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Why a cold frame earns its keep

A cold frame is one of the simplest structures in the garden, and for anyone raising their own seedlings it quickly becomes the hardest working. Strip away the jargon and it is just a bottomless box with a glazed, sloping lid, sitting on soil, gravel or paving. What it does is hold a pocket of warmer, stiller air around young plants, lifting daytime temperatures by several degrees and taking the sting out of a late frost.

That small difference matters enormously in a British spring. Tender seedlings raised on a windowsill or in a heated greenhouse need toughening up before they face the open garden, and a cold frame is the ideal halfway house. It is also perfect for sowing hardy salads in February, overwintering cuttings, bringing on sweet peas, and ripening the last of the tomatoes in September. Build it well once and it will serve you for a decade or more.

Choosing the spot and the size

Siting matters more than construction. Find a level, free-draining spot that faces south or south-west and catches the sun from mid-morning onwards. A position against a south-facing wall or fence is ideal, as the masonry or timber radiates warmth back into the frame overnight. Avoid the bottom of a slope where cold air pools, and keep clear of overhanging branches that will drip onto the lid.

A frame roughly 1.2m wide and 60cm deep is a comfortable size: big enough to hold a decent tray of seedlings, small enough to reach across without treading on anything. The lid should slope from about 45cm at the back down to 30cm at the front, which sheds rain, lets in low winter sunlight, and gives you a natural opening to prop.

• Sit the frame on a base of slabs, gravel or a shallow gravel-filled trench to stop slugs and improve drainage.
• If you plan to plant directly into the soil beneath, dig over and enrich the ground before the frame goes down.
• Set it slightly into the ground if you garden on a windy site, so the wind skims over the lid rather than rattling it.

What you will need

Nothing exotic is required, and much of it can be salvaged. Treat any timber with a plant-safe wood preservative and let it dry thoroughly before planting near it.

• Timber: two side panels cut from 25mm exterior plywood, 60cm long, tapering from 45cm at the back to 30cm at the front.
• Front and back boards: 25mm thick timber boards, 1.2m long, cut to 30cm and 45cm respectively.
• Corner posts: four 50mm x 50mm treated battens, 50cm long, to screw the panels onto.
• Lid frame: 50mm x 50mm battens or 75mm x 50mm timber, made to 1.25m x 65cm so it overhangs slightly all round.
• Glazing: 4mm twinwall polycarbonate is far safer and warmer than glass, and shrugs off hailstones. Horticultural glass works too if you prefer it.
• Fixings: galvanised screws, two galvanised butt hinges, and a sturdy prop stick or notched batten for holding the lid open.

Building the frame

Assemble the sides first. Screw the tapered side panels to the corner posts, keeping the posts flush with the front and back edges, then attach the front and back boards to complete the box. Check everything for square by measuring diagonally across opposite corners; the two measurements should match. Sit the box on its prepared base and firm it down so it does not rock.

Next comes the lid. Build a rectangular frame from the battens to the 1.25m x 65cm dimensions, then fix the glazing to the underside or into a rebate, sealing the edges with a bead of timber. Attach the hinges to the back edge and the frame, so the lid lifts towards you and away from the plants.

• Drill pilot holes before screwing near the ends of timber to avoid splitting.
• Leave a 3mm gap at the lid's lower edge for air to escape, which reduces condensation.
• Fit a small block of timber on each side to stop the lid slamming shut in a gust.
• Mark the prop stick with two or three notches so you can hold the lid at different heights.

Running the frame through the year

Ventilation is the skill that separates a thriving frame from a graveyard of leggy, cooked seedlings. On a bright March day a closed frame can easily reach 30°C, so prop the lid whenever the weather feels mild, and close it again by mid-afternoon to bank the warmth for the night. As a rough rule, if you would be comfortable outside without a coat, the frame wants a little air.

Water sparingly and in the morning. The soil in a cold frame dries out more slowly than you expect, and damp, cold compost is a recipe for rot. Push a finger in to the first knuckle before reaching for the can. In a hard frost, throw an old blanket, hessian sack or a layer of straw over the lid for the night, and stand a couple of straw bales against the sides for extra insulation.

For hardening off, move seedlings into the frame about ten days before planting out. Give them a couple of hours on the first day, then lengthen the exposure gradually, keeping them closed on cold or windy days. By the end of the week they will have thickened up nicely and will barely notice the move to open ground.

Small touches that make a difference

Once the frame is up, a few details will save you trouble. A sheet of fine mesh laid over the top keeps cats off the seed trays and stops birds scratching. Copper tape or a gritty band around the base deters slugs. A max-min thermometer inside tells you far more than guesswork about what is happening overnight, and it is worth keeping a small notebook of readings alongside your sowing dates.

Label everything, including the date, because a cold frame fills up faster than you think and memory is unreliable by April. Wipe the glazing clean a few times a season, since a grimy lid can cut light by a third.

There is something deeply satisfying about a well-made cold frame. It costs very little, takes an afternoon to build, and quietly buys you weeks at both ends of the growing year. Start with one, and you will soon be wondering how you ever managed without it.