Cold Frames
Building and using cold frames
A cold frame is the simplest season-extension structure there is: a bottomless box with a sloped, transparent lid that sits over a bed and traps solar heat. Despite its simplicity it earns its place in a short-season garden twice a year — hardening off seedlings in spring and sheltering cold-hardy greens long after the open garden has frozen.
How a cold frame earns its keep
The lid lets in sunlight, the soil and walls absorb it, and the enclosed air warms several degrees above the surroundings. At night the frame slows heat loss. That modest buffer is often the difference between a salad crop that keeps growing and one that stalls. Because the frame is small and close to the ground, it warms fast and is easy to cover with a blanket on the coldest nights.
Two jobs across the year
- Spring hardening-off. Seedlings raised indoors need a gradual transition to outdoor conditions. A cold frame gives them brighter light and cooler air while still protecting them from wind and late frost.
- Autumn and winter greens. Mache, spinach, claytonia, and many Asian greens tolerate cold well. Inside a frame they can be harvested far later than the same crop grown in the open.
Sizing and materials
Keep a frame narrow enough to reach across from one side — roughly an arm's length front to back. The lid is traditionally an old window sash, but twin-wall polycarbonate is lighter and safer. The back wall should be taller than the front so the lid slopes toward the sun; that slope sheds rain and catches more low-angle light.
Build note: rot-resistant lumber or simple straw bales both make workable walls. Bales add insulation but take up space; lumber is compact and lasts several seasons if kept off constantly wet ground.
Siting the frame
Place the frame where it gets sun for most of the day and where the open side faces toward the midday sun. A spot sheltered from the prevailing wind — against a south-facing wall or fence — gains extra warmth from the structure behind it. Good drainage underneath prevents the bed from staying soggy during thaws.
The one habit that matters most: venting
The most common way to lose plants in a cold frame is not cold — it is heat. On a clear day even in early spring the air inside can climb quickly while the outside still feels chilly. Prop the lid open on sunny mornings and close it again before evening. Gardeners who are away during the day often fit an automatic vent opener, a wax-cylinder device that lifts the lid as the air warms and lowers it as it cools.
Routine: open in the morning, close in the late afternoon, cover with an old blanket on nights with a hard freeze warning. That simple rhythm covers most of what a frame needs.
Further reading
For broader background, see Wikipedia's cold frame article. Continue with the companion guides on greenhouse glazing and the season-extension calendar.