Growing Winter Salads in a Cold Frame or Greenhouse

Why winter salads earn their space

From November through to March, most salad drawers depend on imports. A cold frame or an unheated greenhouse changes that. The cover does less to warm the air than to shelter it: glass and twin-wall polycarbonate keep the rain, wind and hardest frost off the leaves, and the soil inside a frame stays several degrees warmer than the open plot. That is often enough to keep hardy leafy crops ticking over rather than stopping altogether.

The other advantage is that winter salads are cut-and-come-again crops. You are rarely waiting for a heart to form. A sowing the size of a tea towel will give you a handful of leaves every few days for months, and the flavours — peppery, mustardy, nutty and mild — are far livelier than anything that has spent a week in a plastic bag.

Pick crops that genuinely enjoy the cold

Some salad plants merely tolerate winter; a few actively prefer it. These are the ones worth your compost:

• Winter purslane (Claytonia perfoliata): also called miner's lettuce. Succulent, mild and remarkably hardy, with a useful habit of self-seeding once established in a frame.
• Land cress: sharply peppery and hardier than watercress, which it can replace in sandwiches. It tolerates shade and damp far better than most salad crops.
• Mizuna: quick, frilly and mildly mustardy. It shrugs off several degrees of frost under cover and regrows well after cutting. Red varieties add colour to a winter bowl.
• Corn salad (lamb's lettuce): small, nutty rosettes that are among the hardiest things you can sow. Sow it thickly and thin as you harvest.
• Winter lettuces: 'Winter Density', 'Arctic King' and 'Valdor' are the reliable cos types. They are slower and may not heart up in a cold January, but the loose leaves are excellent.
• Mustard greens, wild rocket, chervil, parsley and chard: all worth a row or two, especially if you want something to lift a bowl of plain leaves.

Sow in succession, not in one big flush

The single most useful habit is to sow little and often. From mid-August to late September, sow a short row or a small module tray every two to three weeks. In a cold frame you can push sowings into October and even early November, though growth will be slow; anything sown much later tends to sit and wait for February before doing anything useful.

Sow thinly, about 1cm deep, in drills 10–15cm apart for cut-and-come-again crops, and thin the seedlings to 5–8cm so air can move between them. For lettuces you intend to grow as hearts, allow 25cm. Sowing into modules first is worth the fiddling: it avoids the gaps that slugs and dry spells leave behind, and you can plant out sturdy little clumps rather than coaxing bare seed through wet weather.

Do not overfeed. Rich soil produces soft, sappy growth that collapses at the first hard frost. A dressing of garden compost from the previous crop is plenty, and if your frame soil is heavy or claggy, fork in some grit before sowing.

Keeping the frame sweet through the winter

Ventilation matters more than heat. Damp, still air brings grey mould and mildew faster than cold ever will. Open the lid or a vent for an hour on any mild, dry day, and crack it briefly around midday even in a cold snap. A frame that is sealed shut from December to March is a frame full of rotting leaves.

Water sparingly, in the mornings, and aim for damp soil rather than soggy. Standing water at the roots rots winter crops more reliably than frost does. Water the soil, not the foliage. On the coldest nights, lay horticultural fleece or a sheet of bubble wrap over the plants, but lift it again during the day so light and air get in. Slugs and snails shelter happily under pots and along the frame edges — check these spots regularly, and watch for aphids building up under glass in late winter.

Harvesting so the crop keeps coming

Pick leaves when the plants are dry and the frost has thawed; midday is ideal. Frozen leaves bruise and go limp. Take outer leaves from lettuces and corn salad, or shear whole rows of mizuna, purslane and land cress about 3cm above the crown. They will regrow, though more slowly with each cut, so never strip a plant bare — leave the growing point intact.

Because you have sown in succession, you can simply move along the frame as each row comes into its own. When a sowing finally runs out of steam, pull it, refresh the soil with a little compost, and leave the ground to rest until spring.

Small tweaks that make a real difference

• Site the frame facing south on a free-draining base, and raise pots on slats so air can circulate underneath.
• Clean the glass in late autumn. Light, not warmth, is the limiting factor in a British winter.
• Keep a few spares in modules on a windowsill or in a sheltered corner to plug any gaps left by slugs or rot.
• Record what survives your worst frost. Over two or three winters you will build a list of varieties that genuinely suit your plot.
• Grow a pot of hardy herbs — parsley, chervil, winter savory — alongside the salads, so a midwinter picking feels like a proper harvest.

Get the shelter, the ventilation and the succession right, and you will be cutting fresh leaves on Christmas morning while the rest of the plot sleeps.