Classification of vegetable crops according to their temperature requirements

Air and soil temperature directly govern plant metabolism. Its increase within the optimum range activates photosynthesis, respiration, nutrient uptake, and the movement of assimilates. The agronomist’s task is to maintain the temperature regime within limits that ensure maximum biomass gain. If these limits are exceeded, plants reach the compensation point, where all accumulated carbohydrates are consumed for respiration and growth stops.

• Start of the growing season for frost-resistant ones — 5 °C
• Start of the growing season for cold-tolerant ones — 8–10 °C
• Start of the growing season for thermophilic ones — 13–15 °C
• Optimum for thermophilic ones — 20–30 °C
• Photosynthesis limit for thermophilic ones — 40 °C

Classification of vegetable crops by heat requirement

Vegetable crops are divided into five main groups depending on their temperature regime requirements:

• Frost- and winter-hardy: garlic and all perennials (asparagus, rhubarb, sorrel, tarragon, lovage, perennial onions). They begin their growing season at 5 °C. In spring and autumn, they withstand frosts down to -8...-10 °C, and their underground organs successfully survive harsh winters.
• Cold-tolerant: cabbage, root crops, lettuce, spinach, bulb onion. Active growth begins at 8-10 °C, and the optimum temperature for them is 17-20 °C. They are capable of withstanding short-term frosts down to -3...-5 °C (prolonged ones — down to -1...-2 °C). When the temperature rises to 30-32 °C, growth stops.
• Semi-cold-tolerant: potato. It combines the properties of different groups. The haulm and tubers die at 0 °C, which aligns it with thermophilic crops. At the same time, the optimum temperature for growth and tuber formation lies within the range of 17-20 °C.
• Thermophilic: tomato, pepper, eggplant, cucumber, summer squash, pattypan squash. The growing season starts at 13-15 °C, and the best growth occurs in the 20-30 °C range. Frosts of 0 °C and lower are fatal for them, as is prolonged cooling down to 10 °C. At 40 °C, the expenditure of substances for respiration exceeds their production.
• Heat-resistant: cucurbits (melon, watermelon, pumpkin), beans, corn. In terms of temperature requirements, they are close to thermophilic ones but are able to continue accumulating harvest even at a temperature of 40 °C.

Calculation of heat resources and daily thermoperiodism

For the correct selection of zones and growing methods, it is necessary to compare the growing season of the crop with the sum of active temperatures in the region. Depending on the group, the requirements of plants are distributed as follows:

An important factor in cultivation is thermoperiodism — the response of plants to temperature fluctuations. Seasonal thermoperiodism determines the transition to wintering through the formation of seeds, rhizomes, bulbs, and root crops. Daily thermoperiodism is expressed in the natural need of plants for lower nighttime temperatures compared to daytime ones.

Regulating the difference between day and night temperatures is critically important for greenhouses. By controlling the night regime in the greenhouse, the agronomist manages the growth rate of vegetative mass, the intensity of flowering, fruit set, and the rate of fruit ripening.

How temperature affects individual plant organs

When managing the microclimate in greenhouses or planning sowing dates in open ground, it is important to remember that a plant's sensitivity to temperature changes depending on its age and specific organ. Actively growing plants quickly die from frost, while dry seeds and dormant buds tolerate severe frosts. The agronomist needs to adjust the temperature regime to the current development phase of the crop.

The most rigid requirements for heat are imposed by flowers and ovaries. Pollen is formed and germinates at lower temperatures than those required for fruit ripening. Excessive heat leads to pollen abortion, while the pistil stigma and ovary are most sensitive to cooling and frost.

At the beginning of a plant's life, the temperature regime should be strictly regulated according to its development phases. This helps to form a powerful root system and prevent the seedlings from becoming leggy. During the germination stage and the first days of seedling life, adhere to the following scheme:

1. Seed germination. Maintain the temperature 4–7 °C higher than the optimum for the growing season of an adult plant.
2. Emergence. Immediately after the appearance of sprouts, sharply reduce the temperature to Topt = 7 °C for 4–7 days. This will restrain the growth of the hypocotyl and stimulate root development.
3. Active growth. After hardening the seedlings, the temperature is gradually increased to the standard optimum for the specific crop.

• Difference between root and shoot optimum — 1–3 °C lower for roots
• Temperature excess for seed germination — 4–7 °C above the norm
• Seedling hardening temperature — 7 °C
• Period of temperature reduction after emergence — 4–7 days

Methodology for calculating optimal temperatures by development phases

To calculate the temperature regime, a basic indicator is used — the optimal temperature in cloudy weather (Tcloudy). Knowing this, you can calculate the optimum for any time of the day using the formula: Topt = Tcloudy ± 7 °C. In clear sunny weather, to accelerate photosynthesis, the temperature is raised by 7 °C. At night, when assimilation ceases, the temperature is lowered by 7 °C to reduce respiration costs.

The heat requirements of vegetable crops increase as they develop. During the period of active leaf growth and the transplant stage, the base temperature (Tcloudy) is 3 °C lower than during the budding and harvest formation period. Precise values for different groups of crops are given in the table.

Let's consider the temperature calculation using sweet pepper as an example. During the fruiting period, its base temperature (Tcloudy) is 25 °C. This means that on a sunny day in the greenhouse, it is necessary to maintain 32 °C (25 + 7), and lower it to 18 °C (25 - 7) at night. In the transplant period, the base temperature is lower — 22 °C, so the optimum for a sunny day will be 29 °C (22 + 7), and 15 °C (22 - 7) for the night.

Using the base temperature, it is possible to determine the critical limits beyond which metabolism is disrupted. The growth of most crops ceases when deviating from Tcloudy by 14 °C (Tcloudy ± 14 °C). For the same pepper in the fruiting phase, the critical points will be 11 °C (25 - 14) and 39 °C (25 + 14).

Temperature management by development phases: how to preserve fruit set and increase yield

An agronomist cannot regulate the weather in open fields, but it is within our power to adjust the microclimate or protect crops during critical phases. For each group of vegetable crops, there is its own temperature schedule. To obtain the maximum harvest, it is necessary to adapt the temperature regime to specific stages of the plant's life using sowing dates, ventilation, or irrigation.

• Reduction before flowering of fruit crops — 2–4 °C
• Increase during fruit ripening — 2–3 °C
• Reduction for vegetative organs — 1–3 °C
• Cold-resistance range — 0…+10 °C

For annual fruit vegetables, the period before flowering is critical. At this time, the temperature must be reduced by 2–4 °C compared to the optimal growing temperature. If this is not done and overheating occurs, the pollen will partially or completely lose fertility, which will lead to barren flowers. After the appearance of the first fruit set, the temperature is returned to the optimum, and by the beginning of fruit ripening, it is raised 2–3 °C above the norm.

For crops that form vegetative productive organs (heads, root crops, bulbs), the logic is different. In the phase of active accumulation of storage substances, they require coolness — reducing the temperature by 1–3 °C from the optimum accelerates the growth of productive organs and increases the yield. Biennials and perennials in the first year grow at optimal heat until entering dormancy, overwinter at lowered positive temperatures, and in the second year, when grown for seed, require the same regime as annual fruit crops.

Overcoming extreme temperatures: hardening of transplants and heat tolerance

Frosts and extreme low temperatures block metabolism in a plant. Photosynthesis is disrupted, the transport of assimilates stops, and roots cease absorbing water and nutrients, causing turgor to drop. Even if a plant survives such stress, it is impossible to restore its potential productivity, and further maintenance becomes economically unviable. Since breeding cannot radically change the genetic heat requirements of species, hardening remains the agronomist's primary tool.

Comprehensive preparation of transplants and germinating seed helps increase plant resistance to frost. These practices change cell physiology: they increase plasma viscosity, raise the concentration of cell sap and sugars, and stimulate the production of natural adaptogens. As a result, water in the tissues does not turn into ice during short-term frosts.

1. Exposure of germinating seed or transplants to low temperatures.
2. Maximum improvement of natural light to activate photosynthesis.
3. Limitation of water supply (moderate moisture deficit).
4. Nutrient correction: increasing doses of phosphorus and potassium while strictly limiting nitrogen.

In southern regions, agronomists face the opposite problem — plant overheating. Here, it is critically important to select heat-resistant crops and hybrids that protect themselves from heat at the level of their own physiology. For example, watermelon survives through powerful transpiration, pubescence of leaves, and light spots that reflect light. Pumpkin has higher resistance due to cytoplasm proteins that coagulate only at +60…+65 °C, whereas for most other crops this threshold is +45…+55 °C.

Practical methods for temperature regulation in open and protected ground

To obtain a stable harvest in open ground, it is necessary to match the biological requirements of the crop with the climate of the region. Sowing and planting dates are calculated based on the dates when average daily temperatures cross the thresholds critical for a specific group of plants. If the natural warm period in the region is too short, the transplant method of cultivation is employed.

• Start of growing season for frost-hardy crops — +5 °C
• Start of growing season for cold-hardy crops — +10 °C
• Start of growing season for heat-loving crops — +15 °C
• Temperature difference under protective foam — 10–15 °C
• Effect of smoke screen during a frost — 0.5–1.0 °C

Field relief significantly affects the heat balance. Southern slopes, being the warmest, are allocated for early-ripening and heat-demanding vegetables. Northern slopes are used for early and late cold-hardy plants, as well as late heat-loving cultivars, provided they receive sufficient accumulated active temperatures. In waterlogged areas, the thermal regime is improved by cutting furrows and beds, and to protect against summer soil overheating in the south, windbreaks of tall crops are planted.

To accelerate soil warming, mulching with translucent polymer film is used, which is laid directly over the rows of sown seed. This allows shifting sowing dates one to two weeks earlier than usual. Straw, sawdust, humus, or peat are also used as mulch. Mulching with film allows growing heat-loving peppers and eggplants using the direct sowing method even in the Rostov region and further north, and spring film covers are suitable for early transplanting.

For immediate protection of seedlings from late frosts, smoke screens are used by burning damp straw, peat, sawdust, dry manure, or special smoke candles. This method raises the temperature in the ground layer by only 0.5–1.0 °C. Creating artificial fog by spraying ammonium chloride or chlorosulfonic acid also proves ineffective.

The most reliable method is covering the growing plants before a frost with a special harmless foam. The composition stays on the leaves for more than 10 hours, gradually breaking down, and then completely decomposes in the soil with the help of microorganisms. This method guarantees protection for delicate seedlings from perishing during sudden temperature drops.

In protected ground, the microclimate is regulated artificially. In heated greenhouses, it is necessary to prevent heat loss in winter by carefully sealing the structures. During the spring-summer period, the main problem becomes plant overheating. To reduce the temperature, active ventilation, shading curtains (thermal screens) are used, or glass is whitewashed with lime or clay.

In unheated plastic greenhouses, an optimal thermal regime is created by choosing the right timing for operation and selecting special grades of plastic film. Also, for insulation, a double-film cover is applied, and biological fuel is placed under the soil layer. On hot days, overheating is avoided through maximum ventilation and whitewashing of the roof.