How to Build a Cold Frame (UK): The Thrifty Homestead Season-Extender

A cold frame is one of those humble, low-tech builds that quietly transforms what you can grow. Strip it back and it is just a box that sits on the ground, with a glazed, sloping lid, but that simple shape does real work: it traps the sun, keeps off wind and rain, and buys you weeks at each end of the season. The RHS rates it as one of the most useful gardening accessories, and because the classic version is built from scrap timber and an old window, it costs next to nothing. If you are raising your own plants from seed, it is the piece of kit that stops all that windowsill effort being undone the moment your seedlings meet the open garden.

This guide covers what a cold frame actually does, where to put it, the simple build, and, just as important, how to use it well through the British year so you get warmth without cooking your plants.

What a cold frame is, and why build one

A cold frame is, in the RHS's words, a box that lies flat on the ground with a glazed, sloping lid. There is no heater and no electricity: it works entirely by trapping the sun's warmth and sheltering whatever is inside. That makes it an unheated mini-greenhouse, and on a homestead it earns its place several times over.

• Hardening off seedlings. Plants raised indoors or in a greenhouse are soft, and cannot go straight outside. They need two to three weeks to get used to cooler temperatures, lower humidity and more air movement first, a process the RHS calls hardening off. A cold frame is the ideal halfway house: lid open by day, closed at night, gradually toughening plants up before they are planted out.
• Extending the season at both ends. In spring it warms the soil and air so you can start earlier; in autumn it protects cuttings and young plants from the first mild frosts, stretching the growing year at the tail as well.
• Overwintering tender plants. Through winter it shields alpines from the wet that would rot them, and keeps young or borderline-hardy plants ticking over out of the worst of the weather.
• An early salad and propagation bed. Grown straight in the soil beneath the lid, hardy salads and early crops get going weeks ahead of the open ground, and the frame doubles as a bench for raising seedlings and rooting cuttings.

A cold frame is not a replacement for a greenhouse. Because it holds a small volume of air it heats and cools quickly, so conditions swing more and need closer attention than a walk-in greenhouse, where the larger body of air buffers the changes. But for a fraction of the cost and space, it does an enormous amount, which is why it pairs so well with a simple growing setup. If you are still choosing what to put in it, our guide to the easiest first vegetables for UK beginners is a good place to start.

Where to put it

Siting matters more than the build quality, so choose the spot before you pick up a saw.

• Roughly south-facing. Because the frame is higher at the back than the front, angling that slope towards the south lets the lid catch the most of the low spring and autumn sun.
• Level and firm. A cold frame can stand on soil, gravel or paving, but it wants to sit flat and stable so the lid closes cleanly and rain runs off rather than pooling.
• Sheltered from wind. The RHS is clear that gusts of wind are the main cause of damage. Keep it out of the worst of the wind, and peg or weight down any lightweight frame.
• Against a wall if you can. A south-facing wall soaks up heat during the day and releases it at night, steadies the frame, and shelters it from the wind, all at once.

How to build a simple cold frame

The classic thrifty build is a sloping box made from scrap timber or old scaffold boards, topped with an old window or a sheet of twin-wall polycarbonate. Gardeners' World's version uses a standard pallet, roughly 100 by 120 cm, for the base and an old window, around 115 by 130 cm, for the lid, which gives a sense of a sensible size: about a metre square is easy to manage and covers a good number of plants. The one rule that matters is the slope: build the back higher than the front so the lid sheds rain and tilts towards the sun.

Building a scrap-timber cold frame

1. 1

   Set the base and the back-higher-than-front slope

   Work out your footprint, around a metre square is ideal. The back should stand clearly higher than the front so the lid slopes, roughly half as tall again at the back as at the front, which sheds rain and catches the low winter sun. If you are building against a wall, fix two sloping timber supports to the wall, cut so their tops fall from back to front.

2. 2

   Build the sides and box

   Cut two side pieces from scrap timber or old scaffold boards so their top edge follows the slope from the tall back down to the low front, then fix the front and back boards between them to make a four-sided box. Screws hold up better than nails outdoors. A pallet makes a ready-made base you can build the sides straight onto.

3. 3

   Fit the glazed lid

   An old window makes a perfect lid, or cut a sheet of twin-wall polycarbonate to size and frame it in timber. Polycarbonate is lighter, safer than glass and will not shatter, though glass lets in a little more light. Size the lid so it overhangs the front slightly to throw drips clear of the box.

4. 4

   Hinge it at the back

   Screw the lid to the tall back edge with a pair of sturdy hinges, around 7.5 cm, so it lifts from the front. Hinging at the high side means rain always runs down and away, and the lid opens towards you for easy access.

5. 5

   Add a prop or an automatic opener

   Fix a simple wooden stay or notched batten that holds the lid open at a couple of heights for ventilation. Better still, fit an automatic vent opener: a wax-filled cylinder that expands as the frame warms and pushes the lid open, then closes it as things cool, so ventilation happens even when you are not there.

6. 6

   Draught-proof and finish

   Fill any large gaps so the frame holds warmth, and treat bare timber with a plant-safe wood preserver to slow rot. Stand or peg the finished frame in its spot, weighting a light one down against the wind.

The same box-building logic sits behind a lot of homestead woodwork. If you have not built with outdoor timber before, our guide to building a raised bed covers choosing rot-resistant boards, fixings and finishes in more detail, and the skills carry straight over to a cold frame.

What you need

You can build a cold frame almost entirely from reclaimed materials, but here is the short kit list:

• Timber for the box: scrap boards, old scaffold boards or pallet wood. Search scaffold boards.
• A glazed lid: an old window, or a sheet of twin-wall polycarbonate framed in timber.
• Hinges: a sturdy pair of galvanised butt hinges to hang the lid at the back.
• An automatic vent opener (optional but worth it): automatic cold frame vent opener.
• Screws and a plant-safe wood preserver to hold it together and slow rot.

Using your cold frame through the year

A cold frame rewards a little daily attention. The single habit that matters most is ventilation.

Through spring and early summer, use the frame to harden off seedlings and tender young plants: open by day, closed at night, easing them into outdoor conditions over two to three weeks before you plant out. In late spring and summer it gives extra warmth to tender crops such as tomatoes, aubergines and chillies, and works as a propagation bench for raising seedlings and rooting cuttings, useful if you are getting into saving your own seed.

In autumn, close it up earlier in the day to trap warmth and keep the first mild frosts off cuttings and young plants. In winter it shields alpines from the rain that rots them and overwinters young or borderline-hardy plants.

Cheap and reclaimed material options

The cold frame is a thrifty build by tradition, and there is no need to buy anything new.

• Old windows and doors are the classic lid. A single glazed sash window rested on a timber box is the original cold frame.
• Pallets and old scaffold boards make sturdy, free sides, as does any offcut timber from other jobs.
• Panes of glass on bricks. The RHS notes the old makeshift of raising panes of glass on bricks, or old window lights resting on wooden sides, to cover a row of plants. It is rough and ready but works, as long as you take care with ventilation.
• Straw bales as a temporary frame. A rectangle of straw bales with an old window or a sheet of polycarbonate laid across the top makes a quick, well-insulated seasonal cold frame that can be dismantled and composted afterwards.
• Cut-down plastic bottles cover individual seedlings as tiny one-plant cloches, handy for filling gaps.

Whatever you use, the RHS's advice holds: these inexpensive makeshifts are good enough for most purposes as long as you keep an eye on ventilation.

Common mistakes

• Forgetting to ventilate. The commonest killer. A shut frame in spring sun overheats within the hour and scorches or damps off young plants. Prop it open on warm days.
• Building the lid flat. Without a back-to-front slope, rain sits on the lid and light is lost. Always make the back higher than the front.
• Siting it in the wind. Gusts are the main cause of damage. Keep it sheltered and weight a light frame down.
• Leaving it closed on mild nights but open on frosty ones. Get the rhythm the wrong way round and you undo the frame's whole purpose. Closed holds warmth; open loses it.
• Relying on bubble plastic all winter. It insulates but cuts light and airflow, so plants can grow weak and damp. Save it for the coldest spells.
• Planting straight out of a cold frame into a cold snap. The frame hardens plants off, but still check the forecast and pick a mild spell to plant out.