Building a Cold Frame from Reclaimed Wood

Why a Cold Frame Earns Its Place

A cold frame is the simplest piece of growing infrastructure you can own, and for a kitchen gardener it punches far above its weight. It is, at heart, a bottomless box with a glazed lid. That is all. Yet that lid traps enough warmth to shift your season by four to six weeks at either end of the year, which in a British climate is the difference between a decent crop of lettuce and a disappointing one.

Its uses fall into three groups. First, it brings on early sowings: hardy salads, radish, spring onions, early carrots and spinach can go in weeks before they would survive unprotected. Second, it hardens off seedlings raised indoors or in a greenhouse, easing them into wind, cold and fluctuating temperatures before they go into open ground. Third, it overwinters crops that would otherwise sulk or rot — winter lettuce, land cress, corn salad and parsley all do well under glass through the coldest months. If you have a spare sash window and a few lengths of sound timber, you already have most of what you need.

Sourcing Reclaimed Materials Worth Using

The traditional cold frame is built from exactly the things that get thrown away during a house renovation. Old single-glazed window panes are ideal: they are usually thick, often toughened in later examples, and the wooden frames can sometimes be reused whole as your lid. Salvaged greenhouse glass, cold frame lights and even sturdy shower screens will do the job.

For the timber, rot resistance matters more than beauty. Look for:

• Oak, larch, Douglas fir, cedar or teak — all naturally durable and likely to give you a decade or more with minimal fuss.
• Old floorboards or roof timbers — often dense, slow-grown softwood that outlasts modern pressure-treated stock.
• Reclaimed decking boards — useful for the base rails, though usually too thin for structural corners.

Two cautions. If you are sanding old painted timber, assume any pre-1960s paint contains lead and take proper precautions or seal it instead of sanding. And avoid timber that has been soaked in creosote or heavy wood preservative, as the fumes can scorch young plants — line the inside with a sheet of damp-proof membrane if you are unsure. Check every pane for cracks, and reject anything with a hairline running to an edge.

Size, Siting and the All-Important Slope

A cold frame should face south, or as close to it as your plot allows. Sitting it against a south-facing wall or fence gives you shelter and a little radiated warmth, but keep it clear of overhanging branches and out of the rain shadow of a dripping gutter. Ground that stays waterlogged is fatal; choose a spot that drains freely, or dig in a 10cm layer of gravel as a base.

The classic proportions are simple. Make the back about 45cm high and the front about 30cm, giving a slope of roughly 15 to 20 degrees so rain runs off and low winter sun reaches right to the back of the frame. A length of 1.2m and a depth of 90cm is a good working size, but let your glazing dictate the dimensions — it is far easier to build a frame to fit a window than to cut glass to fit a frame. A single salvaged sash of about 90cm by 60cm makes a perfectly respectable lid for a small frame.

The Build, Step by Step

Work on a level, firm surface. Cut four corner posts from 50mm by 50mm stock, then cut the front and back rails and the sloping side rails. Assemble the base box first, checking it for square by measuring the diagonals — they should match. Fix everything with galvanised screws, which will not rust into the timber, and drill pilot holes to stop old, brittle wood splitting.

Once the box is together, sit it on your gravel bed and check it for level. Add a rebate or a simple ledge along the top edge so the glazed lid rests snugly rather than sliding about. Hinge the lid at the back with two galvanised strap hinges, and fit a prop stick or a notched wooden stay so you can hold it open at different heights. A small wooden peg or a pair of screw-in hooks makes a serviceable handle. Finally, run a bead of exterior-grade sealant along the joints, and consider a strip of foam weather seal where the lid meets the frame to stop draughts whistling through.

Ventilation, Watering and Day-to-Day Care

The commonest mistake with a cold frame is closing it and forgetting it. On a bright February day the temperature inside can easily climb above 25°C, which will cook your seedlings while the outside air is still frosty. Check it daily, and open the lid a crack in the morning, wider as the sun strengthens. If you would rather not think about it, a wax-cylinder automatic vent opener will lift the lid as the temperature rises and close it again at dusk.

Water in the morning, aiming at the soil rather than the foliage, and keep the compost moist but never sodden. Damp, still air is what breeds botrytis and damping off, so ventilation is your best defence. Watch for slugs and snails, which treat a cold frame as a five-star hotel, and check the corners for woodlice and vine weevil. In winter, line the inside with bubble wrap for extra insulation; in summer, you can lift the whole frame onto bricks to use it as a shaded seedling shelter.

Keeping It Going Through the Year

Treat the frame as a rotating space rather than a permanent home. Use it from January for early salads, then for hardening off tender crops through April and May. Once the tomatoes and courgettes are planted out, the frame can raise a catch crop of quick lettuce or basil. Come autumn, clear it out, refresh the soil with compost, and sow winter salad leaves.

Before winter, check the glazing putty, replace any cracked pane, and give the timber a coat of linseed oil if it is looking dry. Reclaimed materials last when they are looked after, and a well-built cold frame made from salvaged wood and old glass will serve you for many seasons — quietly, cheaply, and without a single trip to the garden centre.