How do I protect a garden during a heatwave?

A heatwave is a short, intense event and the useful actions are almost all about reducing demand rather than increasing supply. Trying to water your way through a genuine heatwave is exhausting and largely futile; making the garden need less water works.

Safety

Step by step

1. Soak deeply before it arrives.A hot spell is usually forecast several days out. Water the things that matter thoroughly and slowly so the water reaches root depth, and check with a spade slice. A deep reservoir before the heat is worth more than daily watering during it.
2. Mulch immediately after watering.Mulch on moist ground cuts evaporation substantially. Mulch on dry ground keeps water out. The order matters, and this is the highest-value single job before a heatwave.
3. Prioritise, and say out loud what you are giving up.Containers first — they have the smallest reservoir and dry fastest. Then new plantings from the last two years, then anything cropping. Established trees, shrubs, hedges and lawns get nothing; they have deep roots or go dormant and recover.
4. Move and group containers.Into shade, out of the wind, and clustered together so they shade each other and the air between them stays more humid. A pot standing alone on a sunny patio is in the hardest position in the garden by a wide margin.
5. Shade the vulnerable during the hottest hours.Shade netting, horticultural fleece over hoops, an old net curtain, a parasol, a propped board. You are aiming to break the direct sun in the middle of the day, not to block light all day.
6. Water early in the morning, at the base, slowly.Early morning loses least to evaporation and gives the plant a full reservoir before the day's demand. Water the soil, not the foliage, and slowly enough that it soaks in rather than running off.
7. Stop doing several things.No feeding, which pushes soft growth that then needs more water. No sowing or planting. No pruning, which forces new growth and exposes previously shaded bark to scorch. Raise the mower height or stop mowing, since longer grass shades its own roots.
8. Look after the wildlife.Shallow water at ground level and at height, topped up daily and placed in shade, matters a great deal to birds, insects and hedgehogs in a heatwave, and costs almost nothing.
9. Afterwards, wait before writing anything off.Many plants shed leaves as a drought strategy and regrow from the base or from dormant buds. Do a scratch test on woody stems and give it weeks before removing anything.

Tips

• A lawn browning off in a heatwave is doing what grass does. It greens up within a fortnight of proper rain and needs no intervention, and watering it is the single biggest waste of water available in a garden.
• Fruit and vegetables under stress abort flowers and drop fruit. That is the plant protecting itself; consistent watering at the base of the few crops you care about is worth far more than spreading water everywhere.
• Newly planted trees and shrubs are the real casualties, because their roots are still confined to the original root ball. Water into the root ball itself, not the soil around it.
• Greenhouse temperatures in a heatwave become lethal to plants very quickly. Ventilate fully, shade the glass, and damp down the floor to raise humidity and drop the air temperature.
• Do the physical work at either end of the day. Plants can wait; you should not be lifting and carrying at two in the afternoon in a heatwave.

Common mistakes

• Watering everything a little every evening — It does not reach roots, most of it evaporates, and it consumes the time and water that a few deep soaks of the plants that matter would have used far better.
• Feeding to help plants through the heat — Feeding stimulates growth, and new soft growth needs more water and scorches more easily. A stressed plant needs less demand, not more.
• Cutting back scorched growth immediately — Damaged foliage still shades the stems and buds beneath it, and you cannot tell what has actually died for several weeks. Cutting also stimulates new growth into the ongoing stress.
• Mowing the lawn short in a heatwave — Short grass exposes soil to the sun, dries faster and browns harder. Raising the cutting height, or leaving it entirely, protects the crowns.

Questions people ask