A Small Structure That Changes Your Season

A cold frame is the simplest piece of garden infrastructure you will ever build, and arguably the most useful. It is a bottomless box with a clear lid that sits over your soil. Nothing more. Yet that small pocket of trapped air will lift the temperature inside by several degrees on a sunny day, keep the worst of the wind and rain off tender growth, and give you somewhere to bring plants on weeks before the open ground is ready. In a British garden, that difference matters. It is the gap between an early sowing of hardy salads in February and waiting until April.

Building one from recycled materials is not just thrifty, it is genuinely the right way to do it. A cold frame wants to be low, heavy and slightly rough. Abandoned bricks and an old window are perfect for the job, and if you move house or change your mind, you can dismantle the whole thing and start again.

Choosing the Right Spot and Size

Siting decides how well your frame performs, so get this right before you lay a single brick.

• Face the lid south or south-east to catch the low winter sun.
• Put it against a south-facing wall or fence if you can. The wall absorbs heat during the day and releases it overnight, which is worth a couple of degrees on a frosty night.
• Avoid frost pockets at the bottom of a slope, where cold air collects and sits.
• Keep clear of overhanging trees and shrubs. You want light in winter, and dripping foliage makes the lid filthy.
• Site it near a tap and within easy reach of the house, because you will be opening and closing it constantly.

For dimensions, a classic shape has a back wall around 45cm high and a front wall around 30cm, giving the lid a gentle fall of roughly 15 degrees. That slope sheds rain and lets in more low-angle winter light. Keep the depth front-to-back to about 90–120cm so you can reach the back without kneeling on your seedlings, and match the width to whatever glazing you have scavenged. A standard old sash window, roughly a metre square, sets the size nicely.

Scavenging the Materials

Almost everything you need is already out there. Ask around, check noticeboards, and look at what is being skipped rather than buying new.

• Bricks or blocks: reckon on fifty to eighty for a frame about 1.2 metres wide, with two courses at the front and four at the back.
• Reclaimed timber: scaffold boards are ideal for the top rail and for a timber-only version. Pallet wood works for battens and props.
• Glazing: an old single-glazed window is the classic choice. Avoid sealed double-glazing units, which are far too heavy.
• Clear plastic: if you cannot find glass, use builder's polythene of at least 400 gauge, or rigid polycarbonate sheets from a dismantled conservatory roof.
• Filling: rubble or gravel for the base, plus leafmould or old compost to mix into the soil inside.

One word of caution. Old glazing and painted timber can carry flaking lead paint. Sand it back and seal it, or simply do not use that piece. If you have children or pets, consider fitting safety film to glass or choosing plastic instead.

Building the Frame

Start by digging a shallow trench about 10cm deep around the footprint and filling it with rubble. This gives drainage and stops the walls sinking unevenly. Bed the first course of bricks on a weak mortar mix of six parts sand to one of cement, or lay them dry if you want the frame to be movable. Take your time getting this first course level with a spirit level, because everything above it inherits any error.

Build the two side walls so they step down from back to front, then fill in the back wall and the front wall to match. A dry-laid frame is perfectly sound for this job and has the advantage that you can lift it apart in an afternoon.

If you would rather work entirely in timber, cut four scaffold boards so two are shorter at the front, and screw them to corner posts of 50mm square timber. Set the whole thing on a gravel base, and line the inside with a layer of gravel topped with a mix of garden soil and well-rotted compost. Leave a 5cm gap between the soil level and the top of the frame so air can move around your plants.

Fitting the Lid

The lid is where the heat comes from, so it needs to be clear, close-fitting and easy to prop open. Hinge it at the back so it lifts from the front, or forget hinges altogether and lift the whole window off. Many experienced gardeners prefer the latter, as hinges seize and glass panes crack when a lid slams.

Whatever you use, make the lid overhang the walls by a couple of centimetres on every side so rain runs off clear of the brickwork. If you are building a timber frame for plastic sheeting, sandwich the sheet between two battens and screw through both, pulling it taut as you go. Always fit a prop: a notched batten or simply a spare brick lets you hold the lid open at two or three heights, which you will need.

Managing Your Cold Frame Through the Year

Ventilation is the single most important skill. Even in January, a closed frame in weak sunshine can climb past 30°C and cook everything inside. Open the lid in the morning and close it again mid-afternoon, and on mild days leave it ajar overnight.

Water less than you think. The frame holds moisture, and wet compost in cold conditions rots seedlings faster than drought does. Check for slugs after rain, as they adore the damp shelter, and clear fallen leaves from the lid so light gets through.

Use the frame to harden off plants in spring, opening it a little wider each day over a week. Sow hardy salads, spinach and spring cabbage under it from late February, and overwinter young potted evergreens by plunging the pots into the soil inside. It is also superb for rooting semi-ripe cuttings of rosemary, bay, choisya, hebe and lavender in gritty compost kept just moist. A well-run cold frame quietly doubles the productive capacity of a small UK garden.