How do I protect seedlings from heat?

The difference between a seedling and an established plant is entirely about root volume. A module holds perhaps a few tablespoons of compost; a mature plant in a border has access to a cubic metre of soil. That is why a seedling can be dead by teatime on a day that leaves everything else merely wilted.

Step by step

1. Move seed trays out of the greenhouse.This is the highest-value action and the one people resist because the greenhouse is where seedlings belong. In a heatwave it is the worst place in the garden for them. Bright shade outdoors — under a bench, on the north side of a wall, under a tree — is far safer, and the light level is entirely adequate.
2. Water from below, not above.Standing trays and modules in a few centimetres of water lets the compost draw up what it needs, wets the whole root zone evenly and does not disturb small seedlings or wash seed about. Take them out once the surface darkens rather than leaving them standing all day.
3. Use shade netting or fleece over ground-sown rows.A lightweight fleece or shade net over hoops keeps direct sun off newly emerged rows and cuts soil surface temperature substantially. It also holds moisture at the surface, which is where germinating seed lives. Remove it once the seedlings are established and the weather has broken.
4. Shade transplants for their first fortnight.A seedling planted out has a rootball the size of the module it came from and cannot yet reach water in the surrounding soil. Anything that shades it for the first couple of weeks — a propagator lid propped up, a piece of net, a leafy branch pushed into the soil beside it — dramatically improves survival.
5. Plant out in the evening, never the morning.Planting in the evening gives a transplant the whole cool night to recover and settle before it faces sun. Planting at nine in the morning gives it a full day of heat with a disturbed root system.
6. Water the hole, not just the plant.Fill the planting hole with water and let it drain before putting the plant in. That puts moisture below the rootball where the roots are going, rather than on the surface where it evaporates.
7. Mulch around transplants immediately.A ring of compost or fine bark around each transplant, after watering, keeps the surface cool and holds moisture in the top few centimetres where the young roots are. Keep it clear of the stem itself.
8. Do not sow into hot dry soil.Most vegetable seed germinates poorly above certain temperatures, and lettuce in particular can enter thermodormancy and refuse to germinate at all in warm soil. The sowing-in-hot-weather guide covers the techniques for getting round this.
9. Accept smaller, later sowings.In a heatwave, sowing a smaller quantity in modules in a cool shaded place and planting them out when the weather breaks is far more productive than direct sowing into baked ground and losing the lot.

Tips

• A propagator lid in a greenhouse in summer is a death sentence — it is a greenhouse inside a greenhouse. Take lids off entirely once seed has germinated in warm weather.
• Capillary matting under trays keeps moisture even and buys a day or more, which matters if you cannot water at midday.
• Seedlings in black plastic modules suffer from root heating exactly as larger pots do. Standing trays on a pale surface or inside a larger tray helps.
• Bright shade is not dim shade. Seedlings kept in deep shade for weeks become drawn and weak; the target is full light without direct sun.
• Water in the early morning where you can, so the compost is fully charged before the heat of the day rather than after it.

Common mistakes

• Leaving seedlings in the greenhouse through a heatwave — A greenhouse can pass lethal temperatures for small pots within a couple of hours on a sunny morning, long before an established plant is troubled.
• Watering seedlings overhead in the middle of the day — Much of it evaporates, it disturbs small plants, and water sitting on foliage in a hot greenhouse encourages disease. Bottom-watering is better in every respect.
• Planting out on a hot morning — It gives a disturbed root system a full day of heat before it has settled. Evening planting gives it a whole night.
• Keeping propagator lids on in summer — They trap heat and humidity that were useful in March and are lethal in July. Take them off once germination has happened.

Questions people ask