangerous
To understand when your watering can cause harm, you need to stop relying on your own feelings and start relying on thermometer readings. For tomatoes, two temperatures are decisive: soil temperature and air temperature. Both can become critical in cool weather.
What Happens to Roots When Soil Temperature Drops?
Tomato roots are not a passive "hose" sucking up water. They are a complex mechanism that only works within a specific thermal range. When the temperature drops below the "comfort point," this mechanism starts to malfunction.
Here is a simple cheat-sheet table to help you quickly assess the situation:
| Soil Temperature | What Happens to the Root and Plant | Your Actions |
|---|---|---|
| Optimum: 20–25 °C | Root hairs actively work. Water and nutrients are absorbed most efficiently. The plant is vigorous, leaves are turgid. | Standard watering schedule. Water can be slightly cooler than the air (by 2-3 °C). |
| Below 18 °C | Root hair activity begins to decline. Uptake of water and especially phosphorus (P) and potassium (K) slows down. Plant growth starts to stall. (Swiader and Ware, 1992). | Pay attention. Reduce watering volume by 20–30%. Ensure water for irrigation is warm (around 20–22 °C). |
| 14–16 °C – Critical Threshold | Physiological drought. Roots almost stop absorbing water. The plant starts to wilt even in moist soil. Metabolism is severely inhibited. Phosphorus uptake practically ceases. (Jones, 2008) | Danger Zone. Reduce watering sharply. Water only to sustain life, not to promote growth. Water only when absolutely necessary and only with warm water (25–30 °C). |
| Below 10 °C | The root system is practically non-functional. Uptake of water and minerals is near zero. Growth stops entirely. Prolonged exposure to this temperature can cause irreversible tissue damage and root rots. (Gavrish, 2005) | Stop. Cease watering completely unless the plant is on the verge of dying from dehydration (which is unlikely). The main goal is to protect roots from rotting and allow them to "breathe." |
How Root Activity Declines During Cooling
The graph shows the relative ability of roots to absorb water depending on soil temperature. Hover over the line to see exact values. The color zones indicate how critical the situation is.
Interpretation: green zone (20°C and above) — roots work at 100%. Yellow (16–20°C) — activity begins to decline. Orange (14–16°C) — critical threshold of "physiological drought." Red (below 14°C) — roots practically do not absorb water; watering is useless and dangerous.
It's Not Just Cold Soil That's Dangerous
The temperature of the air around the plant is equally important. Tomato is a heat-loving crop. Its optimal temperature range for vegetation is 18–27 °C (Jones, 2008).
- What happens at temperatures below 15 °C: The process of pollination and fruit set is disrupted. Pollen becomes sterile, flowers drop without forming fruit sets (Akhatov, 2010).
- What happens at temperatures below 10 °C: Plant growth stops entirely. All life processes slow to a minimum. This is a state of "sleep," and watering at this time is not only useless but dangerous.
Why is it important to consider both together?
If you have cold soil (<15 °C) but the air warms up to 20 °C during the day, a dangerous situation arises. Warm air "pulls" water from the leaves, causing them to evaporate moisture (transpiration), but cold roots cannot compensate for this loss. The plant wilts, and the gardener, seeing this, reaches for the watering can again, worsening the problem.
How to Determine Soil Temperature Without Special Instruments?
You may not have an expensive soil thermometer. But there is a simple empirical method: if you find it uncomfortable to kneel on the ground (it feels icy), if the air temperature has stayed around 10-12 °C for several days, then the soil has likely cooled below the critical mark. In this case, it's better to play it safe and reduce watering rather than risk the health of your plants.
Conclusion: The golden rule is: If the soil is cold (below +14 °C), and especially if it's wet — do not water! Your main task now is not to saturate the plant with water, but to preserve its root system. Let the soil warm up.
3. How to Adjust Watering Norms
When cool weather arrives, your main task is not to give the plant water, but to give it a chance to survive. This means you need to completely reconsider your usual watering regime. The rule "more often and more" in the cold turns into "less often and less."
The main principle in cool weather: we water the soil, not the plant. Our goal is not to saturate the plant itself with moisture (it still won't be able to absorb it), but to maintain minimal viability of the soil environment and prevent the roots from drying out to the point where they start dying.
How Much to Reduce the Volume?
There is no universal figure suitable for everyone, as much depends on your soil type and plant age. However, there is a clear practical rule that always works:
Water less frequently, but more deeply.
In cool weather, the intervals between waterings should be increased by 1.5–2 times compared to the summer norm (Heuvelink, 2018). The total volume of water per watering does not need to be reduced, and sometimes can even be slightly increased, to wet a deeper layer of soil. This encourages roots to grow deeper, where the temperature is more stable and where they will be better protected from fluctuations. But! The key word here is — less frequently.
Here's how it looks in practice:
- In warm weather (optimum): You water, for example, every other day with 3-4 liters per bush.
- In cool weather (< 18 °C): You water not every other day, but once every 3-4 days, with the same volume (3-4 liters) or slightly more, to thoroughly wet the deep layers.
Why does this work?
In cold soil, water evaporates much more slowly. In addition, the plant itself needs less water because its metabolism is slowed down. If you continue watering on a summer schedule, the water will simply stagnate in the topsoil.
The Danger of Overwatering: Roots Suffocate and Rot
Tomato roots need not only water but also oxygen. In cold, waterlogged soil, there is practically no oxygen left. Water displaces air from the soil pores, and the roots begin to "suffocate." In such anaerobic conditions, pathogenic fungi causing root rots, as well as bacterial diseases, develop rapidly (Welbaum, 2015; Kotov and Adrițkaya, 2016).
This is why wet soil in cold weather is not a blessing but a deadly threat. Roots deprived of oxygen die, and the plant perishes. At the same time, externally it may look exactly the same as with a lack of water: leaves wilt and droop. But in one case, it's a signal to water; in the other, it's a signal to immediately stop watering.
A Simple Test for Watering Need
How do you determine when it's time to water if the old schedule no longer works? Forget about the top crust. In cool weather, the top layer can remain moist for a long time, while deep down, a moisture deficit may already be starting.
1. Take a handful of soil from a depth of 8-10 cm (about the length of a shovel blade or your index finger).
2. Squeeze it in your fist. If no water drips from the clump and it doesn't crumble into dust but holds its shape — there is enough moisture, no watering is needed.
3. If the clump crumbles in your hand — the soil is dry, and it's time to water. (Jones, 2008)
This simple test is more reliable than any schedule because it takes into account the actual condition of the soil at the depth where the main mass of roots resides.
Approximate Watering Adjustment Table
(Refer to soil temperature, not air temperature)
| Soil Temperature | Watering Frequency (compared to summer norm) | Water Volume per Watering | Note |
|---|---|---|---|
| 20–25 °C (Optimum) | Standard (once every 1-2 days) | Standard (3-5 L) | Full moisture absorption. Plant is actively growing. |
| 16–18 °C | Reduce by 1.5 times (once every 2-3 days) | Standard | Reduce the risk of overwatering. Ensure water is warm. |
| 10–15 °C (Critical Zone) | Reduce by 2-3 times (once every 3-5 days) | Slightly increase (by 20%) | Water only when absolutely necessary. Better to "underwater" than "overwater." Check moisture at depth. |
| Below 10 °C | Stop watering entirely. | — | Roots are not working. Watering can only be harmful. Save the plant with warmth and dryness. |
Important: This is approximate data. Always consider your soil type. Sandy soils dry out faster, clay soils more slowly. In clay soil during cool weather, it's better to water even less frequently and with a smaller volume to avoid crust formation and waterlogging.
Conclusion: Cool weather requires a conscious approach to watering. Your task is not to add more water, but not to overwater. Increasing intervals between waterings is the safest and most effective way to protect tomatoes from root rots and physiological drought.
4. Water Temperature: Why It's Critically Important
Many gardeners believe that water temperature for irrigation is a matter of plant comfort at best. In reality, in cool weather, cold water can become a fatal factor for tomatoes, negating all your efforts.
Cold water is an additional stress, and sometimes a shock.
When you water tomatoes with water from a well, borehole, or straight from the tap, its temperature often does not exceed 10–12 °C. In warm weather, this is not so bad: the water warms up quickly in the soil, and the roots manage to absorb it. But in cool weather, when the soil itself is cold, watering with ice-cold water is a direct path to disaster (Jones, 2008).
Why is Cold Water Dangerous?
1. Sharp temperature drop (thermal shock). When cold water reaches the roots, the soil temperature around them drops sharply. This causes spasms in the root hairs, and they stop absorbing water. The plant experiences shock, which can manifest as temporary wilting (Akhatov, 2010). With regular watering with cold water, the root system gradually weakens.
2. Slowed metabolism. We already discussed that at temperatures below 14–16 °C, roots practically stop working (Jones, 2008). Cold water only worsens the situation, further lowering the soil temperature. This leads to a complete halt in water absorption, regardless of its quantity.
3. Risk of root rots. Cold, poorly warmed soil is an ideal environment for the development of pathogenic fungi that cause root rots (Atherton and Rudich, 1986). Cold water entering the root zone only increases this risk, creating conditions for fungal spore germination and root infection.
What Water Temperature is Optimal?
The ideal water temperature for watering tomatoes is room temperature or slightly warmer, approximately 20–25 °C (Heuvelink, 2018; Welbaum, 2015). Such water does not cause stress, is easily absorbed by roots, and promotes active growth.
In cool weather, the water should be warmer than the surrounding air. This helps warm the soil and stimulate root function. The optimal temperature is 25–30 °C.
How to Properly Heat Water for Irrigation?
Many gardeners don't have the means to heat water for irrigation every time. But there are several simple and accessible methods:
1. Use storage barrels. Place a large container (barrel, tank) of dark color in the greenhouse or near the beds. Fill it with water. During the day, the water will warm up to air temperature, and in a greenhouse, even higher. Do not use cold water from a borehole or well.
2. Water in the first half of the day. If you're watering from a barrel, do it in the first half of the day when the water has already warmed up and the soil is starting to heat up. Evening watering with cold water is especially dangerous because the soil cools down at night, and moisture stagnates (Gavrish, 2005).
3. Use warm solutions. If you combine watering with fertilizing, the solution should be warm (20–25 °C). This improves nutrient absorption and does not create stress for the roots.
What to Do If You've Already Watered with Cold Water?
If you accidentally or unknowingly watered tomatoes with cold water, don't panic. Here's what you can do:
1. Loosen the soil. Gently loosen the soil around the stem to a depth of 3–5 cm. This will improve aeration, and the soil will warm up faster.
2. Temporary cover. If possible, cover the root zone with clear plastic film or agrofabric. This will create a greenhouse effect and help warm the soil faster.
3. Stop watering for a few days. Allow the soil to warm up and "recover." The roots must regain their ability to absorb water.
Basic Rules for Watering in Cool Weather
| Rule | Explanation |
|---|---|
| Do not use cold water | Water from a well, borehole, or tap is typically 10–12 °C. This causes shock and stops roots. |
| Use room temperature water | Ideal temperature is 20–25 °C. Such water does not cause stress and is easily absorbed. |
| In cold weather, use warm water (25–30 °C) | Warm water helps warm the soil and stimulates root function. |
| Water during the day, in the first half | The soil is warm, and the plant actively absorbs moisture. |
| Do not water in the evening during cold weather | The soil cools at night, water stagnates, increasing the risk of rot. |
Conclusion: Water temperature is not a minor detail but one of the key factors affecting tomato health in cool weather. Cold water can cause stress, disease, and even plant death. Warm water, on the other hand, helps them adapt and continue growing even at lower temperatures.
5. Watering in the Greenhouse: Microclimate Peculiarities
A greenhouse is not just a shelter from rain and wind. It is a complex system where temperature, humidity, and air movement interact differently than in open ground. In cool weather, this interaction can create both additional opportunities and serious risks for tomatoes.
How Does a Greenhouse Differ from Open Ground?
1. Temperature inversions. During the day, the air in a greenhouse can warm up significantly more than outside, even in cool weather. But at night, especially in calm weather, the greenhouse cools quickly, and the temperature inside can drop lower than outside (Kotov and Adrițkaya, 2016). This creates sharp fluctuations that weaken plants.
2. High air humidity. In a closed greenhouse, moisture evaporated by plants does not escape into the atmosphere but accumulates. In cool weather, when ventilation is limited, relative humidity can reach 90–95% (Akhatov, 2010). This is an ideal condition for the development of fungal diseases: late blight, gray mold, Cladosporiosis (leaf mold).
3. Stagnant air. Unlike open ground, where wind constantly mixes the air, in a greenhouse during cool weather, air movement is minimal. This leads to moist air stagnating around the plants, especially in the lower tiers.
4. Limited drainage. In a greenhouse, water does not drain into the depths as quickly as in open ground, especially if the soil is compacted or there is a clay layer at the bottom. This increases the risk of water stagnation and root waterlogging.
In Cool Weather, Watering and Ventilation Go Hand in Hand
The main rule of watering in a greenhouse in cool weather: less water — more air. If you've reduced watering but haven't ensured ventilation, you create ideal conditions for diseases. The moisture you give to the plants must be removed through transpiration and evaporation, not stagnant in the air.
This is why not only watering rules but also ventilation rules are important in a greenhouse:
- Water only in the first half of the day. This allows excess moisture to evaporate before nightfall. Evening watering in a greenhouse in cool weather is almost a guaranteed outbreak of fungal diseases (Gavrish, 2005).
- Ventilate after watering. Open vents or greenhouse doors for 30–60 minutes to let moist air escape. Even in cool weather, short-term ventilation is better than stagnant humid air.
- Remove condensation. Water droplets on the film or glass are not just moisture; they are a source of infection. They fall on leaves and create foci for disease development. Regularly shake the film or remove condensation from inner surfaces.
Peculiarities of Watering in a Greenhouse in Cool Weather
| Feature | Practical Recommendation |
|---|---|
| Increased air humidity | Water only when necessary. Better to "underwater" than "overwater." |
| Stagnant air | Be sure to ventilate the greenhouse in the first half of the day. |
| Sharp temperature drops | Water with warm water (25–30 °C) to avoid cooling the soil. |
| Limited drainage | Ensure water does not stagnate. Create drainage ditches if necessary. |
| Condensation | Remove water droplets from the inner surfaces of the greenhouse. |
Practical Tips for Watering in a Greenhouse in Cool Weather
1. Water in furrows, not by overhead sprinkling. Overhead watering (sprinkling from above) greatly increases air humidity and provokes diseases. In cool weather, it's better to use drip irrigation or watering into furrows so that moisture goes directly to the roots, not onto the leaves.
2. Use warm barrels. Place a large container of water inside the greenhouse. The water in it will warm up from sunlight and help maintain temperature in the greenhouse at night (storing heat). In addition, you will always have warm water for watering.
3. Loosen the soil after watering. Even in a greenhouse, loosening helps break the soil crust and improve air access to roots. This is especially important in cool weather when roots are weakened.
4. Monitor the forecast. If a sharp cold snap is expected, stop watering 1–2 days before it. Dry soil warms up faster than wet soil.
How to Tell If the Greenhouse Is Too Humid?
- Water droplets appear on the leaves ("crying" of the plant).
- Condensation constantly hangs on the inner surface of the film or glass.
- Signs of fungal diseases appear: gray coating, brown spots on leaves and stems.
- Plants start to wilt even though the soil is moist.
In these cases, stop watering immediately and increase ventilation!
Conclusion: Watering in a greenhouse in cool weather is a balance between the need to give the plant water and the danger of creating ideal conditions for diseases. Your task is not just to pour water, but to manage the microclimate: combine watering with ventilation, use warm water, and avoid moisture stagnation.
6. Watering After Cold Nights: A Step-by-Step Guide
A cold night is always stressful for tomatoes. But the morning after is a time when the right actions can minimize losses, and the wrong ones can finish off already weakened plants. Here is a step-by-step guide to help you navigate.
Step 1: Assess Plant Condition (before 9:00 AM)
The first thing to do is go to the garden or greenhouse and carefully inspect the plants. Pay attention to the following signs:
| Sign | What It Means | Your Actions |
|---|---|---|
| Leaves are drooping, but turgor is recovering | This is a normal reaction to cold. The plant "survived" the night and is starting to recover. | Do nothing. Do not water! Give the plant time to adapt. By noon, turgor should recover. |
| Leaves are drooping, turgor is not recovering, soil is moist | Physiological drought. Roots are not working due to cold, even though water is present in the soil. | Urgently stop watering! Your task is not to give water, but to warm the soil (see Step 3). |
| Leaves are drooping, turgor is not recovering, soil is dry | Dehydration due to stress. The plant has lost moisture, and roots cannot replenish it. | Cautious watering with warm water and urgent loosening (see Step 2 and 3). |
| Dark, watery spots appear on leaves | Sign of cold damage to tissues. Cells are destroyed, and this is irreversible. | Treat with an anti-stress agent (e.g., Zircon or Epin) and remove severely damaged leaves. |
Important: If you see plants drooping but the soil is moist, do not water! This is the most common and dangerous mistake. Water will only worsen the situation by displacing the last oxygen from the soil.
Step 2: Check Soil Moisture
Before deciding whether to water, always check soil moisture at a depth of 8–10 cm (about the length of your index finger).
- If the soil is moist — watering is not required. Focus on warming the soil (Step 3).
- If the soil is dry — watering is needed, but following all the rules (Step 4).
Step 3: Warming the Soil (Critically Important)
If the soil is cold (below 14 °C), your main task is to help it warm up. Here's what you can do:
1. Loosen the soil. Gently loosen the soil around the stem to a depth of 3–5 cm. This breaks the soil crust, improves aeration, and allows solar heat to penetrate deeper. Loosening in cold weather works like a "blanket" that helps the soil warm up faster (Kotov and Adrițkaya, 2016).
2. Temporary cover. If possible, cover the root zone with clear plastic film or black agrofabric. This will create a greenhouse effect and help warm the soil faster.
3. Water with warm water (if the soil is dry). If you decide watering is needed, use only warm water (temperature 25–30 °C). Warm water not only supplies moisture but also additionally warms the soil (Welbaum, 2015).
Step 4: Performing the Watering (Only if Necessary)
If the soil is dry and you've decided to water, do it correctly:
- Time: Only in the first half of the day, when the sun has already warmed the air. Ideally — from 10:00 AM to 12:00 PM.
- Water: Only warm (25–30 °C). Cold water in this situation is an additional shock to the roots.
- Volume: Slightly less than usual. It's better to give less water to avoid waterlogging the soil and making the roots "suffocate."
- Method: Only at the root. No overhead watering! Water should not get on the leaves, especially cold ones.
Step 5: Foliar Support (Optional)
If the plants look weakened, you can support them with foliar feeding (spraying on the leaves). This helps quickly deliver nutrients and stimulants directly into the cells, bypassing weakened roots.
- Anti-stress agents: Preparations based on epibrassinolide (Epin-Extra) or Zircon help plants recover faster after stress (Akhatov, 2010).
- Calcium nitrate (0.5% solution). Helps strengthen cell walls and restore turgor. Especially relevant during nighttime temperature fluctuations.
- Time: Spraying should be done in the first half of the day so that the leaves have time to dry by evening.
Checklist: What to Do in the Morning After a Cold Night
| Time | Action | Goal |
|---|---|---|
| Before 9:00 | Inspect the plants. Check soil moisture. | Understand if there's a problem. |
| 9:00–10:00 | Loosen the soil. | Improve aeration and warming. |
| 10:00–11:00 | If the soil is dry, water with warm water (25–30 °C). | Supply moisture to roots without shock. |
| 11:00–12:00 | If necessary, apply foliar anti-stress treatment. | Support the weakened plant. |
| Throughout the day | Provide shade (if sun is bright). | Reduce evaporation and lower the load on roots. |
The Golden Rule of a Cold Morning
Better to underwater than overwater. Better to wait until noon than to water early in the morning when the ground hasn't warmed up yet.
If you're unsure whether to water or not, wait 2–3 hours. Observe the plants. If by 11–12 AM the leaves have recovered their turgor — watering is not needed. If the plant continues to wilt and the soil is dry — you can carefully water with warm water.
7. How to Preserve Fruit Set: A Strategy to Save the Harvest
When cool weather arrives, many gardeners notice with alarm that flowers and small fruit sets begin to yellow and drop. This is not just an annoying trifle — it's a direct loss of future yield. But there's no need to panic: understanding the cause of the problem is the key to solving it.
Why Do Fruit Sets Drop in the Cold? The Biology of the Process
The dropping of flowers and fruit sets is a protective mechanism of the plant. When resources (heat, moisture, nutrition) are insufficient, the plant sacrifices its future offspring to preserve its own life. In cool weather, there are three main reasons for this phenomenon:
1. Pollination and fertilization disruption. This is the most common cause. For normal pollination and pollen germination on the stigma, tomatoes need a temperature in the range of 18–24 °C (Jones, 2008). At temperatures below 15 °C, the pollination process is disrupted:
- Pollen becomes sterile and loses viability (Akhatov, 2010).
- Pollen tubes grow slowly or not at all, and fertilization does not occur (Welbaum, 2015).
- As a result, the flower dies and drops without forming a fruit set.
2. Hormonal imbalance. Cold stresses the plant, causing the release of stress hormones (abscisic acid), which can accelerate the dropping of flowers and fruit sets. This is part of the plant's protective strategy (Akhatov, 2010).
3. Nutrient deficiency due to cold. Cold soil blocks the absorption of phosphorus, potassium, and boron — key elements for forming fruit sets (Welbaum, 2015). As a result, even if pollination occurred, the fruit set may not receive the necessary resources for development.
Strategy for Preserving Fruit Set: Practical Steps
Protecting fruit sets from cold means creating conditions for the plant that are as close to optimal as possible, even when it's cool outside.
1. Protection from Cold During Flowering
- Temporary covers. If you know cold weather is coming, cover the plantings with non-woven material (spunbond, Agrotex) at night. This will create a greenhouse effect and raise the temperature around the plants by 2–3 °C.
- Soil warming. Remember Chapter 6: a warm root is the key to success. Loosening and mulching with clear plastic film help warm the soil and activate roots.
- Nighttime "long" watering (only in greenhouses!). In large commercial greenhouses, evening watering is sometimes practiced to use the heat accumulated in the water during the day. For home gardeners, this is a risky technique, as nighttime watering is dangerous due to the risk of rot development.
2. Artificial Pollination Assistance
If the weather doesn't favor natural pollination, it can be stimulated artificially:
- Shaking. Gently shake the flowering clusters in the first half of the day when the air temperature starts to rise. This helps pollen fall from the anthers onto the stigma.
- Vibrator. In greenhouses, special mechanical vibrators are used for this purpose. They can be gently touched to the wire to which the plants are tied, causing vibration and pollen shedding (Nonnecke, 1989).
- Biological help. If possible, place a bumblebee hive in the greenhouse (for large area greenhouses). Bumblebees are excellent tomato pollinators; they work even in cool weather and contribute to better fruit setting (Nonnecke, 1989).
3. Use of Fruit Set Stimulants
When pollination is difficult, preparations containing plant hormones come to the rescue. They help "trick" the plant and trigger fruit formation even without full pollination (this is called parthenocarpy).
- Preparations: "Zavyaz" (Ovary), "Tomaton", "Bud" (Bud), and others containing gibberellins and other stimulants (Gavrish, 2005).
- When to apply: In persistently cool weather (below 15 °C during the day), when natural pollination is unlikely.
- How to apply: Strictly according to the instructions, most often by spraying the flowering clusters in the first half of the day. Important: do not exceed the concentration; this can cause fruit deformation and the appearance of "hollow" fruits (puffs) (Akhatov, 2010).
4. Adjusting Nutrition During Cold Periods
Supporting plants with proper nutrition is critically important. In cool weather, two elements are especially relevant:
- Potassium (K). Potassium strengthens cell walls, increases overall plant resistance to stress, and promotes better development of fruit sets.
- Boron (B). Boron is essential for pollen germination and fruit formation. Its deficiency in cold soils is one of the key causes of fruit set drop (Nonnecke, 1989).
How to apply:
- Root feeding: Use water-soluble fertilizers with a high potassium content (e.g., monopotassium phosphate or complex fertilizers with an N:P:K ratio of 5:15:30). Important! The solution should be warm (25–30 °C) to avoid cooling the soil.
- Foliar feeding (on the leaf): The most effective method in cool weather. Spray the plants with a solution of boric acid (1 g per 1 L of water) or a complex fertilizer with micronutrients. Nutrients are absorbed through the leaves instantly, bypassing the "sleeping" roots.
5. Removing Some Flowers
This may seem paradoxical, but sometimes it's better to remove some flowers to allow the plant to focus its efforts on fruits that have already begun to form. If the bush has many fruit sets, the plant may drop them all in an attempt to survive. Thinning can be the right strategy in difficult conditions (Swiader and Ware, 1992).
How to Recognize Success and Failure
- Success: A few days after flowering, the fruit set begins to increase in size, acquires a bright green color, and does not drop.
- Failure: The flower turns yellow, then dries up and drops. If this happens massively, you need to urgently apply one of the methods described above.
Summary Table: Strategies for Preserving Fruit Set
| Strategy | When to Apply | How It Works |
|---|---|---|
| Covering from cold | When frost or prolonged cold snap is threatened | Raises the temperature around the plant |
| Artificial pollination (shaking, vibrator, bumblebees) | In persistently low temperatures (below 15 °C) | Helps pollen reach the stigma |
| Stimulants (gibberellins) | In unfavorable conditions for pollination | Trigger the fruit formation mechanism without pollination |
| Foliar feeding (boron, potassium) | At lowered temperatures | Quickly delivers necessary substances to leaves |
| Thinning (removing some flowers) | Under severe plant stress | Reduces the load and allows saving some fruit sets |
Important: Combining several methods often gives the best result. For example, foliar feeding with boron + shaking the clusters + covering at night.
Conclusion: Preserving fruit set in cool weather is a complex task. It requires not only revising your watering practices but also actively helping the plant with pollination and supporting its immunity. Remember: cold weather is a temporary phenomenon, and your task is to help the plant survive this period with minimal losses.
8. Mistakes: What You Should Not Do
In cool weather, even experienced gardeners sometimes make mistakes that can cost the plants their health and the harvest its quality. Let's look at the most common ones so you can avoid them.
Mistake 1. Watering with Cold Water
This is perhaps the most dangerous and frequent mistake. Many gardeners use water from a well, borehole, or tap without thinking about its temperature. And it often does not exceed 10–12 °C.
Why it's dangerous:
- Cold water causes thermal shock to the roots, sharply slowing their function (Jones, 2008).
- As a result, roots stop absorbing water, and the plant wilts despite moist soil (physiological drought).
- Cold water creates ideal conditions for the development of root rots and fungal diseases (Akhatov, 2010).
How to avoid: Always use water heated to a temperature of 20–25 °C. Keep a water container in the greenhouse or in the sun.
Mistake 2. Evening Watering
In warm weather, evening watering is acceptable. But in cool weather, it becomes deadly.
Why it's dangerous:
- At night, air and soil temperatures drop, and the water that has reached the roots stagnates.
- High humidity combined with cold promotes the development of fungal diseases: late blight, gray mold, powdery mildew (Kotov and Adrițkaya, 2016).
- In the morning, plants may look wilted and sick.
How to avoid: Water only in the first half of the day (from 8:00 AM to 12:00 PM) so that moisture has time to be absorbed and evaporate before nightfall.
Mistake 3. Excessive Watering ("Overwatering")
This mistake is closely related to physiological drought. The gardener sees the plant wilting and tries to "help" it by pouring even more water. As a result, the soil becomes as wet as a swamp, and the plant continues to wilt or even dies.
Why it's dangerous:
- In waterlogged cold soil, roots suffocate without oxygen and begin to die (Welbaum, 2015).
- Ideal conditions are created for root collar rot and disease development (Akhatov, 2010).
- Overwatering reduces cold tolerance, making plants more vulnerable.
How to avoid: Before watering, always check soil moisture at a depth of 8–10 cm. Water only when the soil is truly dry. Increase the intervals between waterings.
Mistake 4. Overhead Watering (Watering on Leaves)
In cool weather, overhead watering (from above, on the leaves) is especially dangerous.
Fruiting tomato bush with signs of surface wetting inside a ring support.
Tomato bush with ripening fruits inside a metal ring support. The plant is heavily covered with moisture droplets, which may pose risks to crop health.
Why it's dangerous:
- Water that gets on the leaves does not have time to dry, and fungal diseases develop on it (late blight, gray mold).
- Water droplets remaining on the leaves increase evaporation and can lead to tissue cooling, which causes stress (Gavrish, 2005).
- In a greenhouse, overhead watering sharply increases relative humidity, creating ideal conditions for diseases.
How to avoid: Water only at the root (drip irrigation, watering into furrows, or around the stem base). Water should not get on leaves and stems.
Mistake 5. Ignoring Root Condition
In cool weather, roots suffer first. If you continue to pour water without paying attention to signs of stress, it can lead to plant death.
Signs of root problems:
- The plant wilts even though the soil is moist.
- Leaves turn yellow and drop, starting from the bottom.
- The stem at the base becomes dark and soft (sign of rot).
How to avoid: At the first signs of stress, stop watering and loosen the soil to improve oxygen access to the roots. If the condition is critical, treat the plant with anti-stress agents.
Mistake 6. Fertilizing with Nitrogen Fertilizers
Nitrogen stimulates active growth of green mass. In cool weather, when growth is already slowed, excess nitrogen can be harmful.
Why it's dangerous:
- Nitrogen makes plant tissues more watery and loose, reducing their resistance to cold and disease.
- Excess nitrogen against a backdrop of potassium and phosphorus deficiency (which are poorly absorbed in the cold) leads to nutritional imbalance (Swiader and Ware, 1992).
How to avoid: In cool weather, avoid nitrogen fertilizing. Instead, use potassium-phosphorus fertilizers (monopotassium phosphate, wood ash), which strengthen immunity and increase cold tolerance. Apply them as foliar feeds.
Checklist: Main Mistakes and How to Avoid Them
| Mistake | Why It's Dangerous | How to Avoid |
|---|---|---|
| Watering with cold water | Root shock, physiological drought, rots. | Use water at 20–25 °C. |
| Evening watering | Moisture stagnation, fungal diseases. | Water in the first half of the day. |
| Excessive watering (overwatering) | Root suffocation, rots, plant weakening. | Check soil moisture before watering. |
| Overhead watering | Fungal diseases, tissue cooling. | Water only at the root. |
| Ignoring root condition | Plant death from rots. | Loosen soil regularly. |
| Nitrogen fertilizing | Reduced cold tolerance, nutritional imbalance. | Apply potassium and phosphorus. |
The Golden Rule: "Better to Underwater Than Overwater"
In cool weather, this rule should become your main motto. One mistake — and all your efforts to grow healthy tomatoes can be undone. Caution, attentiveness, and knowledge of plant physiology will help you avoid problems and get a good harvest, even if the weather doesn't favor you with warmth.
References
- Boote, K.J. (2017). ‘Modelling crop growth and yield in tomato cultivation’, in Mattoo, A.K., Handa, A.K. (ed.) Achieving sustainable cultivation of tomatoes. Cambridge, UK: Burleigh Dodds Science Publishing, ch. 1.
- Geisenberg, C., Stewart, K. (1986). ‘Field crop management’, in The Tomato Crop. Dordrecht: Springer Netherlands, 511-557.
- Hochmuth, G.J., Sideman, R.G. (2023). ‘Soils and Fertilizers’, in Knott's Handbook for Vegetable Growers. : John Wiley & Sons, pp. 199-302.
- Jones, J.BentonJ. (2008). ‘Plant Characteristics and Physiology’, in Tomato Plant Culture: In the Field, Greenhouse, and Home Garden. Boca Raton, London, New York: CRC Press (Taylor & Francis Group), pp. 55-80.
- Nonnecke, L. (1989). ‘Solanaceous Crops: Potato, Tomato, Pepper, Eggplant’, in Vegetable production. New York, USA: Van Nostrand Reinhold, pp. 175-250.
- Santos, B.M., Torres-Quezada, E.A. (2018). ‘Irrigation and Fertilization’, in Heuvelink, E. (ed.) Tomatoes. Boston, MA: CABI, pp. 180-206.
- Swiader, J.M., Ware, G.W., McCollum, J.P. (1992). ‘Tomatoes’, in Producing Vegetable Crops. Danville, Illinois: Interstate Publishers, pp. 513-536.
- Welbaum, G.E. (2015). ‘Family Solanaceae’, in Vegetable production and practices. Boston, MA: CABI, ch. 11.
- Ахатов, А.К. (2010). ‘Выращивание томатов [Growing tomatoes]’, in Мир томата глазами фитопатолога [The world of tomato through the eyes of a plant pathologist]. Москва: КМК, pp. 78-143.
- Гавриш, С.Ф. (2005). Томаты [Tomatoes]. Москва: Вече.
- Котов, В.П., Адрицкая, Н.А. (2016). ‘Технологии возделывания овощных культур в защищенном грунте [Technologies for growing vegetable crops in protected ground]’, in Овощеводство [Vegetable growing]. Санкт-Петербург: Лань, pp. 429-477.
