Shade, timing and triage: what to do when the forecast hits the mid-30s

A heatwave is not just a hot day. It is a run of days when the soil dries faster than roots can replace the water, and when pollen, flowers and leaves start to fail. The work that saves the crop happens before the heat arrives and in the early hours of each day.

Before the heat

Soak the beds. Give the ground a deep watering two days before the peak, not on the day. The soil will hold that water and the plants will go into the heat with a full tank.

Mulch. A 5 to 7.5 cm layer of compost or straw over wet soil slows evaporation and keeps the root zone cooler. If the soil is already dry, water first, then mulch.

Shade what is vulnerable. Seedlings, lettuce, spinach and greenhouse crops benefit from shade netting, an old bedsheet over hoops, or a temporary lean-to of boards. Shade the greenhouse glass with netting or paint, and open the vents early.

Move containers. Pots dry out far faster than beds. Put them in the shade of a wall or a tree for the duration, and group them so the leaves shade each other.

Harvest what is ready. A courgette, a head of lettuce or a punnet of peas picked before the heat is one less plant asking for water.

During the heat

Water in the early morning. If you miss that, water in the evening. Keep the water at the base and off the leaves. A plant that is wilting at noon is not necessarily dying; check it again at dusk.

Ventilate the greenhouse in the morning and damp down the floor to raise humidity. Tomatoes set fruit poorly in dry air as well as high heat. Tap the flower trusses at midday to help pollen move.

Do not feed a dry plant. Fertiliser on dry roots does more harm than good. Wait until the soil is moist and the heat has passed.

Do not prune or move plants in the heat. Leave the foliage on; it shades the fruit.

What fails, and why

Tomato fruit set. Above about 30°C, pollen production drops and flowers can abort. The damage happens in the buds about nine days before the flowers open, so a truss that drops in the heat was set up by the week before. Night temperatures above 21°C make it worse.

Blossom end rot. The black patch on the bottom of tomatoes and peppers is a calcium problem in the fruit, not usually a lack of calcium in the soil. It comes from irregular watering: the plant cannot move calcium to the fruit fast enough. Keep the soil evenly moist and remove affected fruit.

Bolting. Lettuce, spinach, coriander and rocket run to seed in heat. Once the flower stem is up, the leaves turn bitter. Pick what you can and sow again in cooler weather. See why lettuce bolts.

Courgette and squash flowers. Plants may drop flowers and set less fruit in extreme heat. Keep watering and they usually recover when the temperature drops.

Leaf disease. Wet foliage overnight invites mildew and blight. Water the soil, not the canopy, and leave the leaves to dry before evening.

Triage

If water is short, save the plants you cannot replace: perennials, fruit bushes, a favourite tomato, the seedlings for autumn. Let the lettuce go. It is cheap to resow and it will be bitter anyway.

Containers and raised beds dry first. Check them daily. In-ground beds with a mulch can go two or three days between soaks.

After the heat

Wait for the soil to cool before feeding. Check for aphids and red spider mite, which build up in hot, dry conditions. Remove split tomatoes and bolted salad. Water deeply once the weather breaks, and mulch again if the layer has thinned.

The next heatwave will be easier if you have shade netting, a full water butt and a mulch layer ready before it starts. Water butts and ollas and mulching a vegetable bed cover the preparations.