Why a Cold Frame Earns Its Keep in Winter

There is something quietly satisfying about walking out on a grey January morning and cutting a bowl of peppery leaves while the rest of the garden sleeps. A cold frame makes that possible. It is nothing more than a bottomless box with a transparent lid, but its effect is remarkable: it lifts the soil temperature by a couple of degrees, keeps the worst of the wind off tender foliage, and stops heavy rain battering leaves into the mud.

The key to success is siting. Put your frame somewhere that catches the low winter sun, ideally facing south or south-east and backed by a wall, hedge or fence for shelter. A frame pushed against a south-facing brick wall will hold noticeably more warmth than one standing alone in the open, because the wall releases stored heat overnight. If you have room, sink the frame into the ground a little so the soil inside is insulated by the earth around it.

Choosing Leaves That Shrug Off Frost

Forget the soft butterheads and cos lettuces of summer. Winter salads need attitude — leaves that are either genuinely hardy or so fast-growing that they outrun the weather. The following have all proven themselves in British winters, and most will survive several degrees of frost without protection, let alone under glass.

• Land cress — the closest thing to watercress you can grow in soil, with a fierce, clean pepperiness. Extremely hardy.
• Winter purslane (claytonia) — succulent, mild and spoon-shaped, producing steadily right through the coldest months.
• Corn salad (lamb's lettuce) — nutty, soft little rosettes. 'Vit' is the reliable, mildew-resistant choice.
• Mizuna and red mustard — quick, handsome and spicy. 'Red Giant' adds colour to a winter bowl.
• Rocket — sow a hardy wild type rather than a summer strain; it will regrow after cutting.
• Winter-hardy lettuce — 'Winter Density' and 'Arctic King' will hold in a frame where others collapse.
• Minutina and chicory — for crunch and a pleasant bitterness that lifts a heavy winter plate.

Sowing for a Continuous Supply

The mistake most of us make is sowing everything in one enthusiastic September afternoon, then facing a glut followed by a famine. Stagger instead. Make a first sowing in late August or early September for cutting from October to December, a second in early October for the depths of winter, and a third in late January or February for an early spring flush.

Sow thinly, either direct into the frame or into modules for planting out at about 15cm apart. Closer spacing gives you more leaf but weaker plants that damp off more easily. Water the drill before sowing rather than after, so seeds are not washed into a clump, and cover with no more than a centimetre of fine soil or compost. If you are sowing direct, mix the seed with a little dry sand to help you spread it evenly.

Managing Light, Air and Moisture

Winter growing is mostly about balance. Too much moisture and you invite grey mould and damping off; too little ventilation and the frame becomes a humid breeding ground for disease. Open the lid for an hour or two on mild, dry days, even in December, and shut it firmly before the temperature drops in mid-afternoon. On very cold nights, throw an old rug, a piece of fleece or a straw bale over the top for extra insulation, and remove it in the morning.

Water sparingly and in the morning. In winter, plants need far less than we instinctively give them, and soggy compost is the single biggest killer of frame-grown salads. Water at the base, never over the leaves, and let the surface dry a little between drinks. A layer of grit around the plants keeps the necks of the rosettes dry and discourages slugs.

Keep the glass clean. Light is the limiting factor from November to February, so wash off algae and dirt with warm soapy water in autumn. If you are using a plastic-covered frame, replace cloudy sheeting — the difference in growth is genuinely noticeable.

Harvesting and Keeping the Crop Going

Cut-and-come-again is the technique to master. Once plants have made a decent cluster of leaves, take the outer ones with scissors or a sharp knife, cutting about 2cm above the growing point. The centre continues to push out new leaves, and most of these varieties will give you three or four cuts before they run out of steam.

Harvest in the morning if you can, once any frost has thawed. Leaves cut while still frozen bruise badly and turn to slime within hours. Rinse them in cold water, spin or pat dry, and store in a lidded box lined with kitchen paper in the fridge, where they will keep for several days.

Do not feed plants over winter — soft growth is the first thing frost kills. A light scatter of blood, fish and bone before sowing is plenty. If aphids appear in a mild spell, simply rub them off with your fingers; a frame is far too enclosed for sprays to be sensible. Inspect the crowns every week or so and remove any yellowing leaves at once, before botrytis gets a hold.

Small Troubles, Easy Answers

Slugs are the main thief, hiding under the frame's wooden edges and in the soil. Go out with a torch after dark and deal with them directly, or set a few traps among the plants. Pigeons and sparrows will shred young leaves in hard weather, so drape netting over the frame if you notice damage. Damping off, which shows as seedlings collapsing at soil level, almost always means you have sown too thickly and watered too generously — thin the survivors and ease off the can.

Get these things roughly right and your cold frame will quietly out-produce the vegetable patch all winter. A handful of leaves, still cold from the garden, dressed with good oil and a squeeze of lemon, is one of the small, genuine rewards of growing food in Britain.