Why a cold frame earns its keep

A cold frame is the quiet workhorse of a British garden. It is nothing more than a bottomless box with a transparent lid, yet it gives you weeks of extra growing time at both ends of the season. Sow hardy salads in February, harden off sweet peas in April, overwinter cuttings and alpines, and ripen the last tomatoes in October. Because it traps the sun’s warmth and shields plants from wind, frost and heavy rain, a cold frame acts like a miniature greenhouse without the cost or the clutter.

The best part? You can build one from materials that would otherwise go to waste. Old wooden window frames, reclaimed timber, a few bricks and some hinges are often enough. Here is how to put together a sturdy, practical cold frame that will serve you for years.

Sourcing recycled materials

Start with the lid. A single-glazed wooden sash window is ideal, roughly 90cm by 60cm. Double-glazed units are heavy and rarely fit a simple timber frame, so stick to old single glazing if you can. Check for cracked panes and loose putty; if the glass is sound, it will last for decades. If you are worried about safety, or if the glass is broken, use a sheet of clear polycarbonate instead. It is lighter and safer, though it does not hold heat quite as well as glass.

For the body, you want sturdy, untreated timber. Scaffold boards, old floorboards, or the thicker slats from pallets work beautifully. Aim for 50mm by 50mm posts for the corners and 25mm thick boards for the sides. Avoid any timber that has been treated with creosote or old engine oil, as the fumes can harm plants. Bricks, paving slabs or concrete blocks make a fine base and help to store warmth during the day.

• Old wooden window, about 90cm x 60cm
• Reclaimed timber: 50mm x 50mm for corners, 25mm boards for sides
• Two galvanised hinges and a small handle or latch
• A prop stick, chain or automatic vent arm
• Bricks, slabs or gravel for drainage and base
• Wood screws, 60mm and 40mm, plus a weatherproof wood stain or linseed oil

Building the frame step by step

Plan a size that suits your window. A good working cold frame is about 1.2 metres wide, 60cm deep, 45cm high at the back and 30cm high at the front. That slope lets rain run off and lets low winter sun reach right inside. Cut two side panels as trapezoids, so the back edge is 45cm tall and the front edge is 30cm. Cut the back board 45cm tall and the front board 30cm tall, both 1.2 metres long.

Screw the back and front boards to the side panels using 60mm screws, then add 50mm x 50mm corner posts for rigidity. If you have a spare piece of timber, add a central brace across the back for extra strength. Treat all the cut ends and the outside faces with a coat of linseed oil or a water-based wood stain. Do not paint the inside, as some paints can scorch young leaves.

Attach the window to the back of the frame with two hinges. It should open from the front, like a lid. Fit a small handle and a prop stick, or a chain, so you can hold it open at different heights. A simple wooden peg with notches works well. Finally, sit the whole frame on a layer of bricks or slabs, and line the base with 5cm of gravel or sharp sand. This keeps the frame off the wet ground and gives excess water somewhere to drain.

Siting, ventilation and seasonal use

Place your cold frame in a sunny, sheltered spot, facing south or south-west. It should catch the morning sun but not overheat at midday. A position near a hedge or fence gives wind protection without casting deep shade.

Ventilation is the difference between thriving plants and a mouldy mess. On mild days, prop the lid open a few centimetres. On warm, sunny days, open it wider. Always close it before dusk to trap the day’s warmth. In winter, you can line the inside with bubble wrap for extra insulation, or drape horticultural fleece over the plants on frosty nights.

Use your cold frame through the year. In late winter, sow hardy lettuce, radish, spinach and early carrots. In spring, use it to harden off seedlings before planting them out. In summer, remove the lid entirely and use the frame as a raised bed for herbs or trailing nasturtiums. In autumn, sow winter salads and overwinter spring bulbs or alpines. Water in the morning, and only when the compost feels dry an inch below the surface. Damp, cold soil is the enemy of young roots.

Keeping it in good nick

Once a year, check the glazing putty and replace any cracked panes. Tighten the hinges and brush off algae with a stiff brush and soapy water. Re-oil the timber every couple of years. If you want to make life easier, fit an automatic vent arm that opens the lid as the temperature rises. It costs little and saves a daily job.

You can also turn your cold frame into a hotbed by digging out 30cm of soil and filling it with fresh horse manure, then topping it with compost. As the manure decomposes, it releases gentle heat for weeks, giving you a head start on cucumbers, melons or early potatoes. Whether you keep it simple or tinker, a recycled cold frame is one of the most rewarding things you can build for a British garden. It turns a few old windows and boards into a season-stretching, seedling-saving, money-saving piece of outdoor living.