Building a Cold Frame for Winter Greens

By Emma Johnson · October 8, 2026

What a Cold Frame Does

A cold frame is a bottomless box with a transparent lid that traps solar heat, creating a microclimate 10-20°F warmer than outside air. It extends your growing season by 4-6 weeks in fall and spring, and with proper management, produces greens all winter.

Simple Design with Reclaimed Materials

Build a box 3 feet wide by 6 feet long using 2x10 lumber or stacked concrete blocks. Angle the back wall 12-18 inches higher than the front to capture low winter sun. The lid can be an old window, polycarbonate panel, or heavy-gauge clear plastic stretched over a wood frame.

Location and Orientation

Place the cold frame facing south with its back against a wall or fence for wind protection. Full southern exposure is essential — even partial shade dramatically reduces the heat gain. Avoid low spots where cold air pools.

What to Grow Inside

Spinach, mâche (corn salad), claytonia (miner's lettuce), kale, arugula, and mache all survive winter in a cold frame. Plant them in September-October so they reach harvestable size before daylight drops below 10 hours in November. They will hold through winter and resume growth in February.

Ventilation Is Critical

On sunny days, cold frames can reach 90°F even when outside temperature is 30°F. Open the lid when outside temperature is above 40°F and prop it open 4-6 inches. Close it by mid-afternoon to trap heat for the night. Automatic vent openers use wax cylinders that expand with heat — they work without electricity.

Winter Watering

Water sparingly — plants in cold frames use very little water in winter. Check soil moisture weekly by feeling 2 inches deep. Water on sunny mornings so excess moisture evaporates before nightfall. Overwatering causes fungal disease in the cold, humid environment.