Cold Frame
Structure
A cold frame is an unheated, glazed box that creates a sheltered pocket of warmth close to the ground. It is one of the most useful small-space tools for a UK garden or allotment, helping you sow and grow beyond the short outdoor season without the cost or commitment of a greenhouse.
How it works
Sunlight passes through the clear lid and warms the soil, pots and air inside. The enclosed space then loses heat more slowly than an open bed, giving modest protection from cold, wind and rain. A frame does not make plants frost-proof, but it can advance sowing, protect tender young growth and provide a safer place for plants being hardened off before planting outside.
Best for
Use it for early lettuces, radish, spinach, spring onions and carrots, plus module trays of brassicas, leeks and salads. It is especially valuable for hardening off greenhouse-raised plants, and for overwintering hardy salads, young perennial plants and rooted cuttings. Choose a sheltered, sunny site with easy access for daily checking.
How to use it
Set the frame on level, well-drained soil or a firm base, with its lid facing the strongest available sun. Fill it with free-draining compost in pots, trays or a shallow growing bed. Begin sowing hardy crops a little earlier than outdoors, then cover seedlings on cold nights with horticultural fleece. Open the lid on bright days, even in spring, and close it before temperatures fall. As plants grow, raise pots or transplant them outside after a gradual hardening-off period.
Tips
Ventilation is the essential daily task: a closed frame can overheat surprisingly quickly in sun. Prop the lid open securely, but close it early if frost is forecast. Remove fallen leaves and algae from the glazing to retain light, and protect very young plants with fleece during sharp cold spells.
Cold Frame: frequently asked questions
How does cold frame work?
Sunlight passes through the clear lid and warms the soil, pots and air inside.
How do I use cold frame?
Set the frame on level, well-drained soil or a firm base, with its lid facing the strongest available sun.
What is cold frame best for?
Use it for early lettuces, radish, spinach, spring onions and carrots, plus module trays of brassicas, leeks and salads.
General guidance for UK gardens and allotments. Page updated 19 Sep 2026.