How cold does an unheated greenhouse get?

The honest answer is a range, not a fixed offset

There is no reliable rule that an unheated greenhouse stays a certain number of degrees warmer than the garden. On a bright winter afternoon it can be noticeably warmer; during a long clear cold night it loses that heat. The overnight minimum may approach the outside air temperature and can go below freezing. Glazing type, gaps, wind, foundation, sun exposure and thermal mass all change the outcome.

The RHS greenhouse-heating guidance treats unheated growing as a useful way to shelter suitable plants, not a promise of frost-free conditions. This distinction matters for citrus, tender ornamentals and seedlings. Choose crops for what your greenhouse actually does at dawn, not how warm it felt at lunchtime.

Measure the coldest hours properly

Set a max/min thermometer at the height of the leaves or pots you most need to protect. Put it out of direct sun and away from glazing, doors, a heater or a warm wall. Note the minimum after several cold nights, reset it, and compare with the local forecast. A sensor near the roof will not represent a tray on the floor; a single reading on one mild night does not establish the winter range.

Check how the temperature behaves after a sunny day, an overcast windy day and a clear frost night. These conditions expose different weaknesses. If you use a digital instrument, ensure its outdoor/greenhouse operating range and battery performance are appropriate. For help comparing simple instruments, see best greenhouse thermometers.

Match the protection to the plant

Hardy crops and plants merely needing shelter from winter rain may be well served by an unheated greenhouse. Frost-sensitive plants need a different plan: move them indoors, create a smaller protected inner area, or use a correctly specified heater where a safe supply exists. The frost-protection guide sets out those options in more detail.

Removable bubble insulation can slow heat loss, especially around a smaller zone. It also reduces precious winter light and can increase condensation if the greenhouse is sealed. Close major draught gaps where the maker permits, but retain an ability to ventilate on mild days. A fleece cover over a crop can give an extra layer, provided it does not touch delicate foliage in a way that worsens damage and is removed when light is needed.

Keep a useful winter temperature record

+A simple record needs four things: the indoor minimum at plant height, the nearest available outdoor observation or forecast, the night's conditions and any protection used. Note whether the vents were shut, lining was fitted or a plant cover was in place. A warm sunny day followed by cloud is not a fair comparison with a clear, windy night.

Use that record to plan for the vulnerable plants, not to declare the structure safe after one mild week. A thermometer records air at its own position; leaves near cold glazing and containers on the floor may experience different conditions. When a forecast approaches a plant's known tolerance, allow for uncertainty and move or protect it in advance rather than waiting for damage.

Do not forget the daytime swing

An unheated greenhouse can overheat in sunshine even in winter. Monitor the maximum as well as the minimum, and open vents when conditions allow. Persistent damp under winter lining can encourage plant-health problems. Water less often than in summer and assess compost before each application. The practical goal is a stable, observed environment for the chosen crops—not an unverified claim that a cold greenhouse is always safe.