How do I cool a greenhouse down overnight?

Most greenhouse advice is about the daytime, and the overnight temperature gets ignored. It matters more than people expect: tomatoes and peppers set poorly when the night stays warm, because the plant never gets its recovery period, and a greenhouse that starts the morning already warm reaches the damaging thresholds far earlier the next day.

Step by step

1. Leave the vents open overnight.In a hot spell there is no benefit to closing a greenhouse at night in a British summer, and a real cost to the crop. Roof vents, side vents and the door can all stay open. The reasons to close up are security, wind and keeping animals out — not temperature.
2. Understand what is holding the heat.Air cools quickly; mass does not. The concrete or slab floor, the staging, the pots, the water butts and the soil in the border all absorbed heat during the day and release it slowly overnight. That is why an empty greenhouse cools far faster than a full one, and why the air is still warm at midnight.
3. Open high and low at the same time.Warm air leaves through a roof vent and cooler air is drawn in through a low side vent or the door. One opening alone gives very little flow; two at different heights create a genuine stack draught that empties the building. This is the same principle as any other structure and it works particularly well in a greenhouse because the height difference is large relative to the volume.
4. Damp the floor down in the early evening.Wetting the floor and paths late in the day cools by evaporation for hours as the water evaporates, and raises humidity, which suits most greenhouse crops. It is more effective than misting the plants and it lasts far longer.
5. Add a fan if the site is still.Where a greenhouse sits in a sheltered corner with no natural breeze, a small fan running overnight — either moving air inside or set in a vent to exhaust — makes a considerable difference. It also reduces fungal disease risk, which rises in still, humid air.
6. Reduce the thermal mass where you can.Water butts and full water containers inside a greenhouse are deliberately used to buffer temperature in spring, which is exactly the wrong thing in a heatwave. Move them outside for the summer if overnight heat is the problem.
7. Fit automatic vent openers.A wax-cylinder opener needs no power and opens the vent when the temperature rises, including first thing in the morning before anyone is up. It does not close the vent at night in warm weather, which in summer is exactly what you want.
8. Shade before it gets hot, not after.The overnight temperature is downstream of the daytime one. A greenhouse shaded from late May onwards banks far less heat in its fabric each day and therefore starts each night cooler.

Tips

• A max-min thermometer tells you the overnight minimum, which is the number that matters here and the one nobody measures.
• If security is the reason vents stay shut, louvre vents and roof vents with restrictors are much harder to use as an entry point than an open door, and still ventilate well.
• In a polytunnel the door at each end is the main ventilation, so opening both ends creates a through-draught that empties it quickly.
• Greenhouse borders planted directly into the ground hold and release less heat than a slab floor, and they also root deeper so the plants cope better with heat overall.
• Do not close up in the evening to keep out cold air in summer. British night air in a hot spell is exactly what you want inside.

Common mistakes

• Closing the greenhouse at dusk out of habit — It traps the day's stored heat with the plants all night, costing fruit set and starting the next day warm. Closing up is a spring and autumn habit that does harm in a heatwave.
• Opening only the roof vents — Without a low inlet there is nothing to replace the air leaving, so flow is weak. High out and low in together is what moves the volume.
• Keeping water butts inside through summer — They are thermal mass, put there deliberately to hold warmth overnight in spring. In a heatwave they are working against you.

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