Cold frames — modest, essential, underrated

A cold frame is a wooden or brick box with a glass or polycarbonate lid, usually no more than 45 cm tall. It captures warmth at soil level, where it matters most, and costs almost nothing to run. You can build one from reclaimed timber and old windows in an afternoon. It will outlast most of your tools.

The primary use is hardening off. Seedlings raised indoors or in a greenhouse are soft and pale. Move them straight outside and they scorch, wilt, or stall. The cold frame is the transition zone. You move seedlings into it in mid-April, leave the lid closed for two days, then begin propping it open during the day — a little more each day until it is fully open. After a week, the plants are tough enough for open ground.

The second use is season extension. Sow lettuce, rocket, spinach, and other salads directly into a cold frame in September, and you will harvest through December and beyond. The soil stays warmer, growth is faster, and you are picking leaves when the outdoor garden is frozen. Corn salad, land cress, and winter purslane are especially good for this — they tolerate near-freezing temperatures and keep producing.

The third use is overwintering cuttings. Pelargoniums, fuchsias, penstemons, and salvias all root easily in late summer. Pot them, place them in the cold frame, and they will survive most UK winters with just the lid closed. In hard frosts, drape horticultural fleece over the frame for extra insulation. By spring, you have vigorous young plants ready to fill gaps.