How do I keep a greenhouse from cooking in a heatwave?

The order to do things in when the forecast says thirty degrees — what actually brings the temperature down, what makes no difference, and which plants to move out entirely.

Difficulty: beginner
Time: 30 min
Read: 4 min
Safety: caution

Short answer

Open everything before the sun reaches the glass, get shading on, damp down the floor in the morning and again at midday, add a fan to move air, and move anything in a small pot outside into shade. Ventilation and shading do the work; misting the plants does almost nothing.

In a heatwave a greenhouse is working against you. Above roughly thirty degrees tomato pollen becomes less viable and fruit set fails; above the mid-thirties leaves scorch and growth stops entirely. The aim is not to make it cool but to shave several degrees off the peak and keep the air moving.

Safety

A closed greenhouse in strong sun reaches temperatures that are dangerous to people as well as plants, and it heats up within minutes. Never leave a child or an animal inside one, do not work in one during the hottest part of the day, and if you must, ventilate fully first and take water in with you.

Step by step

Open everything, early.Roof vents, louvres and the door, opened before the sun is on the glass — by seven or eight in the morning, not at eleven. A greenhouse that has already reached thirty-five degrees will not come back down while the sun is on it.
Get shading on if it is not already.External netting or a coat of shade wash on the roof and south side. This is the single biggest temperature reduction available and it can be done in an hour.
Damp down the floor in the morning and again at midday.Throw water over the path and the staging legs. Evaporation absorbs a substantial amount of heat and raises humidity, which reduces plant water stress and discourages red spider mite.
Add air movement.A small fan, mains or battery, moving air along the length of the house. In still conditions the natural stack effect weakens exactly when you need it, and a fan restores it. It also directly reduces leaf temperature.
Move vulnerable plants out.Anything in a small pot, seedlings, cuttings and newly potted plants should go outside into shade for the duration. They dry out fastest and have the least buffer.
Water thoroughly in the early morning.A plant that goes into a hot day with a fully charged rootball copes far better than one watered at midday. Water the compost, not the foliage, and water thoroughly rather than little and often.
Do a second check in the late afternoon.Growbags, small pots and anything fruiting heavily may well need water twice. Check by weight and by pushing a finger into the compost, not by looking at the surface.
Leave the vents open overnight.On a hot night the greenhouse holds its heat and the plants get no recovery period. Leave vents and, if it is secure, the door open until the temperature genuinely falls.

Tips

A max-min thermometer at plant height tells you what actually happened while you were out. Without one you are guessing, and greenhouse temperatures are consistently higher than people assume.
Damping down is much more effective on a solid concrete or paved floor than on gravel, and pointless on bare dry soil that just absorbs it.
In an extreme heatwave, temporarily draping shade netting or even old net curtains over the outside of the roof for a few days is a legitimate emergency measure.

Common mistakes

Misting the plants to cool them — The water evaporates from the leaves in minutes, does almost nothing for air temperature, and droplets on leaves in strong sun encourage scorch and fungal problems. Damp the floor, not the foliage.
Waiting until the greenhouse is hot before opening it — By late morning the mass of the structure and the soil has absorbed heat and the vents cannot shed it fast enough. The vents have to be open before the sun arrives.
Closing up in the evening out of habit — In a heatwave the house needs to lose heat overnight. Closing at five traps the day's heat right through the night.

If it doesn't work

Tomato flowers dropping without setting fruit

Cause: Daytime temperature above roughly thirty degrees, or very low humidity, both of which make pollen non-viable. — Fix: Shade and ventilate to bring the peak down, damp down to raise humidity, and tap the supports at midday to help pollen release.

Pale, bleached patches on leaves facing the glass

Cause: Direct scorch, where a leaf is touching or very close to hot glass or in an unshaded south-facing position. — Fix: Shade that face, move the plant back from the glass, and remove the worst-damaged leaves. New growth will be unaffected.

Plants wilting at midday but recovering by evening

Cause: Transpiration outrunning uptake in extreme heat, not necessarily drought. — Fix: Check the compost before watering. If it is moist, this is heat stress — shade and damp down rather than watering more, which will drown the roots.

Fine webbing and stippled, bronzed leaves

Cause: Red spider mite, which thrives in hot dry greenhouse conditions. — Fix: Raise humidity by damping down twice daily, and introduce the predatory mite Phytoseiulus persimilis early while numbers are still low.

Questions people ask

How hot is too hot for a greenhouse?

Growth in most greenhouse crops slows above about thirty degrees and fruit set in tomatoes fails around there. Above the mid-thirties you get scorch and stalled growth. The aim in a heatwave is to keep the peak below thirty where you can.

Should I close the greenhouse to keep the heat out?

No — that is the opposite of what works. A closed greenhouse in sun gains heat continuously with no way to shed it. Open everything and shade the outside.