Building a Cold Frame from Reclaimed Timber

Why a Cold Frame Earns Its Place

A cold frame is the quiet workhorse of a productive plot. It is not a greenhouse, and it does not pretend to be. What it does is trap a little warmth and a lot of light, buffer the worst of the wind, and give you a sheltered middle ground between the open garden and the kitchen windowsill. In a British climate, that middle ground is where a great deal of good growing happens.

Used well, a frame lets you harden off young plants gradually, so leggy indoor-raised seedlings toughen up before they face the weather. It also keeps winter salads, herbs and hardy leaves cropping when the beds outside have gone quiet. Make one from reclaimed timber and you get something with character, a smaller carbon footprint and a price tag close to nothing.

Sourcing and Preparing Reclaimed Boards

Old boards turn up in all sorts of places: dismantled pallets, skip-bound fencing, shelving from a shed clear-out, the back of a garage. Look for timber that is sound, straight-ish and free of rot, woodworm and flaking paint. A thickness of around 20–25mm is ideal. Anything much thinner will bow; anything much thicker becomes unnecessarily heavy.

Once you have your haul, strip out old nails and screws with a claw hammer and a pry bar, then give the boards a scrub with a stiff brush. Run a hand plane or a belt sander over the worst of the surface where you will be handling it. Avoid reclaimed timber that smells strongly of creosote or has been treated with a heavy-duty preservative, particularly if you will be growing edibles directly beneath it. If in doubt, line the inside with a piece of untreated board or simply use that timber for the base and back rather than anywhere near the soil.

Cut everything to length and lay the pieces out dry before you fix anything. This is the stage where you discover that a board you thought was 1.2m is actually 1.17m, and where you adjust your plan accordingly.

Building the Sloping Frame

The classic cold frame leans forward, with a tall back and a low front, so the glazed lid sheds rain and lets low winter sun reach the plants inside. A useful size for a kitchen garden is roughly 1.2m wide, 90cm deep at the base, with a back wall around 45cm high and a front wall around 25cm. That gives a good working area without being so big that it becomes unruly to move.

Build it as four panels and then join them. Cut two side pieces from your best boards, each shaped as a wedge: 45cm at the back edge, sloping down to 25cm at the front. Cut one back board at 1.2m by 45cm and one front board at 1.2m by 25cm. Butt-joint the panels with exterior-grade screws, predrilling first to stop the old timber splitting, and reinforce the corners with small offcuts of batten screwed inside.

Set the whole frame on a level surface, check it for square by measuring the diagonals, and add a base rail along the bottom edge on all four sides. This rail stops the frame sinking into soft ground and gives the lid something to seat against. For extra insulation, line the inner sides with a layer of cardboard or old woollen blanket before you fill the frame with soil or pots.

• Back height: 45cm
• Front height: 25cm
• Width: 1.2m
• Base depth: 90cm
• Fixings: exterior screws, predrilled

Making and Fitting the Glazed Lid

The lid is where the light gets in, so spend your effort here. Build a simple rectangular frame slightly larger than the top opening, with a cross-brace in the middle for rigidity. Glaze it with whatever you have to hand: a sheet of twin-wall polycarbonate, a piece of salvaged greenhouse glass, or even rigid acrylic from an old light fitting.

Fix the glazing with small galvanised clout nails tapped into the frame and sealed with a bead of exterior silicone. Glass gives the best light transmission but breaks; polycarbonate is lighter and tougher but scratches and clouds over time. Whichever you choose, cut it 5mm smaller than the frame all round to allow for expansion, and bed it on a thin strip of foam or old inner tube to stop rattling.

Attach the lid with two sturdy hinges at the back, positioned about a third of the way in from each end. Add a small latch or hook-and-eye at the front to hold it closed in wind, and fit a short length of chain or a notched prop stick so you can hold it open at various heights. Being able to vent the frame quickly on a bright March morning is the difference between thriving seedlings and cooked ones.

Using It Through the Year

In late winter and early spring, the frame becomes a hardening-off station. Move young plants out from the windowsill on a mild day, leave the lid propped open, and bring them in again at night for the first week. Gradually extend their time outside and close the lid more each night until they are ready for the open ground.

From autumn onwards, sow winter salads directly into the frame: land cress, mizuna, winter purslane, claytonia, and a hardy cut-and-come-again lettuce mix. A few rocket plants will keep you in peppery leaves right through to March. Keep the soil moist but not sodden, and open the lid on mild, dry days for an hour or two to let humid air escape, which helps prevent damping off and botrytis.

A cold frame will not save you from a hard, prolonged freeze, but the difference it makes on a frosty night is genuine. A little extra protection, a little more light, and you stretch your growing season at both ends. Once you have one in the garden, you will wonder how you ever did without it.