How do I stop plant pots getting too hot in the sun?

Everyone knows a plant in a pot needs more water than one in the ground. Fewer people know that the roots themselves are being cooked, which is a separate injury with different symptoms — a plant that wilts despite wet compost, browns at the leaf margins and stops growing has usually had its roots damaged rather than its water supply interrupted. Soil in the ground buffers root temperature; five millimetres of black plastic does not.

Step by step

1. Understand what the roots are experiencing.In the ground, soil a few centimetres down stays remarkably stable even on a hot day, because there is an enormous mass of earth to absorb the heat. In a container the entire rootball is within a few centimetres of a sunlit surface on at least two sides, and dark plastic absorbs radiation efficiently. The roots on the sunny side take the worst of it, and root tips are the most heat-sensitive part of the plant.
2. Shade the pot, not just the plant.This is the key distinction. Shading the foliage helps with transpiration, but it is the container sides that are conducting heat into the rootball. A pot whose sides are shaded by another pot, a low plant, a board or a piece of hessian is doing better than one whose leaves are shaded while its black sides bake.
3. Group containers together.Pots in a block shade each other's sides, and the still, humid air between them slows evaporation. It costs nothing and it is the most effective single change most people can make. Put the small, vulnerable and shallow-rooted ones in the middle.
4. Choose the pot for the position.Thick-walled terracotta, glazed ceramic and stone buffer temperature far better than thin black plastic, simply because there is more mass. Pale colours reflect rather than absorb. Terracotta is porous, which cools by evaporation but also dries faster — a genuine trade-off, and worth accepting for anything heat-sensitive.
5. Double-pot anything precious.Standing a plastic nursery pot inside a larger decorative one creates an insulating air gap, which is remarkably effective. Filling the gap with bark, gravel or compost is better still. This is how to keep an expensive plant alive in a hot courtyard.
6. Get pots off hot paving.A dark paving slab, tarmac or a gravel bed in full sun reaches high temperatures and conducts straight into the base of the pot, where the roots have collected. Standing pots on soil, on grass, on a wooden board or on pot feet that lift them into moving air all reduce it.
7. Mulch the surface of the container.A layer of gravel, bark or slate over the compost reflects and shades the surface, cuts evaporation and stops the top of the rootball baking. It also stops the surface crusting so water penetrates properly.
8. Move rather than mitigate where you can.The cheapest solution is relocation. Morning sun with afternoon shade suits the large majority of container plants in a British summer, and moving a pot two metres is free. Use a trolley or a sack barrow for anything heavy rather than damaging your back.
9. Do not water cold to fix a hot pot.Watering is for water. It cools the rootball briefly, and a plant needing water should be watered whenever you find it, but drenching a hot pot repeatedly to cool it leads to waterlogged compost and root death from the other direction.

Tips

• Black plastic nursery pots are the worst case and the most common. If a plant is staying in one all summer, put it inside something else or bury it in a border.
• Root damage shows as wilting that does not recover after watering, browning leaf margins and a plant that simply stops growing. Tip a suspect plant out and look at the roots — healthy roots are pale and firm, cooked roots are brown and soft.
• A pot pushed into a border and mulched over is a legitimate summer home for a struggling plant, and you can lift it back out in autumn.
• Grouping pots also makes watering quicker, which means it is more likely to actually happen in a hot fortnight.
• South and west-facing walls radiate stored heat back at pots into the evening. A pot against a sunny wall experiences a longer hot period than one two metres away from it.

Common mistakes

• Shading the leaves and leaving the pot in the sun — The injury is to the roots, from heat conducted through the container walls. Foliage shade helps with water loss but does nothing about root temperature.
• Assuming a wilting plant in wet compost needs more water — It is usually the opposite: heat-damaged or waterlogged roots cannot take up water regardless of how much is present. Adding more makes it worse.
• Standing dark pots on dark paving in full sun — The slab and the pot both absorb heat and the base of the rootball, where most roots are, gets the worst of it.

Questions people ask