How to Build a Simple Cold Frame from Scrap
A cold frame is one of the most useful structures a gardener can add to a yard. It costs almost nothing if you use salvaged materials, requires no power, and extends the growing season by 4--6 weeks on each end. In zone 7a, a cold frame allows spinach and lettuce planting in early March -- 6 weeks.
A cold frame is one of the most useful structures a gardener can add to a yard. It costs almost nothing if you use salvaged materials, requires no power, and extends the growing season by 4—6 weeks on each end. In zone 7a, a cold frame allows spinach and lettuce planting in early March — 6 weeks before the last frost date — and extends fall harvests into December with minimal effort.
The principle is simple: a transparent lid traps solar heat, and insulated walls retain it overnight. The difference between a cold frame and a small greenhouse is that a cold frame is unheated — it relies entirely on captured solar energy and thermal mass.
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What a Cold Frame Can Do
Per Penn State Extension, a cold frame with a glass or double-wall polycarbonate lid typically maintains temperatures 10—15°F warmer than outdoor air at night in most of the mid-Atlantic. In zone 7a (average January minimum 10—15°F), this means:
- Spring: Starting cool-season crops (lettuce, spinach, kale) 4—6 weeks before the last frost
- Fall: Extending harvest of cool-season crops through November—December
- Hardening off: The standard use — acclimatizing indoor-started seedlings to outdoor conditions before transplanting
- Overwintering: Protecting marginally hardy plants (borderline zone hardiness) through winter cold
Per Cornell Cooperative Extension, cold frames cannot protect plants through prolonged periods below 10°F without additional insulation (old blankets or bubble wrap laid on top of the lid on the coldest nights).
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