How to build a cold frame that actually survives the winter

You’re standing in your garden in late October, staring at the last of your kale and feeling that familiar sense of impending doom. The first hard frost is coming. You could let the weather take your greens, or you could finally figure out how to build a cold frame that doesn't fall apart when the first gust of wind hits it. Honestly, most people overthink this. They treat it like they’re building a miniature Taj Mahal for their spinach, when in reality, a cold frame is just a bottomless box with a clear lid. It's a low-tech solar heater. That’s it.

If you do it right, you're not just "extending the season." You're basically cheating at nature. You can have fresh salad in January while your neighbors are eating mealy, plastic-wrapped lettuce from the grocery store.

Why most DIY cold frames fail miserably

I’ve seen people spend a weekend building these beautiful cedar structures only to have their plants cook by noon on a sunny Tuesday. Or worse, they use thin plastic film that sags under the weight of three inches of snow, crushing their delicate seedlings. The biggest mistake is poor ventilation. You think you're protecting them from the cold, but the sun is incredibly powerful. Even if it’s 30°F outside, the temperature inside a sealed box can easily spike to 90°F. You’ve basically built a vegetable sauna.

📖 Related: El tiempo en Goodyear: Why Your Weather App Usually Lies to You

You also need to think about the "thermal mass." If you just have a wooden box on top of dirt, it loses heat fast once the sun goes down. To make this work, you want to sink the frame a few inches into the ground or line the inside with dark-colored bricks. These soak up the sun's energy during the day and radiate it back out at night. It's the difference between a cold frame that just stays "not freezing" and one that actually keeps the soil warm enough for growth.

Choosing your materials without breaking the bank

You don't need fancy kits. Look around your garage or a local salvage yard.

The Lid (The "Light")
Old windows are the classic choice. They’re heavy, which is good for wind resistance, and the glass provides excellent clarity. However, glass breaks. If you have kids or clumsy dogs, maybe skip the vintage sashes. Polycarbonate—the corrugated stuff used for greenhouses—is a fantastic alternative. It’s lightweight, nearly indestructible, and has better insulating properties than a single pane of glass. Some people use "twin-wall" polycarbonate, which has a layer of air trapped in the middle. It’s more expensive but worth every penny if you live somewhere like Minnesota or Maine.

The Walls (The "Body")
Cedar is the gold standard because it resists rot without needing toxic chemicals. But it’s pricey. You can use common pine, just know it’ll probably rot out in three or four years. Whatever you do, avoid pressure-treated lumber from before 2004; it contains arsenic, which you definitely don't want leaching into your organic carrots. Modern ACQ-treated wood is safer, but many gardeners still prefer to line it with plastic or stick to untreated wood. Straw bales are another wild-card option. They are incredible insulators. You just plop an old window on top of four straw bales, and you’ve got a temporary cold frame that you can compost in the spring.

The actual step-by-step build process

First, find a spot. It has to face south. If you point it north, you’re just building a refrigerator. You want that low winter sun hitting the glass at as close to a 90-degree angle as possible. This means your box shouldn't be flat. The back should be higher than the front.

1. Cut your sides. Let’s say your old window is 3 feet by 4 feet. You’ll want your back board to be about 18 inches high and your front board to be about 12 inches high. This creates a 6-inch slope.
2. Assemble the box. Use 2x2 stakes in the corners to screw your boards into. This makes the structure much more rigid. Use exterior-grade screws. Rust is your enemy here.
3. The floor situation. You can build this directly over a garden bed. Or, if you want something more permanent, dig out about 6 inches of soil inside the frame and replace it with a layer of gravel for drainage, then top it with high-quality compost.
4. Attach the lid. Use heavy-duty galvanized hinges. You want to be able to prop the lid open at different heights. This is where people get lazy, and it’s where they regret it.

Managing the heat (The "Vent")

You need a way to prop the lid open. A simple notched stick works, but if you’re forgetful, buy an automatic solar vent opener. These are cool gadgets that use a wax-filled cylinder. When it gets hot, the wax expands and pushes a piston that opens the lid. No electricity required. It’s like magic for gardeners who have day jobs and can’t run home at 11:00 AM to vent the lettuce.

According to Niki Jabbour, author of The Year-Round Vegetable Gardener, the goal is to keep the internal temperature between 45°F and 65°F for most cool-weather crops. If it hits 70°F, open that lid.

What to plant when the world is grey

Don't bother with tomatoes. It’s called a "cold" frame for a reason. You want crops that laugh at a light frost.

• Spinach: This is the king of the cold frame. It can actually freeze solid, thaw out, and still taste amazing.
• Mache (Corn Salad): Very underrated. It grows in a little rosette and tastes slightly nutty.
• Claytonia (Miner’s Lettuce): This stuff is bulletproof.
• Carrots: Plant them in late summer, and the cold frame will keep the ground from freezing, allowing you to harvest "candy carrots" all through December. The cold turns the starches into sugars.

Maintenance and survival tips

Snow is actually your friend. It’s a great insulator. If a blizzard hits, don't panic and rush out to scrape the glass unless the weight looks like it's going to break the frame. The snow will keep the plants at a steady temperature. However, once the sun comes out, you have to clear it so the light can get in.

Check for pests. Slugs love cold frames. It's damp, protected, and full of their favorite snacks. A little bit of copper tape around the top edge of the wood or some organic slug bait can save you a lot of heartbreak.

Building for the long haul

If you’re serious about how to build a cold frame that lasts a decade, invest in the hardware. Use stainless steel hinges. Apply a coat of linseed oil to the wood every two years. If you’re using an old window, check the glazing putty. If it’s cracking, scrape it out and put in fresh caulk.

Actionable Next Steps

1. Measure your space: Go outside right now and find a south-facing spot that gets at least 6 hours of sun.
2. Scout for a lid: Check Facebook Marketplace or a local construction reuse store for an old glass window or a scrap of polycarbonate.
3. Source your wood: Buy two 8-foot lengths of 2x12 cedar or untreated pine.
4. Assemble and sink: Build the box this weekend, then dig it 2 inches into the soil to seal the bottom against cold drafts.
5. Sow seeds immediately: Get some spinach and arugula seeds in the ground. Even if it feels too cold, the microclimate you just created will surprise you.

Success with a cold frame isn't about perfection. It’s about creating a little bubble of spring in the middle of a harsh winter. Start simple, keep it vented, and enjoy the taste of a homegrown salad while the rest of the world is stuck with wilted supermarket greens.