Cold Frame

Structure

A cold frame is a low, box-like structure with a transparent top that traps solar heat near the ground. Gardeners use it to protect crops from cold nights, wind, and light frost. It can extend harvest in fall and start hardy plants earlier in spring.

How it works

Sunlight passes through the clear lid and warms the soil and air inside the frame during the day. The enclosed space slows heat loss at night and blocks drying wind. Because the soil beneath the frame also stores heat, temperatures inside usually stay higher than in the open garden. Ventilation is important on sunny days because the space can overheat quickly.

Best for

Cold frames work best for cool-season crops such as lettuce, spinach, scallions, and hardy brassicas. They are useful in late fall, winter in milder areas, and early spring. They also help harden off seedlings before transplanting.

How to use it

Place the frame in a sunny location, ideally with the clear top facing south in the Northern Hemisphere. Set it over well-drained soil and plant directly in the ground when possible. Open the lid or prop it up on warm days to prevent overheating and to reduce excess humidity. Close it again before evening when cold temperatures are expected.

Tips

Check temperatures inside more often during bright weather than during cloudy weather. Water lightly and only when needed, since enclosed soil dries more slowly. A tight fit at the base helps reduce drafts and improve frost protection.

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General guidance for US gardens. Pair with your local frost dates and hardiness zone. Page updated Aug 4, 2026.