How Do I Use a Cold Frame to Extend My Growing Season?

A cold frame is one of the highest-value structures in the kitchen garden. It has no heating system, costs relatively little to build, and yet it can extend your growing season by four to six weeks at each end of the year while also serving as the perfect hardening-off station for greenhouse-raised seedlings.

What a Cold Frame Does

A cold frame is essentially a bottomless box with a transparent lid — called a light. The walls block wind, the transparent lid admits solar radiation and traps heat, and the soil inside warms up faster and stays warmer than open ground. On a sunny late-winter day, the temperature inside a closed cold frame can reach 15–20 °C even when outside it is 2 °C. At night, the frame insulates against frost: a well-sealed frame typically stays 3–5 °C warmer than outside air.

Positioning Your Cold Frame

Always site a cold frame against a south- or south-west-facing wall if possible. The wall absorbs heat during the day and radiates it back into the frame at night — a significant boost. The frame should slope slightly toward the sun so the lid sheds rain and maximises light interception. Avoid low frost pockets — cold air drains downhill and collects in dips, which would negate part of your protection.

Building a Simple Cold Frame

Old wooden pallets or sleepers on three sides, with an old window or sheet of twin-wall polycarbonate as the lid, make a serviceable cold frame at almost zero cost. The back wall should be higher than the front to create the sloped lid angle. Seal gaps with weatherstripping or old carpet offcuts to reduce draught. The lid must be easy to prop open, as ventilation is critical. Automatic vent openers (bimetallic strip type) are inexpensive and open the lid when internal temperature rises above a set level — worth fitting if you cannot check the frame daily.

What to Grow in a Cold Frame

Cold frames are ideal for overwintering lettuce, spinach, land cress, and hardy salad leaves through winter into early spring. In late winter, use the frame for early carrot and beetroot sowings. In spring, it is indispensable for hardening off tomato, pepper, and courgette seedlings. In autumn, pot up herbs like parsley and coriander and place them in the frame to extend harvest into winter. The frame can also protect tender perennials — salvias, penstemons, agapanthus — that need just frost exclusion to survive outdoors.

Managing Ventilation and Watering

Open the lid on any day the temperature inside exceeds 15 °C to prevent overheating and fungal disease. Water sparingly — plants in cold frames grow slowly and cold, wet compost encourages rot. Check plants weekly in winter; they may need no watering at all in cool, overcast conditions. Close the lid each evening before temperatures begin to drop.