What if you could keep harvesting salad greens weeks after the first frost, and start your spring seedlings a month early, without spending anything on a greenhouse?
That’s exactly what a cold frame does—it’s a bottomless box with a clear lid that traps sun and warmth, acting like a mini-greenhouse.
And the classic way to build one costs almost nothing, because the lid is just an old window you’ve probably seen sitting in a garage or a curb pile.
What a cold frame actually buys you
A cold frame typically holds the air inside 5–10°F warmer than outside and blocks drying wind, which in practice shifts your growing calendar in both directions:
| Season | What the cold frame lets you do |
|---|---|
| Late fall / winter | Keep harvesting hardy greens—spinach, kale, lettuce, mâche |
| Early spring | Start seedlings 4–6 weeks before the last frost date |
| Spring transition | “Harden off” indoor seedlings before planting them out |
| Anytime | Protect tender potted plants from a surprise cold snap |
What you’ll need
- One old window or glass door (the lid). Wood-framed windows are ideal—they’re easy to hinge and seal.
- Four boards for the walls. Untreated lumber, old pallet planks, or scrap plywood all work.
- Two hinges to attach the window to the back wall.
- Screws, a drill, and a saw.
- Optional: a stick or a hook to prop the lid open for ventilation.
Build it in one afternoon
- Size the box to your window. The window is the lid, so build the frame’s footprint to match it. A common size is roughly 3 ft by 2 ft.
- Slope the box back to front. This is the one detail that matters most. Make the back wall taller than the front—say 15 inches at the back, 10 at the front. The slope should face south (in the Northern Hemisphere) so it catches low winter sun and sheds rain and snow.
- Assemble the four walls into a bottomless box and screw the corners together.
- Hinge the window to the tall back wall so it lifts open from the front like a lid.
- Set it on prepared soil or over a raised bed, in the sunniest spot you have. No bottom is needed—the plants root into the ground below.
The skill that keeps plants alive: venting
Here’s what surprises most first-timers—the biggest danger to cold-frame plants isn’t cold, it’s overheating. On a sunny day, even in winter, a closed frame can shoot past 85°F and cook your greens.
The rule is simple: prop the lid open a few inches whenever the daytime temperature climbs above roughly 40°F (5°C), and close it again before evening to trap the day’s warmth for the cold night.
A cheap stick with a few notches cut into it gives you three easy vent heights.
Take the guesswork out with a thermometer
Venting by feel works, but a cheap min/max thermometer left inside the frame turns it into a precise tool.
It records the day’s high and low so you can see exactly how hot the frame ran while you were out, and adjust your venting routine to match. Use these rough targets as your guide:
| Inside temperature | What to do |
|---|---|
| Above 75°F (24°C) | Open the lid fully—it’s overheating |
| 45–75°F (7–24°C) | Vent partway; comfortable growing range |
| Below 40°F (5°C) | Keep it closed to trap warmth |
Pushing into the coldest months
To stretch the season even further, you can add cheap thermal mass and insulation.
Jugs or dark bottles of water placed inside soak up heat during the day and release it slowly at night, buffering the frame against a hard freeze.
Lining the inside walls with straw or bubble wrap adds insulation, and throwing an old blanket over the glass on the coldest nights (removed each morning for light) can buy several extra degrees.
None of it costs much, and together it’s the difference between greens that survive a cold snap and ones that don’t.
Mistakes to avoid
- Forgetting to vent. The single most common way to lose cold-frame plants is baking them on a bright day. When in doubt, crack the lid.
- Facing it the wrong way. A north-facing slope gets far less winter sun. Orient the low side toward the sun.
- Using a lead-painted old window near edibles. Very old windows may have lead paint on the frame—if in doubt, seal it or use it only over ornamentals.
- Letting snow pile on the glass. Brush it off so light gets in and the weight doesn’t crack the pane.
What to grow in it
Cold frames shine with low, cold-hardy crops: leaf lettuce, spinach, kale, arugula, mâche, radishes, and hardy herbs like parsley.
In early spring, use it to start brassicas, onions, and other cool-season seedlings before your beds are ready. Save the tall, heat-loving crops like tomatoes for the open garden—they’ll outgrow the frame fast.
Turn your cold frame into a hotbed
Here’s an old-school trick that pushes a cold frame through even hard winters: turn it into a hotbed.
Dig out about a foot of soil under the frame and pack in a 6–8 inch layer of fresh horse or cow manure mixed with straw, then replace 6 inches of soil on top.
As the manure decomposes, it gives off gentle, steady heat from below—enough to keep the frame frost-free and growing when the air outside is freezing.
One caution: fresh manure heats up sharply at first, sometimes past 100°F. Wait a few days for that first spike to settle before you sow or transplant, or the heat will cook your seedlings. Once it levels off, you’ve got a heated growing bed that costs nothing to run.
For a broader primer on how cold frames fit into a season-extension plan, the Old Farmer’s Almanac has a solid free guide.
Does a cold frame need any heat or electricity?
No—a true cold frame is unheated and runs entirely on trapped solar warmth, which is what makes it so cheap to run.
If you want to push into colder weather, you can line the inside with straw bales or add jugs of water that store daytime heat and release it at night, but you don’t need any power.
Where should I put it?
Against a south-facing wall of your house or shed is ideal—the wall blocks cold north wind and radiates a little stored heat back at night.
Otherwise, any spot with full winter sun and shelter from the harshest wind will do.
Olivia Bloom has gardened for over a decade — from a cramped balcony to a full backyard — and writes only what she has tested in real soil. Her hands-on guides on plant care, DIY projects, composting, and sustainable gardening prove a thriving garden takes the right steps, not a big budget or a green thumb you were born with.