rootwise

How-to guide

Watering in a heatwave: what to water first and how

By Hannah Price · Updated

In a heatwave, water early in the morning while the soil is cool, and aim the water at the roots, not the leaves. Give priority to the plants that will die without help: anything planted in the last year, containers, seedlings and vegetables that are flowering or fruiting. Water these thoroughly every few days rather than a little every day. Established trees, shrubs, many perennials and lawns are tougher than they look and can usually be left alone.

Hot spells are becoming a regular feature of British summers, and the Met Office now defines a heatwave by county: at least three days in a row reaching a local threshold of 25 to 28°C. The plan below assumes you have limited time and limited water, which is the reality in most gardens.

When to water

Early morning is best. The soil is at its coolest, little water evaporates before it soaks in, and plants go into the hottest part of the day fully charged. Leaves that get splashed have time to dry, which reduces the risk of fungal problems.

Evening watering is the next best option, especially for pots that dried out during the day. The downside is that damp soil overnight brings out slugs, and wet leaves stay wet for hours. Water the soil rather than the plant and you avoid most of the trouble.

Watering in the midday sun is not harmful to leaves, despite the old warning about scorch from droplets, but much of the water evaporates before it reaches the roots. If a plant has collapsed at lunchtime, give it a drink anyway.

What to water first

When water or time is short, work down this list:

  1. Anything planted in the last twelve months, including trees, shrubs and hedges. Their roots have not yet spread beyond the planting hole. See our bare-root planting guide for first-year watering.
  2. Pots, hanging baskets and window boxes. They hold very little water and can dry out in a single afternoon.
  3. Seedlings and recent sowings, which have tiny root systems.
  4. Vegetables and fruit at flowering and fruiting. Tomatoes, courgettes, runner beans and strawberries all need steady moisture to set and swell their crops.
  5. Leafy crops such as lettuce, which bolt quickly when dry.
  6. Thirsty ornamentals like hydrangeas, hostas and newly planted perennials.

Low priority: established trees and shrubs, and Mediterranean plants such as lavender, rosemary, sedum and achillea. These have deep roots or leaves built for drought. Lawns go at the very bottom of the list.

How much to water

One thorough soak does far more good than several light sprinkles. A light watering only wets the top centimetre or two, which evaporates within hours and encourages roots to stay near the surface where they are most vulnerable.

In beds and borders, aim for the equivalent of about 2 cm of rain: roughly two full 10-litre cans per square metre. Pour slowly or in two goes, so the water soaks in rather than running off. For individual plants, a full can poured around the base of a shrub or a trug-sized fruit bush is a good dose. Then leave the soil alone for several days, even a week, before watering again.

To check whether you need to water, push a trowel into the soil and look at the soil 10 cm down. If it is damp, the plant has water within reach. If it is dry and dusty, it is time.

Forming a shallow ring of soil or sinking an upturned plastic bottle with its base cut off next to thirsty crops helps the water get straight to the roots instead of spreading across the surface.

Containers and hanging baskets

Pots need daily checks in hot weather and sometimes watering twice a day, especially small terracotta pots, hanging baskets and anything in full sun. Water until it runs out of the drainage holes. If the compost has shrunk away from the sides of the pot, water runs straight down the gap; stand the pot in a tray of water for half an hour to re-wet it.

To reduce the work, move pots together into light shade during the worst of the heat, where they shade each other’s sides. Stand them on saucers in a heatwave only, and remove the saucers when normal weather returns so the roots do not sit in water. Bigger pots dry out more slowly than small ones, which is worth remembering when you plant up next year.

The vegetable plot

Irregular watering is the cause of several common summer problems. Tomatoes that swing between dry and soaked develop blossom end rot, a dark, sunken patch at the base of the fruit. Courgettes and squash under stress are more prone to powdery mildew. Potatoes in dry soil give small crops and can crack when rain finally arrives.

Concentrate water where it counts. Potatoes benefit most once the tubers start to form, roughly at flowering. Beans need moisture at the roots once flowering begins to set their pods. Onions and garlic that are nearly ready can be left to dry off.

Once the soil is soaked, a mulch of garden compost, straw or grass clippings 5 cm deep greatly reduces evaporation. Our mulching guide explains when to put it down; never spread mulch over dry soil, as it will keep the rain out later.

The greenhouse

Glasshouses can pass 40°C on a sunny day, which damages plants even when they are well watered. Open every vent and door in the morning and leave them open through the day. Shade the glass with shade netting or a wash-off shading paint. Damping down, which means soaking the floor with a can in the morning, raises humidity and cools the air. It also discourages red spider mite, which thrives in hot, dry conditions.

Lawns

An established lawn that turns straw-coloured is not dead. Grass goes dormant in drought and greens up again within a couple of weeks of decent rain. Watering a whole lawn uses an enormous amount of water, so it is usually best to let it go. Raise the mower blades, mow less often and leave the clippings on the surface to shade the soil. Newly sown or turfed lawns are the exception and do need watering. Our guide to lawn care through the year covers summer and recovery in autumn.

Saving and reusing water

A water butt fed from a shed, garage or house roof fills surprisingly quickly in a summer storm, and rainwater is ideal for acid-loving plants. Fit as many butts as you have downpipes.

Used bath, shower and washing-up water, often called grey water, is fine on ornamental plants and lawns for short periods. Let it cool, use it soon rather than storing it, avoid water with bleach or strong cleaning products, and keep it off leafy salad crops and anything you will eat raw.

If your water company brings in a temporary use ban, often called a hosepipe ban, you will not be able to use a hose connected to the mains on the garden. Watering cans filled from the tap are normally still allowed. Check the details for your area, as rules and exemptions vary.

Signs of trouble

Drooping leaves in the afternoon heat are normal for many plants and they perk up by evening. If plants are still wilting in the cool of the morning, they are short of water. Brown, crispy leaf edges, flowers dropping and fruits failing to swell are later signs. See drought stress for how to tell this from root damage or disease.

Questions people ask

Should I water plants in the evening or morning in a heatwave? Morning is best, because less water evaporates and leaves dry quickly. Evening is fine for pots that dried out during the day; just water the compost, not the foliage.

How often should I water pots in hot weather? Check daily. Small pots and baskets in sun often need water every day and sometimes twice; large pots in shade may manage two or three days.

Does a sprinkler waste water? Usually, yes. A lot evaporates or lands where it is not needed. A can, a hose with a trigger aimed at the roots, or a seep hose under mulch puts water exactly where plants use it.

Will my brown lawn recover? Almost always. Leave it alone, avoid feeding or heavy use, and it will green up once the rain returns.

Sources and further reading

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