Why a Cold Frame Earns Its Place in a Small Garden

A cold frame is one of the quiet workhorses of a productive garden. It is essentially a bottomless box with a transparent lid, and it does three jobs beautifully: it shelters seedlings from wind and frost, it lets you sow earlier in spring and harvest later in autumn, and it gives tender plants a gentle halfway house when moving from windowsill to open ground. In a small garden, where every square metre has to pull its weight, that is a lot of value from a piece of kit that costs almost nothing if you build it from salvage.

Old windows are ideal for the lid. They are rigid, they transmit plenty of light, and they are often available free or cheaply when someone replaces their glazing. Pair one with a few lengths of reclaimed timber and you have a cold frame that is sturdier and more characterful than most shop-bought versions. You can build one in an afternoon with a saw, a drill and a tape measure.

What You Will Need

Gather your materials before you start so you are not hunting for screws halfway through. A little planning here saves a lot of fiddling later.

• Old windows: two matching sashes around 60–90cm wide work well. Check for rot in the frame and cracks in the glass. If the panes are broken, replace them with acrylic or polycarbonate sheet cut to size, which is lighter and safer.
• Reclaimed timber: 50x50mm battens for the internal corners, plus 150mm-wide boards for the sloped sides and front. Pallet wood is fine if you plane it and treat it.
• Fixings: galvanised or stainless-steel screws, two or three exterior hinges, and a prop stick or automatic vent opener.
• Finishing: wood preserver, exterior sealant, a bag of gravel and some good compost.

Tools are simple: a hand saw or circular saw, a drill, a screwdriver, sandpaper, a brush and a tape measure.

Preparing the Old Windows

Old glazing deserves a bit of respect. If the paint is flaking, it may contain lead, so do not dry-sand it. Use a chemical stripper or seal it with a suitable exterior paint. Wear a dust mask and gloves, and work outside. Wash the glass with warm soapy water and a scraper to remove years of grime, then re-putty any panes that have worked loose. A bead of clear sealant around the frame will stop draughts and keep rain out.

If the window frame itself is soft or crumbling, cut out the rotten sections and splice in new timber, or simply reject that window and find another. The lid needs to be sound because it will be opened and closed hundreds of times.

Building the Frame, Step by Step

Size the frame slightly smaller than your windows so the lid overhangs the sides and sheds water. A useful size is about 120cm long and 90cm wide, sloping from 45cm high at the back to 30cm at the front. That slope lets in more low winter sun and sheds rain.

• Cut two side rails from 150mm board: front edge 30cm, back edge 45cm, so the top edge slopes neatly.
• Cut front and back rails to 90cm. Screw the sides to the front and back using 50x50mm battens fixed inside the corners.
• Add a base rail of 50x50mm batten around the bottom to sit on soil or paving. This keeps the timber off wet ground and stops it rotting.
• Attach the windows to the back rail with two or three galvanised hinges. Fit a prop stick with notches cut into it, or an automatic vent opener that lifts the lid as the temperature rises.

Brush wood preserver onto every cut end and any bare timber, then leave it to dry. A coat of paint on the outside will extend its life by years.

Siting, Venting and Everyday Use

Put the frame in full sun, facing south, and shelter it from strong winds. If your soil is heavy, stand it on a layer of gravel or raise it on bricks so water drains away freely. Line the bottom with gravel, then a layer of sand, then your compost.

Ventilation is the difference between a thriving cold frame and a graveyard of cooked seedlings. On mild days above about 10°C, prop the lid open a few centimetres. Close it again in mid-afternoon so the frame retains warmth overnight. An automatic vent opener is worth the small cost if you are out at work all day.

Use the frame for sowing hardy salads in late winter, for overwintering lettuce and parsley, and for hardening off courgettes and tomatoes in late spring. Water sparingly in winter — damp, cold compost rots roots faster than drought does.

Small Details That Make It Last

Check the frame every few weeks. Slugs love the damp corners, so look under pots and along the base rail. In summer, when the frame is empty, remove the glass and replace it with shade netting so it does not become an oven. Repaint hinges and oil the vent opener. If wasps or mice move in, clear them out before they settle.

A cold frame will not replace a greenhouse, but it will stretch your growing season by weeks at either end and give you a somewhere to nurture plants without taking up much room. Build it well, keep it ventilated, and it will earn its place for a decade or more.