Gardening for extreme heat: keeping plants alive through a heatwave

Heat and drought often arrive together, but they damage plants in different ways. Drought means there is not enough water in the soil. Heat can damage leaves, flowers and roots even when the soil is still moist. That is why a plant that tolerates drought may still struggle in a heatwave.

Heat extremes are becoming more frequent and more intense as the climate warms. The IPCC reports this with high confidence. In a garden, an occasional hot week is becoming a recurring problem. This article is about heat. For water shortage, see drought-tolerant gardening. For the wider pattern, see gardening in a changing climate.

What heat does to a plant

On a hot day, a plant loses water through pores in its leaves and pulls more up from the roots. If the leaves lose water faster than the roots can replace it, the plant wilts even in damp soil. If the shortfall goes on, the leaf edges and tips die and turn brown. This pattern is known as leaf scorch.

Above certain temperatures, photosynthesis slows down and then stops, so a plant in extreme heat uses its energy to survive instead of grow. Flowering is even more sensitive. Many plants drop their flower buds, or fail to set fruit, when temperatures rise beyond what their pollen can tolerate. That is one reason a tomato or a runner bean can flower well in a heatwave and set almost nothing. Blossom drop is the result.

Strong sun can damage exposed tissue as well as heating the plant. Fruit and bark that are normally shaded can burn when a canopy thins or a plant is suddenly exposed. The damaged area turns pale and sunken on the sunward side, which is a form of sunscald. Container roots are especially vulnerable. A pot in full sun can warm through until the roots against its sides cook.

Water deeply, and early

Water early in the morning during a heatwave. The plant starts the hot part of the day with water available, and the soil surface dries before evening instead of sitting warm and wet overnight. A deep soak every few days encourages roots to grow down into cooler, damper soil. A daily splash keeps roots near the hot surface, where they dry out fastest.

Aim the water at the soil, not the leaves. Wet foliage in strong sun does no real harm, but water on the ground reaches the roots, and a thorough soak that wets the whole root zone beats a light sprinkle that dampens only the top. Established plants can often go longer between waterings than they appear to need. A plant that wilts at midday may be losing water faster than it can move it, then recover by evening without extra watering.

Shade over the hottest hours

A plant that would scorch in open sun may survive under light shade for the middle of the day. Shade netting, horticultural fleece, an old sheet over canes or a parasol all take the sharpest heat off seedlings, salad crops and anything planted in the last few weeks. The shade need not be permanent. A few hours of cover from late morning to mid-afternoon is when it counts.

Permanent structure gives the same protection over years. A tree, a hedge or a wall provides shade and a pocket of cooler air. A pot stood at the foot of a larger plant gets some of that shade too. Group containers together so their sides shade each other and dry more slowly. A single pot in open sun heats on every side.

Build the resilience into soil and planting

Soil rich in organic matter holds more water and buffers the roots against heat, and a layer of mulch over it keeps the surface cooler and slows evaporation. Bare soil in a heatwave bakes hard and sheds water when it finally rains. Covered soil, whether under mulch or under low planting, stays in better condition through summer.

Choosing plants for heat matters as much as protecting them during a heatwave. Plants from hot, sunny regions often have small, tough, waxy, silver or aromatic leaves that lose less water and reflect more light. Lavender, rosemary and thyme belong to that group, and succulents such as agave store water in their tissue against the next dry heat. An olive takes summer heat that would exhaust a plant bred for a cool, damp climate.

Lawns and containers show heat quickly

A lawn often browns first in a heatwave, and usually needs the least attention. Most lawn grasses go dormant in heat and drought. They turn straw-coloured while the crowns stay alive underground, then green up again when rain returns. Cutting high, mowing less often and leaving the clippings helps the grass survive. Watering a lawn through a heatwave uses a great deal of water for grass that would usually recover anyway.

Containers are the opposite case, and the ones to watch. A pot has a small volume of compost that heats and dries far faster than open ground, and a plant in it cannot send roots deeper to find water. Move vulnerable pots into shade, stand them together, and check them daily, because a container plant can go from healthy to collapsed in a single hot afternoon.

Plan for the hottest day, not the average

A hardiness zone measures the coldest night and says nothing about the hottest day. The American Horticultural Society built a separate heat-zone map for exactly that gap, counting the number of days a year above 30 deg C, from fewer than one in the coolest zones to more than 210 in the hottest. A plant can survive your winter and still fail in your summer.

Read your own garden for heat. How hot does it get in the worst week? Which corners bake, and which stay cool? Which plants and pots will need shade and water when that week comes? Heat that used to arrive once a decade now arrives more often in many places. Plan for those days, and you lose fewer plants.