Heat stress
Damage caused when plants cannot cool themselves or take up water fast enough in very hot weather, leading to wilting, scorch and poor flowering.
Also known as: Heat injury, Heat wilt
Symptoms and identification
Leaves wilt in the afternoon, curl or fold, and edges turn brown and crisp. Flowers and buds drop, fruit may fail to set, and leafy crops such as lettuce turn bitter and bolt. Fruit exposed to sun can develop pale, papery sunburn patches.
Causes
Air temperatures much above 30 to 35 degrees Celsius, strong sun, dry or reflected heat and drying winds. Plants in shallow soil, small pots and dark containers heat up and dry faster.
Organic treatment
Water in the early morning or evening, keep the soil evenly moist and shade plants in the hottest part of the day. Do not fertilise heavily during a heatwave, and do not prune scorched growth until the weather cools.
Prevention
Water deeply and mulch soil to keep it cool and moist. Give afternoon shade with cloth in the hottest weeks, group pots out of reflected heat and choose heat-tolerant varieties for hot climates.
Chemical treatment (use with care)
No chemical treatment is needed. Anti-transpirant sprays exist but have limited use in home gardens.