Temperature is the make-or-break variable in a greenhouse, and the biggest surprise for beginners is how fast it swings — a closed greenhouse can go from cold at dawn to plant-cooking by mid-morning. Managing that range, not hitting one magic number, is the goal. Different plants have different needs, so this is a framework.
Why greenhouse temperature swings so fast
Glass and polycarbonate trap solar heat efficiently, so on a sunny day the interior heats rapidly; at night, without that sun, a single-glazed greenhouse loses heat almost as fast. This large day-night swing stresses plants and is why ventilation and, in cold spells, heating matter so much.
“Cold”, “cool” and “warm” greenhouses
- Cold (unheated) — frost protection is limited to a degree or two; suits hardy plants and season extension. See unheated greenhouse.
- Cool / frost-free — heated just enough to stay above freezing; overwinters many tender plants.
- Warm — held at higher minimums for tender and tropical plants; costs more to run.
Choose the level that matches what you want to grow and your budget — heating a warm greenhouse through winter is expensive.
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Day vs night
Most plants tolerate a natural drop at night, but extremes at either end cause trouble: daytime overheating and night-time cold both stress plants and can stall growth or damage tissue. Aim to blunt the extremes rather than hold a flat temperature.
How to keep it in range
- Measure it — a max-min thermometer reveals the real highs and lows.
- Stop overheating — ventilate and shade (see cooling and shade cloth).
- Hold a minimum — insulate (bubble wrap) and heat in winter (see heating).
Quick questions
What temperature should a greenhouse be? There’s no single number — it depends on your plants. Most aim to prevent daytime overheating and hold a safe minimum (frost-free, or warmer for tender plants).
