How do I help a plant recover from heat stress?

Heat stress and drought stress arrive together and look similar, but they are not the same injury. A plant short of water is failing to supply its leaves. A heat-stressed plant may have plenty of water and still be shutting down, because above a certain leaf temperature photosynthesis stops, the stomata close to conserve water, and the plant stops functioning until it cools. Knowing which you are looking at changes what you should do, and in several common cases it changes it to nothing.

Step by step

1. Check the compost before doing anything else.Push a finger in to the second knuckle, or lift a pot to feel the weight. Dry means it is a water problem. Wet, with the plant wilting anyway, means either heat stress, root damage or waterlogging — and in every one of those cases more water makes it worse.
2. Separate afternoon wilt from real wilt.Many plants, courgettes and hydrangeas especially, wilt dramatically in afternoon sun and are completely recovered by morning. That is normal regulation, not damage, and it needs no action at all. A plant still wilted at seven in the morning has a real problem.
3. Recognise what heat damage looks like.Leaf margins and tips browning and going crisp while the middle of the leaf stays green. Bleached or papery patches on the upper surfaces of leaves that face the sun. Flowers and small fruit dropping. Growth stopping entirely. Leaves shed wholesale, which looks catastrophic and is often a survival strategy rather than death.
4. Water once, properly, then stop.If it is dry, soak it thoroughly at the base until water runs from the bottom of the pot or the ground is wet to depth — then leave it. Repeated light watering of a stressed plant keeps the surface wet, encourages shallow roots and does not reach where it is needed.
5. Reduce the load on the plant.Take off flowers, buds and developing fruit. These are the most expensive things a plant does and removing them redirects what capacity it has into staying alive and regrowing roots. On a badly hit plant, removing some foliage is also legitimate, because less leaf area means less water demand.
6. Move it into shade and leave it there for a fortnight.Shade removes the ongoing injury and lets the plant repair. Reduced light for a couple of weeks costs a plant far less than continued heat does. Reintroduce it to sun gradually, over several days, so the new growth does not scorch.
7. Do not feed it.This is the most common and most damaging mistake. Fertiliser salts in dry compost draw water out of already-damaged roots and make the injury worse, and feeding pushes a plant to produce growth it cannot currently support. Wait until it is visibly growing again, which may be several weeks.
8. Do not prune off the damaged leaves yet.Brown leaves still shade the stems and the crown beneath them, and a plant recovering from heat needs every scrap of functioning tissue. Wait until new growth appears and you can see what is genuinely dead, then tidy up. Pruning also stimulates growth the plant cannot afford right now.
9. Do not repot or move it into a bigger container.Repotting disturbs exactly the root system that is already damaged. If the pot is genuinely too small, wait until autumn. The exception is compost that has become completely water-repellent, where the guide on reviving a dried-out plant applies.
10. Wait several weeks before deciding anything is dead.Scratch a stem with a thumbnail: green underneath means it is alive. Many shrubs and perennials drop everything in extreme heat and reshoot weeks later from the base. Plants dug up in August that would have recovered in September are one of the most common losses in a hot year.

Tips

• The single most useful rule: if the compost is wet and the plant is wilting, adding water makes it worse. That is true for heat damage, root rot and waterlogging alike.
• Plants recently bought from a nursery or moved outdoors are far more vulnerable, because they have not hardened to the light and their roots are confined to the original rootball.
• A heat-stressed plant in a pot is usually also a root-temperature problem. Shading the pot itself matters as much as shading the leaves.
• Recovery is slow and undramatic. Expect several weeks before new growth, and do not judge progress daily.
• Trees and shrubs planted within the last two years are the most at risk in the ground, because their roots are still confined to the planting hole.

Common mistakes

• Feeding a stressed plant to help it recover — Fertiliser salts pull water out of damaged roots and force growth the plant cannot support. It is the most common way a plant that survived the heatwave is killed afterwards.
• Cutting off all the damaged foliage immediately — Those leaves shade the crown and stems, and pruning stimulates growth the plant cannot currently fund. Wait for new growth before tidying.
• Watering a little every day — It wets the surface, never reaches the root zone, and encourages shallow rooting that makes the next hot spell worse. One thorough soak beats seven light ones.
• Digging up and replacing anything that looks dead in August — Many plants shed everything and reshoot weeks later. Waiting until autumn costs nothing and saves a great many plants.

If it doesn't work

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