Heat and drought in the vegetable bed

Long hot spells are now part of every gardening year. For vegetables that is doubly difficult: they consist of more than 90 per cent water and, unlike a tree, have no deep root system to fall back on.

If you only react when the leaves hang, you have already lost yield. The measures that work start earlier — with the soil, the watering rhythm, and a little shade at the right time.

In short

Watering rarely and thoroughly beats a little every day.
Mulching cuts evaporation from the soil by up to 70 per cent.
Midday wilting is normal — only if the plant still hangs in the evening is water actually short.
From about 30 °C tomatoes and peppers stop setting fruit; shading helps.
Even watering prevents blossom end rot, split fruit and hot radishes.

Watering properly

The commonest mistake is watering too often and too shallowly. It keeps the moisture in the top few centimetres, and that is where the plants then form their roots — precisely the layer that dries out first. The result is a plant that cannot manage without a daily ration.

Thoroughly: 15 to 20 litres per square metre at a time, but only two or three times a week. That way the water reaches 20 to 30 cm down and the roots follow it.
Water in the morning. In the evening leaves and soil stay wet overnight — which encourages fungal diseases and attracts slugs.
At the root, not over the leaves. A watering ring or a buried flower pot next to the plant directs the water where it belongs.
Do not water in full midday sun. A large share evaporates unused, and droplets on leaves act like burning glasses.
Collect rainwater. It is softer, warmer and free. A butt per downpipe is quickly set up.

Mulching: the single most effective measure

A mulch layer works like a blanket: it keeps the sun off the soil, slows evaporation and stops the surface capping over. The difference is so great that the watering interval can often be halved.

Grass clippings: let them dry off, then apply in thin layers of 2 to 3 cm. Applied fresh and thick they turn slimy.
Straw: ideal for strawberries, tomatoes and squash. Lasts a long time and keeps fruit clean.
Leaves: good for soft fruit and over winter.
Not on vegetable beds: bark mulch. It takes nitrogen from the soil as it rots.
Water thoroughly before mulching. Laid on dry soil, mulch locks the dryness in rather than the moisture.

Heat: when shade becomes necessary

Not every heat problem is a water problem. Above certain temperatures plants simply shut processes down, no matter how moist the soil is.

Above 30 to 32 °C the pollen of tomato, pepper and aubergine becomes infertile. Flowers drop and no fruit sets — even with perfect watering.
Above 25 °C lettuce, spinach and rocket bolt and turn bitter.
Sunscald on peppers and tomatoes: pale, leathery patches on the sunny side of the fruit, especially after leaf removal.
Remedy: shade netting with 30 to 50 per cent shading over the hottest hours, or shading paint or netting in the greenhouse. Not full shade — the plants still need light.
Do not strip too many leaves. Leaves shade the fruit. On tomatoes remove only the lower, yellowing ones.

Tip

Midday wilting is not an alarm signal. Squash, courgette and cucumber let their leaves hang in full sun to throttle evaporation, and pick themselves up again in the evening. Only if a plant is still limp in the morning or evening is water genuinely short. Watering in response to midday wilting usually means watering too much.

What drought stress does to the fruit

Blossom end rot on tomato and pepper: a dark, sunken patch at the flower end. Not a fungus but a calcium shortage in the fruit, triggered by fluctuating water supply.
Split tomatoes: long dry spell, then plenty of water — the fruit grows faster than its skin.
Hot, pithy radishes and woody kohlrabi: classic consequences of drought stress.
Bitter cucumbers: irregular watering raises the level of bitter compounds.
Common denominator: it is almost always the fluctuation, not the absolute amount.

Caution

During heatwaves, do not water tomatoes and cucumbers with ice-cold tap water straight onto the roots. The temperature shock slows water uptake exactly when the plant needs it most. Rainwater from a butt is at ambient temperature and much better.

Frequently asked questions

How do I tell whether water is really short?

Finger test: 5 cm into the soil. If it feels dry there, water. The state of the leaves at midday says little.

Is drip irrigation worthwhile?

Yes, particularly for fruiting vegetables. It puts water precisely at the root, loses little to evaporation and above all delivers the evenness that is otherwise so hard to achieve.

Should I feed more during heat?

No, rather less. In dry conditions roots absorb nutrients poorly, and salt in the soil makes the stress worse. Feed again once there is enough water.

What do I do before a week away in high summer?

Water thoroughly, mulch thickly, group pots together and move them into part shade. For beds, a simple irrigation timer on the tap is worth it.

Sturdy, well-rooted young plants cope with heatwaves considerably better than freshly germinated seedlings — they reach deeper soil layers where water is still available even in dry spells.