A cold frame is a simple, unheated box with a transparent lid, set on the ground in the garden. It is one of the oldest and most useful garden structures: it shelters plants from frost, wind and heavy rain, warms up in the sun during the day, and lets you sow earlier in spring, grow salads through winter, and harden off seedlings before they go outside. It needs no electricity, no fuel and almost no maintenance, and a basic one can be made from old windows, timber and a little time. This guide covers what a cold frame does, how to site and use one, and what to grow in it.
A cold frame works by trapping the sun's heat during the day and sheltering plants from cold wind and frost at night. On a sunny day, even in early spring, the inside of a cold frame can be noticeably warmer than the air outside, which is enough to get seeds germinating weeks earlier than they would in open ground. At night, the lid and the walls cut the wind and hold in some of that warmth, so a light frost that would damage seedlings in the open may leave plants in a cold frame untouched. A cold frame will not protect tender plants from a hard, prolonged frost on its own — for that you need a heated greenhouse or bringing plants indoors — but for hardy plants, early sowings and hardening off, it is often all you need. Our greenhouse heating guide covers heated protection, and frost cloth covers the lightest form of frost protection.
Siting and building a cold frame
Site a cold frame in the sunniest sheltered spot you have, ideally facing south, against a wall or fence that shelters it from cold wind and reflects a little extra warmth. The ground should be level and free-draining, because a cold frame that sits in a puddle will rot plants. A cold frame is usually a low box, about 30 to 45 centimetres high at the back and lower at the front, so the lid slopes towards the sun and rain runs off. The lid is traditionally an old window or a sheet of glass or clear polycarbonate in a wooden frame, hinged at the back so it can be opened. The box can be made from timber, brick or straw bales, and it should sit firmly on the ground with no gaps at the base where cold air can blow in.
If you build one, make the lid easy to open and prop, because you will open it most days in spring and summer. A simple stick, a chain or a notched prop holds the lid open at different heights, which lets you control ventilation. Make sure the lid is heavy enough not to blow away in wind, or fit a catch to hold it shut. If you buy a ready-made cold frame, check that the lid fits well, that the hinges are sturdy, and that it is heavy enough to stay put in your garden. Whatever you use, keep the glass or polycarbonate clean, because dirty glazing lets in less sun, and winter sun is the heat you want most.
Ventilation: the most important job
Ventilation is the most important part of using a cold frame, and the easiest to get wrong. On a sunny day, a closed cold frame can overheat quickly, and seedlings can be scorched or cooked in a matter of hours. Open the lid on mild and sunny days, a little at first and wider as the day warms up, and close it again in the late afternoon so the warmth stays in overnight. In spring, open the lid a crack on cloudy days too, because fresh air prevents mould and disease. Close the lid before a frost is forecast, and on very cold nights, cover the lid with an old carpet, a sheet of bubble wrap, or a piece of frost cloth for extra insulation. The rule is simple: open in the morning, close in the evening, and never leave a cold frame shut on a sunny day. If you are away during the day, leave the lid open a crack so the frame cannot overheat.
What to grow in a cold frame
Cold frames are ideal for salad leaves, spinach, radishes, spring onions and herbs through autumn, winter and early spring, when the open garden is too cold or too wet. Sow salad leaves in late summer and autumn for winter picking, and sow again in late winter for the earliest spring salads — see growing lettuce, growing spinach and growing radishes. They are also ideal for starting seeds early: sow peas, broad beans, lettuce and flowers in pots inside the frame in late winter, and they will germinate weeks earlier than outside. Use the seed starting calculator to plan sowing dates, and the frost date calculator to check your local frost dates.
A cold frame is the best place to harden off seedlings — the process of gradually accustoming indoor-grown plants to outdoor conditions before planting them out. Put seedlings in the cold frame for a week or two before planting out, opening the lid wider each day, and they will be far tougher than plants moved straight from a warm windowsill to the open garden. Tender plants such as tomatoes, peppers and courgettes should not live in a cold frame long-term, because a hard frost will damage them, but they can harden off in one in late spring. Alpine plants, succulents and other plants that hate winter wet also do well in a cold frame, because the lid keeps the rain off them while they still get light and air.
Soil, watering and care
Fill the base of a cold frame with good soil or compost, or stand pots and trays inside it on the ground. If the soil in the frame is poor, dig in compost before sowing, and refresh it each year. Water sparingly, because a cold frame dries out more slowly than open ground, and wet soil in cold weather rots roots and encourages slugs. Water in the morning, so the surface dries before night, and water the soil rather than the leaves. Keep the frame clean: remove dead leaves promptly, because they harbour mould and slugs, and check for slugs and snails regularly, especially in damp weather — our slug control guide covers organic-first methods. If you are unsure what will grow in a cold frame in your area, check your local extension service for advice.
Cold frames through the year
In late winter and early spring, the cold frame is for sowing: salads, peas, beans and flowers started early. In spring, it is for hardening off seedlings raised indoors. In summer, it is mostly empty or used for plants that like heat, and the lid should be left open or removed so plants do not overheat. In autumn, it is for sowing winter salads and for sheltering tender plants before the first frosts. In winter, it is for winter salads and for overwintering hardy plants that need shelter from wind and wet. A cold frame that is used all year repays its small cost many times over, and it extends the gardening season by weeks at both ends.
Common mistakes
The most common cold frame mistakes are leaving the lid shut on a sunny day so plants overheat, leaving it open on a frosty night, siting it in shade where it never warms up, overwatering, and forgetting to check for slugs. Another is expecting a cold frame to protect tender plants from a hard frost: it softens light frosts, but it is not a heated greenhouse. The fixes are simple: open in the morning, close in the evening, site it in sun, water sparingly, and check it daily in spring. A cold frame rewards a gardener who looks at it every day, and it punishes one who forgets it for a week.
Frequently asked questions
What is the difference between a cold frame and a greenhouse?
A cold frame is a small, low, unheated box with a transparent lid, set on the ground. A greenhouse is a larger, walk-in structure, usually glazed, that may be heated. A cold frame is cheaper, simpler and needs no fuel, but it holds fewer plants and gives less frost protection than a heated greenhouse.
What can I grow in a cold frame in winter?
Hardy salad leaves, spinach, radishes, spring onions and herbs are the best winter crops for a cold frame. Sow them in late summer and autumn for winter picking. Tender plants such as tomatoes and peppers will not survive winter in an unheated cold frame.
Do I need to open a cold frame every day?
In spring and summer, yes — a closed cold frame can overheat quickly on a sunny day, even when the air outside feels cool. Open the lid in the morning and close it in the late afternoon. In winter, open it on mild days for fresh air, and close it before frost is forecast.
Can I make a cold frame from an old window?
Yes, an old window makes an excellent cold frame lid. Build a low box from timber or brick, hinge the window at the back so it opens, and make sure the box sits firmly on level ground. Check that the glass is sound, and fit a catch so the lid does not blow open in wind.
Will a cold frame protect plants from frost?
It protects against light frosts and cold wind, and it warms up in the sun during the day, which is enough for hardy plants and early sowings. It will not protect tender plants from a hard, prolonged frost. On very cold nights, cover the lid with a carpet, bubble wrap or frost cloth for extra insulation, and bring tender plants indoors.