DIY Cold Frame: How to Build and Use One (Materials + Venting Guide)

Search interest in cold frames climbs every September, and for good reason. Your first frost is coming, but your lettuce and spinach aren't done yet.

A cold frame buys you weeks on both ends of the season. It costs almost nothing to build, and most of the parts are already sitting in your garage or a neighbor's trash pile.

What a cold frame actually does

A cold frame is a box with no bottom and a clear top. You set it over a bed or a row of pots, and the sun does the rest.

Sunlight passes through the clear lid and warms the soil and air inside. That heat gets trapped, the same way a car heats up with the windows closed. On a cold, sunny day, the inside of the frame can run well above the outdoor air temperature.

It's not a heated greenhouse. There's no power, no thermostat, nothing plugged in. It's a passive box that makes the most of whatever sun shows up, which is exactly why venting it correctly matters so much (more on that below).

Materials and options

You don't need a kit. Most cold frames are built from whatever's on hand, and the trade-off is usually cost against how long the thing lasts.

The lumber-and-plastic route is the most flexible since you size the box to your bed. The hay-bale version is the cheapest and adds insulation on all four sides, but the bales break down by spring and need replacing.

A simple build, step by step

1. Pick the spot. A south-facing wall, fence line, or open bed with no shade from November through February.
2. Build the box. Four sides, no bottom, sized to your window or plastic sheet. Make the back wall taller than the front, usually by 6 to 12 inches over a 3-foot depth, so the lid slopes toward the sun.
3. Attach the lid with hinges on the tall back edge, so it props open from the front.
4. Seal the gaps around the base with soil or mulch banked against the sides, so cold air can't sneak in underneath.
5. Add a prop stick or an automatic vent opener to hold the lid open partway on warm days.

Which way to face it, and why

Face the frame south. That's the whole rule, and it's worth doing right, since it's the difference between a cold frame that actually works and one that just sits there cold.

A south-facing lid catches the most sun over the course of a day, including the low winter sun angle that skims past everything facing east or west. The frame works as a small passive solar collector: light comes in through the clear top, warms the soil and air, and that warmth stays trapped longer than it would in the open.

A taller north-facing back wall adds a second benefit. It blocks cold wind from the north while the sloped, south-facing lid keeps soaking up sun. Between the two, a well-oriented cold frame runs noticeably warmer than an identical box facing any other direction.

Venting temperatures (don't skip this)

This is the part people miss, and it's the fastest way to cook a bed of lettuce on an otherwise mild day.

Because a cold frame traps heat so effectively, it can overheat fast even when it's cold outside. Per Iowa State University Extension and University of Missouri Extension guidance, vent the frame once the inside temperature climbs to about 70 to 75°F, and definitely by the time it passes 85°F.

Keep a simple thermometer inside the frame so you're not guessing. On a sunny day, even when it's 30°F outside, the inside of a closed frame can climb into the 80s or 90s within an hour or two.

Close the lid again in the late afternoon, before the outside temperature starts to drop. That traps the accumulated daytime heat and carries the plants through the colder overnight hours. University of Missouri Extension specifically recommends closing it promptly in late afternoon or early evening rather than waiting until dark.

If you're tired of checking the thermometer every hour, an automatic vent opener (a wax-cylinder piston that expands with heat) will lift the lid on its own once it warms up and close it again as it cools. They run $20 to $40 and need no power.

What to grow in a cold frame by month

A cold frame is built for cool-season crops, not heat lovers. Think of it as a way to push spinach and lettuce later into fall, or start them earlier in spring, plus a spot to toughen up transplants before they go into open ground.

For exact in-ground spacing once your seedlings graduate out of the frame, the plant spacing calculator will size the bed for whatever you're transplanting. If you're timing a fall spinach or lettuce planting to fill the frame, when to plant spinach and when to plant lettuce cover the sowing windows for each.

Cold frame vs frost cloth vs a full greenhouse

A cold frame sits in the middle of the season-extension toolbox. Frost cloth is cheaper and faster to throw over a bed, but it does nothing on its own without an air gap and offers no real heat trapping. A greenhouse extends the season further and can support warm-season crops, but it costs far more and usually needs some form of heating or ventilation control.

For the specific job of carrying cool-season crops through the shoulder seasons, a cold frame is the cheapest structure that reliably works, and it's simple enough to build in an afternoon.

If you want the exact numbers for how far your cold frame's crops need to be spaced once they're out in the garden, run your bed size through the plant spacing calculator and get the count for your exact layout.