Sophie Barnes
Why a cold frame earns its space
A cold frame is the simplest piece of gardening kit you can own and, ounce for ounce, one of the hardest working. It is a bottomless box with a transparent lid. That is all. Yet it traps the sun's warmth, sheds rain and keeps the worst of the frost off, creating a pocket of ground that runs three or four weeks ahead of the rest of the plot. In a British growing season, where a warm April can turn into a freezing May, that head start is worth its weight in courgettes.
What a decent frame gives you:
- A nursery for seedlings that would otherwise sulk on a windowsill
- Protection for autumn sowings of hardy salads right through the winter
- Somewhere to harden off tender plants before they face the open garden
- A dry, sheltered spot for potting on and for overwintering tender herbs
- Earlier crops of radish, lettuce, carrots and spring onions
It suits any plot, from a balcony to a half-plot, because you can build it to whatever size fits.
Building one from what you've already got
You do not need to buy a thing. The classic cold frame is a timber box with a sloping lid facing south, and almost every component can be salvaged.
- The frame: pallet wood, old scaffold boards, fence panels or even a course of bricks. Aim for roughly 1.2m wide and 90cm deep, with the back board about 45cm tall and the front about 30cm. That slope, facing the sun, lets in far more low winter light than a flat lid.
- The lid: an old double-glazed unit, a reclaimed conservatory panel, a sheet of polycarbonate, or a wooden frame with heavy-duty polythene stapled over it. Rigid glazing holds heat better and lasts far longer than plastic sheeting, which shreds in the wind.
- The base: dig out 10-15cm and backfill with gravel or broken pots for drainage, then top with a mix of garden soil and well-rotted compost.
- The fixings: galvanised hinges at the back so the lid opens upwards, plus a prop or a stout stick to hold it open.
Avoid anything that has held creosote, engine oil or unknown chemicals. Weathered timber is fine.
Siting it properly
Position beats construction every time. Put the frame somewhere that gets full sun from mid-morning onwards, ideally against a south-facing wall or fence, which will radiate stored warmth back into the frame overnight. Raising the whole thing on a shallow gravel bed keeps the timber off wet ground and stops it rotting.
Keep it within easy reach of a tap, because you will be watering more often than you expect. And leave room to open the lid fully: a frame jammed against a hedge is a frame you will not ventilate.
Using it through the year
A cold frame is not a one-season tool. Treat it as a rotating workspace.
- Late winter to early spring: sow hardy lettuce, radish, spinach and early carrots, then prick out seedlings and grow them on before planting out.
- Spring: the busiest time. Harden off tomatoes, courgettes, sweetcorn and bedding plants over seven to ten days, opening the lid a little more each day and closing it at night at first.
- Summer: use it for salad successions, sweet peppers or aubergines if you have the space, and for drying off onions and garlic.
- Autumn: sow winter lettuces, corn salad, land cress and rocket. Pot up parsley, mint and chives and bring them in for cut-and-come-again leaves.
- Winter: overwinter hardy herbs, protect a few tender perennials, and keep the lid clear of fallen leaves.
Ventilation, watering and the usual pests
This is where most cold frames go wrong. On a bright February day the inside can hit 30°C while the outside sits at 5°C, and that sudden heat followed by a cold night will kill seedlings faster than frost will. Open the lid whenever the temperature inside climbs above about 15°C, and close it in the late afternoon while there is still warmth to trap. A maximum-minimum thermometer takes the guesswork out of it, and an automatic vent opener does the job for you on days when you are at work.
Water in the morning and water the soil, not the leaves. Damp foliage in a still, enclosed space invites botrytis and damping off. If you spot grey mould or collapsed seedlings, remove them at once and ventilate more.
Slugs adore a cold frame: it is warm, damp and full of tender leaves. Patrol after dark with a torch, and run a gravel collar around the base of the frame. Vine weevil and aphids also enjoy the shelter, so check pots carefully before you bring them in.
Finally, keep a few bricks or a length of timber on the lid in winter. A gale will lift a lightweight frame, and a lid that flies off in January is a lid you will be repairing in February. Looked after properly, a well-built cold frame will give you a decade of earlier harvests and stronger plants for very little cost.
