Why hot spells catch tomatoes and courgettes out

Tomatoes and courgettes are the two crops that most often let a small grower down in July and August, and it is rarely the heat itself that does the damage. It is the gap between what the plant loses through its leaves and what its roots can find in the soil. Both crops have large, soft leaves and fast-growing fruit, and in a hot spell a single plant can lose several litres of water a day through transpiration. A grow bag or a 30cm pot simply cannot hold that much.

The tricky part is that the symptoms arrive late. You water erratically for a fortnight, everything looks fine, and then the fruit tells the truth: a leathery brown patch at the base of the tomatoes, courgettes that swell at one end and shrivel at the other, split skins, tough leaves curling inwards by mid-afternoon. By then the plant has been under stress for a while. The good news is that the fix is simple, free, and mostly about timing rather than effort.

Water deeply and less often

A daily drizzle from a watering can feels kind and does very little. It wets the top centimetre or two of compost, which then dries out within hours, and it encourages roots to stay near the surface where they are most vulnerable. Deep watering does the opposite: it pushes moisture down to 20–30cm, where roots can find it through the hottest part of the day.

Before you water anything, push a finger into the compost up to the second knuckle. If it is damp, walk away. If it is dry, water until it runs freely from the drainage holes, then stop and come back in ten minutes to top up. That second pass is what soaks the whole root ball rather than just the outside of it.

• Tomatoes in the ground: 2 litres per plant every two or three days in normal weather, daily in a heatwave.
• Tomatoes in grow bags: two plants per bag, 4–5 litres per bag per day once the fruit is swelling.
• Tomatoes in 30cm pots: up to 2 litres per plant per day in hot, breezy weather.
• Courgettes in the ground: 5 litres per plant every other day, and daily in a heatwave.
• Courgettes in pots: a 40-litre container is the sensible minimum, and it will want 3–4 litres a day.

Water the soil, never the leaves. Wet foliage in still, warm air is an open invitation to blight and powdery mildew. Early morning is ideal, because the plant has water available before the sun gets to work. Early evening is the next best choice, while the soil is still warm and there is time for it to soak in before dark.

Mulch is the quiet hero of hot weather

If you do one thing after reading this, mulch everything. A 5cm layer over the surface of beds and containers can cut evaporation by half and keeps the compost underneath noticeably cooler, which matters because roots stop working properly when they cook.

Any of these will do the job: straw, dried grass clippings applied in thin layers, homemade compost, leafmould, wool pellets, or a gravel topping on pots. Leave a small gap around the stem of each plant so it does not sit damp against the bark, and top the mulch up mid-season as it settles. For containers, a mulch plus a saucer underneath is almost as good as a self-watering pot, and it costs nothing.

Blossom end rot is about consistency, not calcium

Almost every gardener meets blossom end rot at some point: a sunken, dark, leathery patch on the flower end of a tomato. It is usually blamed on calcium, and that sends people out to buy sprays that rarely help. The real cause is irregular watering. Calcium moves through the plant in water, so if the compost dries out, the fruit at the end of the chain gets starved of it even though the soil is full of the stuff.

So the cure is boring and effective. Keep moisture even. Mulch. Water deeply rather than little and often. Do not let plants dry to the point of wilting and then flood them, because that is exactly the cycle that triggers the problem. Go easy on high-potassium tomato feeds while the first trusses are setting — a rush of potash can make the situation worse — and keep feeding to a steady weekly dose once fruit is properly swelling. Pick off any affected tomatoes so the plant is not wasting energy on them, and the later trusses should come through cleanly.

Courgettes need more than you think

Courgettes are generous plants and greedy drinkers. Their roots are shallow, their leaves are enormous, and a plant that dries out will sulk, drop flowers, and produce fruit that rots at the tip. Water at the base rather than over the foliage, and be generous: a courgette given 5 litres every other day will out-crop one given a daily sprinkle every time.

In very hot weather, pollination can also fail, because pollen gets sticky and the flowers close early. Watering well in the evening helps the plant keep going, and planting something flowering nearby brings in the bees that do the work. A shallow dish of water with a few pebbles set among the pots gives bees and hoverflies somewhere to drink without drowning, and a stick left leaning into your water butt gives hedgehogs and birds a way out if they fall in.

A simple hot-weather routine

None of this needs to be complicated. In a heatwave, the whole job takes ten minutes a day.

• Check containers every morning with a finger test — small pots dry out fastest.
• Water deeply and slowly, at the base, until it runs through.
• Top up mulches and move pots into afternoon shade if you can.
• Harvest tomatoes and courgettes regularly, because ripening fruit draws heavily on water.
• Keep the water butt topped up and use grey water from the washing-up bowl on established plants, not on seedlings.

Do that and your plants will sail through the hottest fortnight of the year with fruit that is firm, unblemished and full of flavour — which is, after all, the whole point.