How-to Guide
Using a Cold Frame to Extend the Season
The short answer
A cold frame is a low, bottomless box with a clear lid that works like a miniature greenhouse: the sun heats the ground and frame by day, and that heat radiates at night, keeping the inside a few degrees warmer than the outdoor air. Cold frames typically extend the season by two to four weeks in both spring and fall, letting you start cool-season crops like lettuce, spinach, and radishes earlier and harvest later.
Place the frame where it gets direct sun at least half the day, ideally facing due south. The biggest risk is overheating on sunny days, so vent by propping the lid open, and close it again as temperatures drop.
Siting and building
Choose a south-facing spot with sun at least half the day. Keep the frame no more than about three feet front-to-back so you can reach the back without stepping in; a sloped lid (lower in front, higher in back) sheds water and catches more sun.
Venting and temperature
Expect the interior to run roughly five to ten degrees warmer than outside. On sunny days heat builds fast.
- 1Open on warm days — Prop the hinged lid with a stick, or offset/remove the sash to let cool air in and prevent overheating.
- 2Close as it cools — Lower the lid in late afternoon to trap the day's heat for the cold night ahead.
⚠ A closed cold frame in full sun can overheat and cook plants within hours; vent on mild and sunny days.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Iowa State University Extension — All About Cold Frames — Cold frame construction, siting, venting, and 2-4 week season extension.
- University of Missouri Extension — Building and Using Hotbeds and Cold Frames (G6965) — Cold frame design, placement, and temperature management.
- University of Minnesota Extension — Extending the growing season — Cold frames extend the season for cool-season crops in spring and fall.