Cold Frame

A cold frame is a low box with a clear lid, warmed by nothing but the sun. Light passes through the glazing, the soil inside absorbs it, and the lid holds that warmth overnight, which keeps cool-season crops growing two to four weeks longer at each end of the season. That is the whole return. It is not a small greenhouse and will not carry tender plants through a winter; add a heat source beneath it and it becomes a hotbed.

It is also a propagation bed, probably its commoner use. Seedlings raised indoors have to be hardened off, meaning gradually accustomed to outdoor conditions, and a frame does that without anyone carrying trays in and out each night. Build it facing the midday sun on ground that drains, with a wall or hedge on the cold side set so it never shades it. Keep the depth front to back near 90 cm (3 ft) so the back is reachable without standing in the bed, slope the lid towards the sun, and put 10 to 15 cm (4 to 6 in) of soil inside. Plain lumber and a salvaged window will do.

What kills a cold frame is heat, not cold. A closed glazed box cooks its contents on any bright day, a cool one as readily as a warm one, which is when nobody thinks to look. Vent once the inside passes about 29 C (85 F), take the cover off for heat-sensitive crops if it will sit above 32 C (90 F) for hours, and close it before sunset. Water early, so leaves dry before dark: a humid closed box with wet foliage breeds fungal disease. Against a hard freeze, bank straw or soil along the sides. In a hot season it is useless or harmful unless shaded or unglazed. The timber outlasts the glazing, so replacement is a glazing cycle: film renews yearly, rigid panels last years. It sits beside a vegetable garden, belongs with a seed bank and nursery in one propagation chain, and hands on to a heated greenhouse where its protection ends. Pack manure under the soil and the box heats itself.