What Is a Cold Frame and How to Use One
A cold frame is a low, transparent-topped enclosure that captures solar warmth and protects plants from wind and light frost. It can extend spring and fall growing, harden off seedlings, and shelter hardy greens without the cost or scale of a greenhouse.
A basic cold frame is a bottomless box set over garden soil, with a clear lid made from durable glazing material. Position it where it receives strong winter and early-spring sun, commonly facing south in the Northern Hemisphere, and shelter it from prevailing wind if possible. A sloped lid that is higher at the back helps shed rain and directs more light into the frame.
Cold frames work through solar gain, so temperature can rise quickly even on a cool day. Use a thermometer inside the frame and vent it whenever conditions become warm, often when interior temperatures approach 70 F for cool-season plants. Prop the lid open in stages, then close it before evening as temperatures fall.
Use a cold frame for hardy crops such as spinach, lettuce, mâche, kale, scallions, radishes, and Asian greens. In many climates, sowing or transplanting in late summer to early fall allows plants to establish before winter; the frame then slows growth but can keep leaves harvestable. The exact season depends on local winter lows, snow cover, and available sunlight.
A cold frame is also useful for hardening off seedlings. Put seedlings inside for short daytime periods at first, increasing exposure over about a week while closing the lid during cold nights. This transition is gentler than moving tender plants directly from indoor conditions into wind, intense sun, and temperature swings.
Manage moisture carefully because rainfall does not reach plants under a closed lid. Check soil with a finger several times each week and water when the root zone is drying, preferably earlier in the day so foliage and surfaces are not wet overnight. Ventilation reduces condensation and helps limit the persistently damp conditions that favor disease problems.
Cold frames provide protection, not absolute freeze-proofing. On very cold nights, add a layer of row cover over plants inside, place insulating material around the exterior, or use containers of water to moderate short temperature swings. Do not seal the frame tightly for long periods, and avoid using combustion-based heat sources in such a small enclosed space.
Frequently asked questions
How much warmer is a cold frame than outside?
The difference varies with sun, wind, lid material, and nighttime conditions. It may be much warmer in direct sun but only modestly warmer during a cloudy, very cold night, so monitor temperatures rather than relying on a fixed number.
Can I grow tomatoes in a cold frame?
A cold frame can help harden off tomato transplants in spring, but tomatoes still need warm conditions and protection from frost. It is usually better suited to cool-season crops for extended growing.
Do I need to open a cold frame every day?
Check it every sunny day. Vent when the interior gets too warm, and close it before cold evening temperatures arrive.
General guidance for US home gardens. Conditions vary by region and season; check with your local Cooperative Extension office for site-specific advice. Page updated Aug 4, 2026.