Cold frame

Cold frames are box like items with a slanted, glazed roof or roofs. They offer protection to plants in a wide range of situations, including hardening off (which can save greenhouse space), raising plants from cuttings and overwintering seedlings or vulnerable plants. The roof section is usually hinged or sliding, to allow you to ventilate the frame to ensure good air circulation.

Cold frames are generally at least 1.2m by 60cm in size, although it’s usually best to get the largest one you can afford to spend the money and space on. If you are needing to accommodate tall plants then you can raise your cold frame on bricks to the required height.

Cold frames should, as a rule, be placed in a location which maximises the light reaching them. However, in the height of summer shading may need to be applied to prevent plants overheating. If you have an aluminum frame then you can move the cold frame to appropriate locations at different times of the year.

The base frame is either made from wood, brick or aluminium struts and a glazing material (ie glass, plastic or polycarbonate). Wooden and brick frames provide the best insulation, while aluminium frames provide a cheaper alternative which has the added advantage that it is easily moved (eg so it can be placed over plants growing in the ground).

The glazing of the frame is usually horticultural glass, which transmits light well and is a good insulator. However, plastic or polycarbonate glazes are a cost effective alternative, and also useful if children or animals use the area.

To provide additional insulation, for example on frosty nights, hessian or old carpet can be placed over the cold frame – just remember to remove it in the morning or you risk starving your plants of light. Bubble wrap can be used for daytime insulation as it should let some light through.