What is a cold frame and do I need one?

A cold frame is a low, unheated box with a transparent lid — traditionally a wooden or brick frame with a glass "light" — that traps solar heat and shelters plants. It is the cheapest and most useful piece of protected growing equipment most gardeners can have.

What it does. It creates a microclimate a few degrees warmer than outside, with protection from wind, heavy rain and frost. It is unheated, so it relies entirely on the sun and on the thermal mass of the ground beneath.

What it is genuinely useful for:

Hardening off, which is its most valuable role. Seedlings raised indoors or in a greenhouse cannot go straight outside — the shock of wind, temperature swing and direct sun kills or checks them. A cold frame provides a controlled intermediate step: closed at first, then opened progressively during the day over a week or two, then left open at night. Without one, hardening off means carrying trays in and out twice daily.

Extending the season. Sowing earlier in spring and later in autumn, and overwintering hardy salads and herbs.

Protecting overwintering plants from wet more than from cold — many alpines and Mediterranean plants are killed by winter wet rather than low temperature, and a frame keeps rain off.

Rooting cuttings, where humidity and shelter matter.

Storing dormant plants and forcing rhubarb or chicory.

Do you need one? If you raise plants from seed indoors, it is close to essential and will improve results more than most purchases of similar cost. If you buy plants ready-hardened, it is optional.

Practical points:

Ventilation is the main skill. A closed frame in spring sun can reach lethal temperatures within an hour. Prop the lid open on any bright day.

Site it facing south, sheltered, on free-draining ground.

Insulate in hard frost with fleece or bubble wrap.

Cheap alternatives work — a simple box with an old window, or a plastic-covered frame.