Why cedar is the right timber for a cold frame

A cold frame does unglamorous work. It sits low in the garden, takes the worst of the weather on its lid, and spends most of its life holding trays of seedlings that would otherwise sulk. That is precisely why the timber matters. Western red cedar is naturally resistant to rot, light enough to shift around the plot single-handed, and stable enough that it will not twist itself out of shape after a wet British winter. It also ages beautifully — that warm honey colour fades over two or three seasons to a soft silver-grey that settles comfortably among the borders. You can leave it bare, or give it a coat of plant-safe oil if you prefer to keep the depth of the grain.

Try to avoid pressure-treated softwood on the inside faces. Cedar costs more, but it lasts for years, and there is no risk of anything leaching into the compost near tender roots.

Deciding the size and the slope

The classic cold frame is a bottomless box with a sloping lid. The slope sheds rain, but its real job is to let low winter light in at a useful angle rather than bouncing it off a flat pane.

Work to a footprint of roughly 1.2 metres wide by 60 centimetres deep. That is small enough to reach right across from the front without kneeling on anything, and generous enough for two standard seed trays side by side. Make the back board about 45 centimetres high and the front about 30 centimetres. That gives the lid a pitch of roughly 15 degrees — ample for drainage, gentle enough that lifting it stays easy.

If you garden on heavy clay, stand the frame on a shallow bed of coarse grit or a couple of spare slabs. Keeping the timber clear of permanently wet ground will add years to its life.

The cutting list

Use 20mm cedar boards for the sides and 38mm x 38mm cedar for the corner posts. Cedar is often sold as cladding or decking stock, so check the nominal sizes before you cut.

• Four corner posts: 38mm x 38mm x 450mm
• Back rail: 20mm x 450mm x 1200mm
• Front rail: 20mm x 300mm x 1200mm
• Two side rails: 20mm x 600mm, tapered on the top edge from 450mm deep at the back to 300mm at the front
• Lid frame: four pieces of 38mm x 38mm — two at 1300mm, two at 700mm
• Glazing: 12mm twinwall polycarbonate cut to 1300mm x 700mm, or 4mm horticultural glass if you have a glazier nearby
• Brass or stainless steel screws, 40mm, plus four 25mm for the glazing beads
• Two galvanised strap hinges and a lid stay or short prop

Always pre-drill cedar — it is soft enough to split if you drive a screw straight in. A 3mm pilot hole takes seconds and saves a board.

Assembling the frame

Lay the four posts out and build the back panel first, screwing the 450mm rail between two posts with the top edges flush. Repeat for the front with the 300mm rail. Now join front and back with the tapered side rails, checking for square by measuring the diagonals — they should match to within a few millimetres. If they do not, rack the frame gently before driving the final screws.

Set the whole box on its prepared base and check it sits level. A frame that rocks will let in slugs and draughts alike. Pack the corners with grit or a sliver of slate until it is solid.

Fitting the lid, hinges and ventilation

Build the lid frame as a simple rectangle, then glaze it. Twinwall polycarbonate is the sensible choice for a British garden: it is light, will not shatter if a branch comes down, and insulates noticeably better than a single pane. Fit it with the flutes running down the slope so condensation can drain. Leave a couple of millimetres of movement all round, as polycarbonate expands in warm weather, and hold it in place with cedar beads rather than silicone.

Hinge the lid to the back rail so it opens away from the prevailing wind, and fit a stay that lets you hold it open at two or three heights. This matters more than anything else on the build. On a bright February afternoon a closed frame can hit 30°C inside and cook everything in it.

Using and looking after your cold frame

A cold frame buys you roughly two to four degrees of frost protection, which is often the difference between a surviving seedling and a blackened one. It is not a greenhouse, so use it accordingly.

• Harden off properly. Start two weeks before planting out, opening the lid a little further each day and closing it at night.
• Ventilate every mild day. Damping-off fungus thrives in still, humid air. Open the lid in the morning and shut it mid-afternoon.
• Water sparingly. Rain cannot get in, so the compost dries far more slowly than you expect.
• Watch for slugs. Check under the bottom rim and along the inside of the posts after rain; a torchlit evening patrol works wonders.
• Empty it in summer. Lift the lid off entirely and store it flat. Use the base for lettuce, cuttings or a late sowing of winter salad.
• Clean it each autumn. Warm soapy water on the glazing restores light transmission, and a light sanding with a wipe of oil keeps the cedar in good heart.

Treat it well and a cedar cold frame will outlast most of the tools you used to build it — quietly earning its keep every spring.