Gardening glossary
Cold frame
A cold frame is the simplest piece of season-extending kit a gardener can own. Picture a rectangular box, knee-high to waist-high, with sloping clear-glass or twin-wall polycarbonate lids. No heating. No artificial light. Just glass and a south-facing position. Inside, daytime temperatures can run 5–10 °C above outdoor air, and the frame buffers minimum temperatures by 2–4 °C — enough to stretch the growing season by 4–8 weeks at either end.
What to use a cold frame for:
- **Hardening off seedlings.** Move spring-raised seedlings from the windowsill or grow light into the cold frame for 7–10 days before transplanting. Open the lid wider each day. By the end, the lid is off entirely during the day and you can plant out without check. - **Autumn and winter salads.** Mustard, mizuna, lamb's lettuce (corn salad), winter purslane, claytonia, oriental greens, and overwintered spring onions all crop through a UK winter under a cold frame. - **Overwintering tender perennials.** Pelargoniums, half-hardy fuchsias, agapanthus in pots, and dahlia tubers can spend winter in a cold frame in zones 7–8 instead of being moved indoors. - **Forcing rhubarb and chicons.** Crowns lifted into a frame covered with an opaque lid produce blanched, tender stems weeks earlier than in the open. - **Direct-sowing early veg.** Carrots, radishes, lettuce, and broad beans sown in February under a cold frame mature 3–4 weeks ahead of their open-ground equivalents.
Ventilation is the key skill. On a sunny March day the inside can hit 30 °C even when the air outside is 5 °C, cooking seedlings. Prop the lid open in the morning and close it before evening. A simple wax-cylinder automatic lid opener takes the guesswork out for around £25.
Position a cold frame on a south-facing aspect against a south-facing wall if possible — the wall absorbs and re-radiates heat overnight, adding another 2–3 °C of minimum temperature protection. Lined with bubble wrap on the inside in deep winter, a well-sited cold frame gives you a near-zone-shift in growing capability for under £100 of materials.