Cold Frames

A cold frame is an unheated, low enclosure with a transparent lid that captures solar heat to extend the growing season. Growers use them to harden off seedlings, overwinter cold-hardy crops, start plants weeks early in spring, and protect tender plants through fall frosts without the cost of a heated greenhouse.

Site it facing south or southwest for maximum winter sun, ideally against a wall or other windbreak that adds reflected heat and blocks cold wind. Good drainage underneath is essential, since standing water rots roots and stems. Easy access matters too, because the lid needs daily attention during temperature swings.

Cold frames heat up fast on sunny days, even in winter, and can cook plants if left closed. Vent the lid on sunny days once interior temperatures rise, propping it open and closing it before evening to trap heat overnight. Automatic vent openers that respond to temperature remove the guesswork for growers who cannot check constantly.

Cold-hardy crops thrive: leafy greens, spinach, and many cool-season vegetables hold through winter in most climates, and the frame protects against the harshest cold and wind. It is also the standard tool for overwintering dormant nursery stock and hardening off spring transplants before they go in the ground.

A cold frame relies only on solar heat, while a hotbed adds a heat source underneath, traditionally decomposing manure and now often electric cables, to keep soil warm enough for germination in deeper cold. Start with a cold frame; add bottom heat only if you need reliable winter propagation.

Look for durable glazing that transmits light while resisting weather, a lid that opens easily for venting, and a frame material suited to your climate and budget. Size it to the bench space or bed you actually need, and confirm the lid can be propped at multiple heights for graduated venting.