Building a Cold Frame for Winter: Your Secret Weapon Against the Cold

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Right, I’m going to let you in on something that genuinely changed my entire approach to winter gardening. A cold frame. That’s it. Sounds boring, I know. Not exactly the most glamorous bit of kit on the allotment. But honestly? My cold frame has probably saved me more plants, extended more growing seasons, and produced more early harvests than anything else I own — and that includes the greenhouse that cost me ten times as much. If you haven’t got a cold frame yet, you’re missing a trick, and I’m going to show you exactly how to build one for next to nothing.

I built my first cold frame about four years ago from some old scaffolding planks and a knackered window that was going in a skip. Total cost? About three quid for the hinges. And it’s still going strong. JB reckons it looks like something out of a post-apocalyptic movie, but I don’t care because the lettuce coming out of it in February while his plot is still frozen solid tells its own story. You know what I mean?

What Actually Is a Cold Frame and Why Do You Need One?

For those who aren’t sure, a cold frame is basically a bottomless box with a transparent lid. That’s it. Nothing fancy. It sits on the ground, captures heat from the sun, and creates a microclimate that’s several degrees warmer than the air outside. Think of it as a mini greenhouse, but without the expense, the planning permission worries, or the glass panels that always seem to get smashed by next door’s football.

The beauty of a cold frame is its simplicity. There’s no heating, no electricity, no complicated ventilation systems. Just passive solar energy doing its thing. On a sunny winter’s day, the inside of a cold frame can be 10-15°C warmer than outside. That’s the difference between your seedlings surviving and them turning to mush overnight.

You can use a cold frame for all sorts. Hardening off seedlings in spring before they go into the ground. Overwintering tender plants that wouldn’t survive outside. Growing salad leaves right through winter. Starting seeds earlier than you could outdoors. Protecting autumn sowings from the worst weather. I even use mine for drying onions in summer when the weather’s rubbish, which let’s face it, is most of the time up here.

Materials — What You Need (and What You Can Scrounge)

Here’s where I get proper excited, because building a cold frame is one of those projects where being a bit of a scavenger really pays off. You do NOT need to go to a garden centre and spend fifty quid on a fancy pre-made job. Half the fun is bodging one together from whatever you can lay your hands on.

For the frame itself, you need something solid that won’t rot too quickly. Old scaffold boards are perfect — they’re thick, sturdy, and usually free if you ask nicely on building sites. Pallet wood works too, though it’s thinner so you might need to double it up for insulation. Railway sleepers are brilliant but bloody heavy. Old bricks or breeze blocks work if you want something more permanent. I’ve even seen people use straw bales as the walls, which is actually genius for insulation even if it looks a bit mad.

For the lid, you need something transparent that lets light through. Old windows are the classic choice — check skips, Freecycle, Facebook Marketplace. Double-glazed units are the jackpot because the insulation is miles better than single glass. If you can’t find windows, clear polycarbonate sheeting from any DIY shop works brilliantly and it’s lighter and safer than glass. You can also use thick clear polythene stretched over a wooden frame, though it’ll need replacing every year or two.

Building Your Cold Frame — Step by Step

Right, here’s how I built mine, and I promise if I can do it, anyone can. I am not what you’d call a skilled carpenter. Audrey once described my woodworking as “enthusiastic but alarming,” which I think is fair.

Step 1: Decide on size. The size depends on what window or lid you’ve got. Work backwards from the lid — there’s no point building a frame and then trying to find something to cover it. My main cold frame is about 120cm wide by 80cm deep, which is the size of the old window I scrounged. That gives me enough room for two rows of lettuce or about 20 pots of seedlings.

Step 2: Cut your sides. You want the back wall to be taller than the front wall — this creates an angle that lets rain run off and catches more sun. I made my back wall 45cm high and the front wall 30cm. The two side pieces are cut at an angle to bridge the difference. Use a saw and take your time. Or don’t take your time, like me, and end up with wonky cuts that somehow still work.

Step 3: Assemble the box. Screw the four sides together to make a bottomless box. Use decent screws, not nails — nails work loose over time, especially with frost heave moving things around. I use 75mm decking screws because they grip like mad and don’t rust. Pre-drill your holes if the wood is thick or it’ll split. Ask me how I know.

Step 4: Attach the lid. Fix your window or polycarbonate sheet to the back wall with a couple of sturdy hinges. Make sure it opens easily because you’ll be propping it open on warm days for ventilation. I use an old bit of wooden batten as a prop stick — just wedge it under the lid to hold it open at different heights.

Step 5: Treat the wood. Give the whole thing a coat of exterior wood preserver. Not creosote though — that stuff is toxic to plants. Use something water-based and plant-safe. Ronseal do a fence paint that works brilliantly. This’ll add years to the life of your cold frame.

Positioning and Setup — Getting the Most Heat

Where you put your cold frame matters more than how you build it, honestly. South-facing is the golden rule. You want maximum sun exposure, especially during winter when the sun is low in the sky. Avoid anywhere that’s shaded by trees, buildings, or fences during the middle of the day.

The angle of the lid should ideally face the sun at roughly the same angle as your latitude. For most of us in the UK, that’s around 50-55 degrees. But honestly, don’t get too hung up on exact angles — the sloped back-to-front design I described above works perfectly well and that’s only about a 15-degree angle. Close enough.

Sink your cold frame slightly into the ground — about 5-10cm — to improve insulation and stability. The earth around the edges acts as a natural insulator. Some people dig down a foot and fill the base with gravel for drainage and then a layer of fresh compost to plant into. This is called a sunken cold frame and it’s even warmer than a surface one because the soil temperature underground is more stable than the air temperature.

Using Your Cold Frame Through the Seasons

Autumn (September-November): This is when your cold frame really starts earning its keep. Sow winter lettuce, lamb’s lettuce, spinach, and rocket directly into the cold frame. These will grow slowly through autumn and then sit dormant during the coldest weeks before picking up again in late February. I also use the cold frame to protect late-sown carrots and keep them going a few weeks longer than they’d survive outside.

Winter (December-February): Not much active growing happens now, but your cold frame is protecting everything inside from the worst frosts, wind, and rain. On sunny days — even in January — open the lid a crack during the day for ventilation, then close it before 3pm to trap heat overnight. Water very sparingly. Plants in cold frames over winter need almost no water because there’s very little evaporation happening.

Spring (March-May): This is prime time. Start sowing seeds in the cold frame 4-6 weeks earlier than you’d sow outside. Tomatoes, peppers, courgettes, cucumbers — all can be started in a cold frame and transplanted out when the weather settles. This is also when you use it for hardening off — gradually getting indoor-raised seedlings used to outdoor temperatures before planting them out.

Summer (June-August): Prop the lid fully open or remove it entirely. Use the frame for growing melons or cucumbers if you want the extra warmth. Or use it as a cloche for ripening peppers and aubergines that need a bit more heat than our summers usually provide. I grew some belting padron peppers in mine last August.

Advanced Cold Frame Tricks

Once you’ve got the basics sorted, there are some clever ways to squeeze even more performance out of your cold frame. One thing Ronny does is line the inside walls with aluminium foil — the reflective surface bounces light back onto the plants from all angles, which is especially useful during short winter days when every photon counts.

Another trick is thermal mass. Put a few dark-coloured water bottles or even a couple of bricks inside the cold frame. They absorb heat during the day and release it slowly overnight, keeping the temperature more stable. On really cold nights, you can drape an old blanket or piece of carpet over the lid for extra insulation. Just remember to take it off in the morning or your plants won’t get any light.

If you want to get really fancy, you can create a hotbed inside your cold frame by filling the base with fresh horse manure topped with a layer of compost. As the manure decomposes, it generates heat — sometimes up to 20°C worth of free heating. This is properly old-school gardening and it works brilliantly. Just make sure the manure is fresh, not well-rotted, because it’s the decomposition process that generates the heat. The smell is… distinctive, shall we say. JB made a hotbed last year and Audrey refused to go near his plot for a fortnight.

My Cold Frame Mistakes (So You Don’t Have to Make Them)

I’ve made pretty much every cold frame mistake going, so let us save you the trouble. First one — I didn’t make the lid openable enough. My first attempt had the lid screwed on permanently because I was too lazy to buy hinges. Which meant every time I needed to water or harvest, I had to lift the entire lid off and prop it against the shed. Lasted about two weeks before I gave in and bought proper hinges.

Second mistake — no drainage. I built the frame on flat, compacted soil and after the first heavy rain, everything inside was sitting in two inches of water. Now I always put a layer of gravel or broken crocks in the bottom before adding compost. And I make sure the site has a slight slope so water runs away from the frame rather than pooling inside it.

Third mistake — forgetting to ventilate. On a sunny March day, the inside of a closed cold frame can hit 30°C or more. I killed a batch of lettuce seedlings by leaving the lid closed on a warm day when I was at work. Now I use an automatic vent opener — it’s a simple wax-filled cylinder that expands when it gets warm and pushes the lid open. Cost about fifteen quid and worth every penny for the peace of mind alone.

Look after yourselves. Take good care. 🌱