Heat stress
Above about 30 °C (86 °F) many crops close down: leaves wilt to save water, pollen fails, and flowers drop without setting. Cool-season crops bolt. Most plants recover when the heat breaks.
Light & temperature
What it looks like
Heat damage comes and goes with the temperature.
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Afternoon flop Leaves wilt in the heat of the day and recover by the next morning. -
Flowers falling Blossoms dry at the stalk and drop without setting fruit. -
Rolled leaves Tomato leaves roll upward along the midrib to reduce water loss. -
Early flowering Lettuce, spinach and coriander bolt in a hot spell.
How to confirm it
Rule out thirst first; then look at the thermometer.
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Soil is moist
If the soil is damp and the plant still flags at 3 pm, it is heat, not water.
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Morning recovery
Heat-stressed plants stand up again overnight; drowning or diseased ones do not.
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Hot nights
Tomatoes and peppers stop setting fruit when nights stay above about 22 °C (72 °F).
What to do
Help the plant ride out the heat.
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Shade at midday
Throw shade cloth or fleece over crops, or move pots into afternoon shade.
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Water in the morning
Water deeply early in the day so plants start the heat full.
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Damp down
In a greenhouse, open everything and wet the paths to cool the air.
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Wait for new flowers
Flowers set again once temperatures fall; do not pull plants in a heatwave.
Prevent it next time
- Grow heat-tolerant varieties where summers run hot.
- Sow cool-season crops early or late, not for high summer.
- Mulch to keep roots cool.
- Shade greenhouses from late spring.
What not to do
Do not feed or prune hard during a heatwave. Both push new growth the plant cannot support.
In the growing guides
Where the guides mention it, with plant-specific advice.
Sources
- Blossom drop in tomato, Alabama Cooperative Extension System
- Weather damage, Royal Horticultural Society