Cold Frame

A cold frame is a simple, unheated growing structure: a bottomless box with a transparent lid, placed over a garden bed or raised planting area. The lid - typically old window glass, salvaged storm windows, or polycarbonate panels - allows sunlight in while trapping the resulting heat and blocking wind. It functions as a miniature greenhouse without any heating system.

The name is slightly misleading. Cold frames don’t stay warm in hard freezes; they moderate temperature changes, slow the rate of heat loss on cold nights, and provide solar gain on sunny days. The soil mass inside a cold frame stores heat during the day and releases it overnight - soil that accumulated heat through a summer and early fall provides substantial thermal mass through the first frosts.

What Cold Frames Protect Against

Light frosts (28-32°F): A cold frame with good solar gain during the day will often hold temperatures 10-15°F warmer than outside on still, clear nights. A 32°F night with a warm soil base may stay above 40°F inside a well-built frame.

Wind: Eliminates wind chill, which matters as much as air temperature for plant damage in early spring.

Desiccation: Protected crops experience less moisture loss from winter wind and cold.

What Cold Frames Don’t Protect Against

Extended hard freezes (below 20°F for multiple nights) will freeze an unheated cold frame. For protection against sustained cold, supplemental heating (a single low-wattage heat cable or even a string of incandescent bulbs) is needed. This transitions the cold frame toward a “hotbed.”

Crops for Cold Frame Production

The best cold frame crops can tolerate temperatures near or slightly below freezing and actively grow in cool (35-50°F) conditions:

• Spinach: extremely cold-hardy; will overwinter in zone 5 with cold frame protection and resume growth in February
• Mache (corn salad, Valerianella locusta): exceptionally cold-tolerant; produces through winter in frames in zone 6
• Claytonia (miner’s lettuce): cold-hardy winter green
• Asian greens (pac choi, mizuna): tolerate light frosts
• Lettuce (heading types): tolerates frost to about 24°F with protection
• Arugula: cold-tolerant; may overwinter in frames
• Scallions: slow growth through winter, harvestable
• Parsley: biennial; holds well through winter in frames

Construction

The simplest cold frame: four pieces of lumber forming a box, taller in the back than the front (so the lid slopes toward the sun), with an old storm window or piece of polycarbonate as the lid.

Standard dimensions: a 3x6 foot frame accommodates a standard storm window. Height: 8-12 inches at the back, 4-6 inches at the front. Orient with the lid sloping toward south to maximize solar gain.

Frame materials: untreated rot-resistant wood (cedar, locust), cinder block, brick, bales of straw (excellent insulation). Commercial metal and polycarbonate cold frames are available but not necessary for function.

Polycarbonate (corrugated or twin-wall) transmits more total light over a season than glass because it doesn’t shade as severely on cloudy days, is lighter, and doesn’t shatter. Twin-wall polycarbonate has better insulating value than single-layer glass.

Ventilation

Cold frames trap heat quickly on sunny days. A 40°F morning can become a 90°F interior by noon under full sun. Without ventilation, plants heat-stress and bolt. Vent by propping the lid open several inches during sunny days above 40°F outside temperature; close before dark to retain day’s heat. Automated prop arms that open based on temperature (bimetallic spring mechanism) are inexpensive ($20-40) and eliminate the need to manually open and close the frame.