cold frame

A cold frame is a low, box-like structure with a transparent roof, typically 12 to 24 inches tall, that sits directly on the ground or over a prepared bed. The transparent cover, historically made from window sash or glass panes and now often polycarbonate or plastic film, allows sunlight to enter while trapping radiant heat inside. The structure protects seedlings and tender plants from frost, wind, and heavy rain, and can raise the internal temperature by 5 to 15 degrees Fahrenheit above the outside air on sunny days.

The simplest cold frames are built from wood, concrete blocks, or metal frames with the sides open to the soil. More permanent installations have brick or stone walls sunk slightly below grade to improve insulation. The transparent top is hinged or removable to allow access for planting, watering, and ventilation. Traditional designs use old window frames salvaged from buildings; modern versions often use aluminum frames with polycarbonate panels that are lighter and less fragile than glass but less transparent and prone to yellowing.

Operation and limitations

A cold frame works passively, relying on the greenhouse effect alone, with no heating equipment. On mild days the interior can overheat quickly, so the lid must be propped open or left fully raised to prevent scorching seedlings. This manual ventilation is essential in spring and autumn. The structure captures enough warmth to extend the growing season by 2 to 4 weeks at each end, allowing early sowing of cool-season crops like lettuce and spinach, or hardening off of seedlings before transplanting. In severe winter conditions, however, a cold frame offers no protection; freezing temperatures inside are common in northern climates.

Cold frames are most effective in temperate zones where daytime temperatures in early spring and late autumn stay above 40 degrees Fahrenheit, and where the site receives at least 6 hours of direct sun daily. Poor drainage, inadequate ventilation, and rodent entry are common problems. The transparent cover accumulates dust and algae that reduce light penetration, requiring regular cleaning. Condensation inside can encourage fungal disease on seedlings if air circulation is poor.

The term "cold frame" distinguishes this unheated structure from a "hotbed," which uses fermenting manure, electric heating cables, or warm water pipes beneath the growing medium to provide additional warmth. Cold frames have been used in market gardening for centuries and remain standard in small-scale nurseries and organic vegetable operations where the initial cost is low and the benefit in season extension justifies the labor of manual venting.