What is a cold frame?
A cold frame is a low, unheated box with a clear, hinged lid, usually glass or polycarbonate, that sits directly on the ground and uses the sun's warmth to shelter seedlings, cuttings and tender plants from frost, wind and heavy rain.
It's often described as a halfway house between a greenhouse and the open garden, and gardeners use it to harden off young plants, overwinter the not-quite-hardy and stretch the growing season at both ends of the year.
If you've ever found yourself with a windowsill full of seedlings in April and nowhere sensible to put them, or watched a tray of cuttings struggle through a cold, wet autumn, then a cold frame may be exactly what your garden has been missing. The name comes from the fact that there's no heating involved at all. Everything a cold frame does, it does with sunlight, shelter and a sensible bit of ventilation, which naturally raises the question of how something so simple can make such a difference.
Why a cold frame earns its place in the garden
For something that takes up so little room, a cold frame works remarkably hard, taking on a slightly different role with each passing season and repaying its modest footprint many times over.
This is the job most gardeners buy a cold frame for, and the one it does best. Seedlings raised indoors or in a heated greenhouse have lived a sheltered life, and moving them straight out into the garden can leave them scorched, wind-battered or checked in their growth for weeks.
A cold frame lets you acclimatise them gradually, so that when they're finally planted out they settle in with barely a flicker of complaint.
The gentle, even conditions inside a frame suit early sowings of salads, brassicas and hardy annuals in spring, while semi-ripe cuttings of shrubs and herbs taken in late summer will root happily in pots, sheltered from drying winds and heavy downpours.
When darkness comes early and temperatures plummet, a cold frame becomes a safe haven for young perennials, autumn-sown sweet peas, alpines that dislike winter wet and the cuttings you took earlier in the year.
If you already have a greenhouse, you'll know how quickly the staging fills up in spring. A cold frame alongside it gives your seedlings somewhere to go once they've outgrown the staging and frees up valuable space for the next round of sowing.
Because the soil inside warms earlier in spring and stays warmer later into autumn, you can start sowing sooner and keep harvesting longer, which, for a vegetable grower in particular, can add weeks to the year at each end.
Should a cold frame be in the sun or the shade?
For most of the year, and certainly from autumn through to late spring, your cold frame wants all the light it can get. A bright, open position, ideally facing south, will catch the low winter sun and warm the frame through, while a wall, fence or hedge behind it will soften cold winds without casting shade across the glass. Tucking it close to your greenhouse or back door also makes daily checks far easier, and you'll want to check on it most days.
In high summer the balance shifts. A frame in full sun can become too hot for young plants and cuttings, so a little shade, whether from a nearby shrub, some shade netting laid over the lid or simply keeping the lid wide open, will stop tender leaves from scorching. Cuttings in particular prefer bright but indirect light, so if rooting cuttings is a big part of how you'll use your frame, a spot that catches the morning sun but is shaded through the hottest part of the afternoon is ideal.
Once your frame is in place and filling up with plants, the next thing most gardeners wonder is how long everything should actually stay in there.
How long plants can stay in a cold frame
That depends entirely on why they're there. Plants being hardened off are only passing through, and a week or two is usually enough, sometimes a little longer for tender crops such as courgettes and squash. When the time comes to plant them out, our guide to moving plants from a cold frame into the garden walks you through it, including the signs your plants are ready to go.
Overwintering plants, on the other hand, can happily stay put for the whole winter, from the first frosts of autumn until the weather settles in spring. Cuttings will generally stay until they've rooted and put on some new growth, which for semi-ripe cuttings often means through the winter and into the following spring, and summer crops such as chillies can live in the frame for the whole season if you give them room.
Growing vegetables in a cold frame
A cold frame is one of the simplest ways to eat more of your own produce for more of the year. Early in the season it gives your sowings a head start, through summer it lends heat-loving crops a little extra warmth, and in autumn and winter it keeps a steady supply of fresh leaves coming long after the rest of the plot has slowed to a halt. A frame placed over a raised bed in late winter is a lovely way to warm the soil ahead of your first sowings.
To plan exactly when each crop should go in, our monthly seed sowing guide is a helpful companion, and if you'd like to make every inch of the frame count, we've shared our favourite companion and succession planting ideas for cold frames too. All of which might leave you picturing a greenhouse in miniature, so it's worth knowing how the two fit together.
A cold frame and your greenhouse
People often think of a cold frame as a greenhouse in miniature, and while the two share the same principle of catching the sun's warmth, they do quite different jobs and are at their best when they work together. A cold frame beside the greenhouse takes the overflow of spring seedlings and gives them a gentle stepping stone on the way to the garden, leaving the greenhouse free for the next round of sowing and the crops that need room to grow tall.
If you're still deciding which is right for your garden, or wondering whether something in between such as a patio greenhouse might suit your space, our guide to the differences between a cold frame and a greenhouse goes through it all in detail.
Choosing a cold frame
Cold frames come in all shapes and materials, from home-made boxes of timber and old windows to lightweight plastic frames and sturdy aluminium and glass designs, and the one you choose will shape how well it serves you over the years.
Glass lets in more light and holds on to warmth better than plastic, and it won't cloud or turn brittle in the sun. Toughened safety glass adds strength and peace of mind, particularly if children or pets are about.
A cold frame lets you acclimatise them gradually, so that when they're finally planted out they settle in with barely a flicker of complaint.
Timber looks lovely but needs regular treatment to stop it rotting, and plastic frames can be too light to stay put in a strong wind. Aluminium won't rot or rust, needs next to no maintenance and gives the frame the rigidity to stand up to years of British weather.
Look for a lid that can be opened to several heights and held securely in place, so you can fine-tune the airflow to the day rather than choosing between fully open and fully closed.
The Rhino Cold Frame
We designed our cold frame to the same standard as our greenhouses, because we believe the plants in it deserve nothing less. It's made in Britain from the same high-quality box section aluminium, finished with marine grade stainless steel and glazed with Grade A, 4mm toughened safety glass that's around a third thicker than most cold frame glass, and it's covered by our 25 year guarantee right down to the powder coated finish.
The roof pane locks at three heights, 5, 16 and 43 inches, so you can go from a gentle crack of air on a chilly spring morning to fully open when you're planting and tending. At 2.5ft x 3ft it sits happily on soil, paving or hardstanding, whether that's beside your greenhouse or somewhere else entirely, and it comes in 6 powder coated finishes so you can match your Rhino or add a contrasting touch of colour. It needs no planning permission and very little upkeep beyond the occasional clean of the glass.
