Building a Cold Frame for Autumn: Extend Your Growing Season with Ease

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A cold frame. Now this is one of those things that sounds fancy and complicated but is actually one of the simplest and most useful structures you can build for your garden. It’s basically a box with a transparent lid. That’s it. And yet this incredibly simple contraption can extend your growing season by weeks at both ends, protect tender plants from frost, harden off seedlings safely, and keep winter salads going when everything else has given up. For what it costs — which can be basically nothing if you use reclaimed materials — it’s probably the best investment in your garden after decent soil and a sharp hoe.

I built my first cold frame about five years ago out of some old timber from a skip and a window someone was chucking out. Took about two hours and cost me the grand total of zero pounds. Zero. And it’s still going strong. Bit tatty around the edges, but functional. Which is basically my approach to most things on the allotment. Functional beats pretty every time in my world.

JB took one look at it and said “That looks like it was built by a drunk carpenter.” Which is unfair because I was completely sober. It’s just that my DIY skills are… developing. Let’s say developing. The important thing is it works. My winter lettuce doesn’t care that the corners aren’t perfectly square. It cares about being warm and dry. And it is. So there.

What You’ll Need

The beauty of a cold frame is that you can build one from almost anything. Here’s what you need:

For the base/walls — timber is the classic choice. Old scaffolding boards, pallet wood, fence panels, anything that’s reasonably thick (at least 20mm) and not completely rotten. The walls need to be about 30-40cm high at the back and 20-25cm at the front, creating that all-important slope. You could also use old bricks, breeze blocks, straw bales, or even those deep plastic storage boxes with the bottom cut out. I’ve seen someone use an old filing cabinet drawer. That was creative.

For the lid — an old glass window is ideal. Double glazed is even better because it retains more heat. Check local skips (always ask first, mind), Facebook marketplace, or Freecycle. People are constantly replacing windows and they’re often free if you collect. If you can’t find glass, a sheet of clear polycarbonate works brilliantly and it’s safer because it won’t shatter if something falls on it. Or heavy-duty clear plastic sheeting as a budget option — not as durable but gets the job done.

You’ll also need some hinges to attach the lid to the back wall so you can prop it open for ventilation. And a stick or prop to hold it open at different heights. That’s literally it. Hinges, a prop, and something to keep the lid from flying off in the wind. I use a hook and eye catch on the front. Dead simple.

Building It — Step by Step

Right, here’s how I built mine. And I promise you, if I can do this, anyone can. I am not a carpenter. I am barely competent with a screwdriver. But this is within the abilities of anyone who can use a saw and drive a screw.

Cut your timber for two side pieces that are taller at the back and shorter at the front — this creates the angled slope. Cut the angle on the top edge so the lid sits flush. Then cut a back piece at the taller height and a front piece at the shorter height. Screw the four sides together to make a box. That’s your frame.

Now attach the lid with hinges along the back edge. Two standard gate hinges work fine. Make sure the lid overhangs the frame slightly on all sides so rain drips off rather than into the frame. Add your hook catch on the front to keep it closed in windy weather. Cut a piece of wood about 30cm long to use as a prop for ventilation.

Place the whole thing in position — on level ground, facing south, ideally against a wall or fence that provides shelter from the north wind. If you want to get fancy, you can line the inside of the back wall with reflective material (old foil survival blankets work brilliantly) to bounce extra light onto the plants. But that’s optional. The basic frame will work perfectly well without it.

Total build time: about two hours if you’re methodical, three hours if you’re me and you have to redo one of the cuts because you measured wrong. Twice.

Using Your Cold Frame in Autumn

Right, so you’ve built it. Now what do you do with it? In autumn, your cold frame becomes a salad factory. Sow winter lettuce varieties, lamb’s lettuce, winter purslane, and mizuna directly into the soil inside the frame (or in pots placed inside it). These crops will grow steadily through autumn and, with the protection of the cold frame, keep producing well into winter.

You can also use it to overwinter tender herbs like parsley and chives. They’ll die back a bit but come through the winter much better than they would unprotected. And if you’ve got any late-season seedlings that need a bit more time — late-sown spinach, maybe some pak choi — the cold frame gives them that extra warmth to get established before the cold really bites.

The critical thing with a cold frame is ventilation. On sunny autumn days, the temperature inside can rocket up surprisingly fast. You need to prop the lid open on warm days to prevent the plants from cooking. I’ve lost more plants to overheating in the cold frame than I have to cold. Which is ironic and annoying. A thermometer inside the frame helps — if it’s above about 20°C, prop it open. Close it again in the evening to trap the heat overnight.

Winter Use

Through winter, keep the lid closed most of the time. Only open on mild, sunny days for a couple of hours to let fresh air circulate and prevent mould. Water very sparingly — plants hardly grow in winter so they don’t need much, and overwatering in cold conditions leads to rot. I water my cold frame crops about once every two weeks in winter. Sometimes less. Just enough to stop them drying out completely.

In really cold spells — below minus five or so — drape an old blanket or bubble wrap over the lid at night for extra insulation. This can raise the inside temperature by several degrees, which can be the difference between your plants surviving and not. I keep an old sleeping bag in the shed specifically for this purpose. It goes over the cold frame on freezing nights and comes off in the morning. Bit of a faff but it works.

Spring Use — Getting Ahead

Come spring, the cold frame really earns its keep. You can start seeds in there weeks before it’s warm enough outside. Early lettuce, peas, broad beans, brassica seedlings — all of these can be started in the cold frame from February onwards. The soil warms up earlier than open ground because of the trapped heat, and the glass protects from frost and cold wind.

It’s also perfect for hardening off seedlings that you’ve started on the windowsill. Move them from the windowsill to the cold frame for a week or two before planting out. The cold frame is like a halfway house between indoors and outdoors. Gradually prop the lid open more and more each day, and by the time the plants go in the ground, they’re properly toughened up.

Maintenance

Keep the glass clean — dirty glass blocks light, which you need as much of as possible in autumn and winter. A wipe with a damp cloth every couple of weeks does the trick. Check the timber for rot annually — a coat of wood preservative in summer extends the life considerably. Replace any cracked glass promptly because a broken lid is worse than no lid at all (the crack lets cold in while trapping moisture).

And that’s honestly about it for maintenance. A cold frame is about as low-maintenance as any garden structure can be. It just sits there being useful. Mine has survived five winters, two storms, and an incident where Daisy sat on the lid (she’s a big dog and the glass held, remarkably). It’s been one of the best things I’ve ever added to the allotment. Dead simple, dead cheap, dead useful.

Look after yourselves. Take good care. 🌱