How to Build a Simple Cold Frame and Extend Your Growing Season
- A cold frame extends your growing season by 4–8 weeks on each end
- Build one for under $30 with lumber, an old window, and hinges
- Slope the lid south — back wall 4–6 inches taller than front
- Prop open on sunny days to prevent overheating (above 90°F)
- Grow salad greens well into winter and start spring crops early
A cold frame is essentially a miniature greenhouse — a bottomless box with a transparent lid that traps solar heat and protects plants from frost, wind, and cold rain. It is one of the simplest and most effective tools for extending your growing season by weeks or even months on both ends. You can build one in an afternoon with basic materials.
I built my first cold frame from an old storm window and some scrap lumber. Total cost: about $12. That little box let me harvest lettuce and spinach straight through December, and it is still going strong years later. Easily one of the best returns on investment in our garden.
What Can You Do with a Cold Frame?
A cold frame lets you start crops 4–6 weeks early, harden off seedlings, grow greens into winter, and overwinter tender perennials. Here are the main uses:
- Start cool-season crops 4 to 6 weeks earlier than normal in spring
- Harden off seedlings in a protected environment
- Grow salad greens and herbs well into winter
- Overwinter tender perennials and fall-planted garlic
- Root cuttings and start new plants from divisions
How Do You Build a Cold Frame Step by Step?
Screw four boards together to form a sloped box, hinge an old window on top, and set it in a south-facing sunny spot — done for under $30.
Materials: Four boards (2×10 or 2×12 lumber), an old window or a sheet of polycarbonate, hinges, and screws. Straw bales can also work as the frame walls in a pinch.
Size: Match the size to your window or glazing material. A common size is 3 feet by 6 feet, but even a 2 by 4 foot frame is useful.
Key detail: The back wall should be 4 to 6 inches taller than the front wall. This creates a slope that sheds rain and angles the glass toward the sun for maximum heat gain. Face the cold frame south for the best solar exposure. The University of Maryland Extension recommends a 30–45 degree lid angle for optimal heat capture.
Screw the boards together to form a box, attach the window to the back edge with hinges so it opens like a lid, and set the whole thing on level ground in a sunny spot. That is genuinely it. You have just built a season extender for under 30 dollars.
| Cold Frame Use | When | Best Crops | Temp Range |
|---|---|---|---|
| Spring start | Feb–Mar | Lettuce, spinach, radishes | 35–75°F |
| Hardening off | Apr–May | All transplants | 45–80°F |
| Fall extension | Oct–Dec | Kale, chard, carrots | 30–65°F |
| Overwintering | Nov–Feb | Garlic, perennial herbs | 25–55°F |
How Do You Manage Cold Frame Temperature?
The biggest mistake is overheating — prop the lid open on sunny days because interior temps can exceed 90°F even in winter. On a sunny day, the interior can climb dangerously high and cook your plants. Prop the lid open a few inches on sunny days and close it before evening. Some gardeners use automatic vent openers that use wax cylinders to open and close the lid based on temperature. They cost about 30 dollars and are worth every penny.
A cold frame pairs perfectly with raised beds and is a natural complement to your composting setup. Use it alongside a container garden to maximize your growing options. You will be eating fresh salad in December while your neighbors wonder how you pulled it off.
Frequently Asked Questions
A: Generally no — it will be too hot. Some gardeners remove the glass and use shade cloth in summer to create a cool, shaded nursery for starting fall crops.
A: A basic cold frame adds about 10–15°F of frost protection. Below about 15°F outside, even cold-hardy crops may need extra insulation like blankets or straw bales around the frame.
A: Yes, and many gardeners prefer it. Polycarbonate is lighter, shatterproof, and provides better insulation than single-pane glass. Twin-wall polycarbonate is ideal.
A: Yes. The lid blocks rain, so you will need to water manually. Check soil moisture every few days, especially during sunny periods when evaporation increases.
