Prolonged extreme heat, dry winds and water shortages place fruit trees and vineyards under severe stress. Under these conditions, plants lose more water through their leaves, photosynthesis slows down, and the risk of sunburn, fruit drop, drying and cracking increases.

When temperatures rise above 35°C, photosynthetic efficiency in grapevines may decline considerably. When extreme heat is combined with insufficient soil moisture, the negative impact becomes even more severe.

During a heat wave, the main objective is not to provide a one-time “emergency treatment.” The priority is to maintain stable moisture in the root zone, preserve natural shade around leaves and fruit, and avoid practices that place additional stress on the plants.

1. Schedule irrigation for the evening and night

During extremely hot weather, avoid flooding the area around a tree with a large amount of water at once. Excess water may reduce soil aeration, while shallow irrigation may fail to supply moisture to the main root zone.

Whenever possible, begin irrigation after 8:00 p.m. and complete it before 7:00 a.m. With drip irrigation, water can be delivered in several short cycles with pauses in between. This pulse-irrigation approach allows water to penetrate the soil more evenly.

Water should not be applied directly against the trunk. It should be delivered to the active root zone, which is generally located beneath the spread of the tree canopy.

After irrigation, check that moisture has reached the deeper soil layers rather than remaining only near the surface. A soil auger, moisture meter or a small inspection hole can be used for this purpose.

Irrigation intervals should be based on soil moisture and soil type rather than on a fixed calendar:

•  sandy and sandy-loam soils lose moisture more quickly and usually require more frequent irrigation; 
•  heavy clay soils absorb water more slowly, so overwatering must be avoided; 
•  frequent shallow watering does not encourage the development of a strong, deep root system. 

Slow irrigation that thoroughly moistens the root zone is generally more effective than short, shallow watering.

2. Protect the soil with mulch

Mulch reduces excessive heating of the soil surface, slows moisture evaporation and limits competition from weeds.

Suitable mulching materials include:

•  straw; 
•  chopped stems and crop residues; 
•  wood chips or bark; 
•  well-matured compost. 

A mulch layer of approximately 5–8 cm is generally suitable. Do not pile mulch directly against the trunk. Leave an open space of at least 10–15 cm around the base of the tree to prevent the bark from remaining constantly wet and to reduce the risk of decay.

Do not place an airtight plastic sheet beneath the mulch.

When mulch is unavailable, the soil crust can be loosened carefully and shallowly after irrigation. Avoid deep cultivation during hot weather because the fine active roots of young trees and grapevines may be located close to the soil surface.

3. Protect trunks and major branches from direct sunlight

If the white protective coating applied in spring has washed away or deteriorated, it should be renewed. A light-coloured surface reflects part of the solar radiation and reduces excessive heating of the bark.

Suitable materials include:

•  specialised tree-trunk paint; 
•  an agricultural whitewash mixture; 
•  a plant-safe, white, water-based protective coating. 

Priority should be given to the trunk, major scaffold branches and surfaces exposed to the south and southwest. Young trees with thin bark and trees whose branches have become exposed after pruning require particular attention.

Do not use oil-based paint or other coatings that are not intended for living trees.

4. Temporarily stop heavy pruning and leaf removal

Before and during an extreme heat event, avoid heavy pruning, removing large numbers of leaves, sharply thinning the canopy or cutting back shoot tips.

Leaves act as a natural sunshade for fruit and grape clusters. Fruit that has remained shaded for a long period may not adapt quickly when suddenly exposed to direct sunlight and may develop sunburn.

In vineyards, temporarily stop:

•  removing leaves from around grape clusters; 
•  heavy shoot thinning; 
•  intensive shoot topping; 
•  exposing clusters on the western and southwestern sides of the row. 

It is especially important to preserve leaves that protect clusters from intense afternoon sunlight.

Protecting Vineyards During Extreme Heat

5. Maintain stable soil moisture

Part of the active grapevine root system is located in the upper soil layers. When row surfaces remain bare and dry, the soil heats up quickly and moisture loss increases.

Using straw or another plant-based mulch can:

•  reduce soil-surface temperature; 
•  improve the efficiency of irrigation water; 
•  help retain moisture between the rows and beneath the vines. 

Do not allow grapevines to enter a severe water deficit before a heat wave. At the same time, do not suddenly flood soil that has been dry for a long period. Soil moisture should be restored gradually and evenly.

6. Use kaolin-based protectants carefully

Purified kaolin products create a thin, white, reflective film on leaves and fruit. This coating may help reduce the surface temperature of leaves and grape berries.

The effectiveness of kaolin depends on:

•  local climate; 
•  vine vigour; 
•  the level of cluster exposure; 
•  product concentration; 
•  the uniformity of coverage. 

Kaolin should not be viewed as a universal solution. It is best used as a supplementary measure alongside proper irrigation and the preservation of natural canopy shade.

When applying kaolin:

•  use only a purified product intended for agricultural use; 
•  follow the application rate and pre-harvest interval stated on the label; 
•  test the product on a small area first; 
•  consider the possibility of visible white residue close to harvest; 
•  check compatibility before mixing it with other products. 

7. Apply foliar nutrition with caution

Foliar feeding can provide additional support during hot weather, but it cannot replace proper irrigation.

Applying fertilisers or biostimulants to plants already suffering from severe water stress may fail to produce the expected benefit and may increase the risk of chemical injury.

Potassium and calcium

Potassium plays an important role in plant water regulation, while calcium contributes to cell-wall development and the strength of fruit tissues.

Foliar application rates vary depending on the formulation, crop, variety, water quality and growth stage. For this reason:

•  do not use one universal rate for every product; 
•  apply only the concentration stated on the product label; 
•  do not increase the rate because of hot weather; 
•  conduct a trial application on 3–5 plants first; 
•  do not spray plants that are severely wilted. 

Boron products

Boron should be applied only when a genuine need has been confirmed through leaf, soil or irrigation-water analysis.

Plants require only a very small amount of boron, and the difference between deficiency and toxicity is narrow. Excess boron may cause leaf-edge burn, tissue death, reduced growth and lower yields.

Repeated preventive applications of boron are not recommended without prior diagnosis.

8. Amino acids and seaweed extracts are supporting tools

Biostimulants based on amino acids, protein hydrolysates and seaweed extracts may support the plant’s natural response to heat stress.

In some studies, these products helped grapevines maintain chlorophyll levels and photosynthetic activity under high-temperature conditions. However, performance can vary considerably depending on the formulation and the conditions under which it is used.

When selecting a biostimulant, consider more than the trade name. Check:

•  the exact composition; 
•  whether it is recommended for the intended crop; 
•  the plant’s growth stage; 
•  the application rate; 
•  compatibility with other products; 
•  results from trials conducted under similar climatic conditions. 

These products are generally more useful when applied before the heat wave, while the plants are not yet severely wilted. Repeat applications only at the interval stated on the label.

What Should Be Avoided During Extreme Heat?

Do not apply fresh manure or highly concentrated fertilisers

During the hottest period, avoid applying fresh manure, high rates of nitrogen fertiliser or strongly concentrated nutrient solutions.

Stimulating rapid new growth increases the plant’s demand for water and may intensify heat stress. Major fertiliser applications should be postponed until temperatures become more moderate.

Well-matured compost may be used as mulch, but it should not be placed directly against the tree trunk.

Do not apply chemical products during the hottest part of the day

Spraying during peak daytime temperatures causes the solution to dry rapidly, increases product concentration on the leaf surface and raises the risk of phytotoxicity.

Treatments should be carried out in the evening or early morning, when temperatures are lower and winds are lighter. Weather conditions must also comply with the instructions on the product label.

The common claim that every water droplet acts like a magnifying glass is not a complete explanation of plant injury. The main risks are high temperature, rapid evaporation, increased solution concentration and the heat-related toxicity of certain active ingredients.

Do not apply sulfur before severe heat

Sulfur is an effective treatment against grapevine powdery mildew, but it may burn leaves and berries when temperatures are extremely high.

Particular caution is required when temperatures are expected to approach or exceed approximately 38°C. Some products may specify a lower maximum temperature on their labels. In such cases, the manufacturer’s instructions must take priority.

Sulfur should not be mixed with oil-based products or applied within a short period before or after an oil treatment.

Do not exceed the recommended application rate

Never increase the product rate during hot weather in an attempt to obtain a stronger effect. Rapid evaporation may already cause the solution to become more concentrated on the surface of leaves and fruit.

Mixing several products without first checking compatibility may cause sediment formation, reduce effectiveness or result in chemical burns.

UFarmer Heat-Wave Action Plan

Before the heat wave

Inspect the irrigation system, filters and drip emitters. Evaluate soil moisture, renew the mulch layer and protect exposed trunks and major branches.

Stop heavy pruning, leaf removal and intensive shoot topping.

During the heat wave

Maintain stable moisture throughout the root zone. Irrigate gradually during the evening or night.

Inspect trees and vines in the morning and evening. Look for leaf wilting, changes in fruit colour and the first signs of sunburn.

Do not apply fertilisers or chemical treatments during the hottest hours of the day.

After the heat wave

Do not suddenly increase or reduce irrigation. Assess the damage, but do not immediately remove every affected leaf or shoot, as some plant tissue may recover.

Once the plants have stabilised, revise the nutrition programme based on soil or leaf analysis.

Increase monitoring for pests and diseases, as heat-damaged tissues may be more vulnerable to secondary infections.

Important note

Irrigation volume, nutrient application rates and product selection depend on the age of the tree or vine, variety, rootstock, soil composition, water quality, crop load and the formulation of the product being used.

Before applying any product, carefully read the label, conduct a small-area trial and, whenever possible, consult a local agronomist.

During extreme heat, the UFarmer team recommends prioritising efficient water use, preserving the natural shade around fruit and avoiding sudden agricultural practices that place additional stress on plants.