People get a greenhouse to protect plants from cold, so they fixate on keeping the cold out — heating, insulating, sealing it up tight. That instinct is not wrong, but it blinds new owners to the more common and more sudden danger, which is the exact opposite. On a sunny day, a closed greenhouse becomes an oven, temperatures inside soar far above outside, and plants cook. More greenhouse plants are lost to overheating than to cold, and the skill that actually saves them is not heating — it is ventilation.
Why the greenhouse overheats
A greenhouse works by trapping the sun's heat — that is the whole point — but on a bright day, even a cool one, that trapping is ruthlessly effective, and a closed greenhouse can heat up astonishingly fast to temperatures that stress or kill plants. The danger is its speed and its sneakiness: a sunny morning can cook a closed greenhouse while the owner assumes the cool outdoor air means the plants are fine. The very mechanism that protects against cold is what cooks the plants when the sun comes out and the vents are shut.
Why beginners miss it
Because the reason they bought the greenhouse was cold protection, so cold is what they watch for, and the idea that the greenhouse itself is the threat on a nice day is counterintuitive. The overheating is invisible from outside and fast, so it catches people who are diligently guarding against the wrong danger. The greenhouse protects against cold and threatens with heat, and beginners only expect the first half.
The takeaway
Respect overheating as the greenhouse's most common killer, not cold. Prioritise ventilation — open up on warm and sunny days, use automatic vent openers so it self-regulates when you are away, and shade in peak summer — because a sunny day can cook a closed greenhouse fast. Heating matters in genuine cold, but for much of the year the real craft is letting heat out, not keeping it in. Master ventilation, and your greenhouse protects your plants instead of cooking them.