Building and using a cold frame — print version.
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SUS Farms · Utah Gardening
Building and using a cold frame
season · intermediate · ~6 min read
A cold frame is an unheated glazed box over a garden bed. It works as a passive solar collector — sun through the glass during the day, trapped warmth at night. A simple cold frame extends Utah's growing season 4–6 weeks on each end.
Basic geometry
4 ft wide × 6 ft long is a good single-bed size. Back wall 18" tall, front wall 12" tall — that 6" slope southward catches winter sun. Glazing tilted 30–45° from vertical maximizes solar gain in winter.
Materials
Walls: 2x12 untreated cedar or recycled storm windows. Lid: an old aluminum-frame storm window or twin-wall polycarbonate. Hinges and a stick prop for venting. Total cost ~$80 from local stock.
Venting is the secret
On a sunny February day, an enclosed cold frame can hit 90°F inside even when it's 30°F outside. Cooked seedlings. Prop the lid open 6" anytime daytime temps rise above 50°F. Close at sunset.
What to grow
Spring (Feb–April): lettuce, spinach, radish, hardened-off tomato/pepper transplants. Fall (Sept–Nov): the same plus mâche, claytonia, mizuna for fresh winter greens.