Greenhouse overheating: what to do

Diagnose the overheating, not just the weather forecast

An enclosed greenhouse can heat rapidly on a bright spring day, even when the garden air feels comfortable. The RHS guidance on ventilation and shading recommends ventilation as the first defence, with shading when airflow is insufficient. The question is what happens inside your structure, at the height of the leaves, not just what the forecast says outside.

Put a max/min thermometer in a shaded position among the crop canopy. Read its daytime maximum over several sunny days and compare it with how plants look in the afternoon. A sensor against sunlit glass or directly under a roof panel can exaggerate the temperature experienced by the plants. Different crops tolerate heat differently, so avoid treating a single number as a universal safe limit.

A same-day rescue order

Open roof vents fully and provide a lower inlet—often the door, a side vent or a louvre. Check that every vent actually moves freely and that netting, bubble lining or tall tomatoes are not obstructing it. If the weather is windy, secure doors and vents as the maker instructs rather than leaving them to slam. Move especially vulnerable trays out of intense direct sun where possible.

Check the compost below the surface. A wilted plant may be dry, but watering already wet compost can make root problems worse. Water the root zone if it needs it, preferably in the cooler part of the day. Ventilation and water management solve different problems: the former removes hot air; the latter replaces moisture used by the crop. See the greenhouse watering systems guide if keeping pots evenly supplied is the repeated failure point.

If the greenhouse is still overheating with its vents working, add removable shade to the sunniest elevation or roof. External shade intercepts solar energy before it passes through the glazing, but it must be secured against wind and kept clear of opening vents. Start with moderate coverage and reassess plant growth and thermometer readings. The shade-netting guide explains percentage, fitting and removal.

Fix the recurring cause

An automatic roof opener helps with mornings when no one is available to open the vent, but it is not a complete cooling system. It has to fit the exact vent and frame, and its wax mechanism opens progressively rather than instantly. A small greenhouse with one roof vent may still need a dependable low inlet. Read the ventilation guide before buying more hardware.

Plant density also matters. A wall of mature tomatoes can cut off airflow to seedlings behind it; crowded shelving creates hot, still pockets. Thin out old leaves where horticulturally appropriate, leave a path around the crop and avoid placing a fan where it merely blows hot air against one group of plants. A circulation fan moves air inside; only an open vent or designed extractor exchanges it with outside air. See circulation versus extraction.

Check that your changes worked

After each change, reset the max/min instrument and compare a few similar sunny days. Record the roof and door position, shade coverage and which crop first showed stress. This is more useful than buying several gadgets at once. On dull days or as autumn approaches, remove or roll back shade so plants regain useful light. Keep a hot-day routine in the greenhouse: open, measure, water based on the root zone and recheck in the afternoon.