Best Winter Vegetables to Sow in a Cold Frame

A cold frame is one of the quiet heroes of the winter garden. It is not heated, yet it traps enough warmth and blocks enough wind and rain to turn a small patch of ground into a productive little larder. In the UK, a well-sited frame can keep you in fresh leaves from October right through to April, even when the veg patch outside is frozen solid. The trick is to sow the right hardy crops at the right moment, then manage air and moisture carefully through the darkest weeks.

Why a cold frame changes the winter rules

A cold frame works because it creates a microclimate. Glass or twin-wall polycarbonate lets light in, holds daytime heat, and keeps the worst of the frost off tender leaves. The soil inside stays a few degrees warmer than the ground outside, and it drains better after heavy rain. That small difference is often enough to keep hardy salads, spinach and Asian greens growing slowly rather than sulking or rotting.

Position your frame somewhere bright and sheltered, ideally facing south or west. Free-draining soil or compost is essential — winter wet is a bigger killer than cold. On mild, sunny days, the temperature inside can rise surprisingly fast, so be ready to open the lid and let air through.

Hardy salads for cut-and-come-again pickings

Winter salads are not just summer lettuce in a coat. Look for varieties bred to shrug off cold and low light. Good choices include corn salad, also called lamb’s lettuce, along with claytonia, land cress and winter purslane. For proper lettuce, try ‘Winter Density’, ‘Arctic King’, ‘Valdor’ or ‘Rouge d’Hiver’.

Sow thinly in modules or direct into the frame from late August to mid-October. Cover seed with about half a centimetre of compost — too deep and germination will be slow and patchy. Aim for plants 15–20cm apart if you want full heads, or closer if you plan to cut young leaves. Water with a fine rose and keep the soil moist but never soggy.

• Sow little and often: small batches every three weeks give a steady supply rather than a glut.
• Harvest outer leaves first: the heart keeps growing for later pickings.
• Use fleece inside the frame: a double layer on bitter nights can save tender leaves.

Spinach and leaf beet: substantial winter leaves

Spinach is made for cold frames. It prefers cool, moist conditions and actually suffers in summer heat. Sow ‘Giant Winter’, ‘Atlanta’ or ‘Toscane’ in August and September for overwintering. You can also sow in January under glass for an early spring crop. Leaf beet, often sold as perpetual spinach, is even tougher and will keep producing through mild spells.

These crops need fertile soil, so dig in plenty of well-rotted compost before sowing. Space plants about 15–20cm apart and keep them watered in dry spells, though be careful not to waterlog the frame. Harvest the outer leaves while they are young and tender, leaving the crown to produce more. A light feed in late winter will encourage a flush of new growth as the days lengthen.

Asian greens: fast, spicy and generous

Asian greens are some of the most rewarding winter crops under glass. Mizuna, tatsoi, komatsuna, pak choi and mustard greens such as ‘Red Giant’ or ‘Green in Snow’ germinate quickly in cool soil and can be cut within four to six weeks. They bring welcome heat and texture to winter salads, stir-fries and soups.

Sow from September to October, or again in January and February if your frame is empty. Scatter seed thinly and cover lightly. These greens are ideal for cut-and-come-again harvesting: snip leaves 2–3cm above the soil and they will regrow several times. Flea beetle is far less of a problem in winter, but slugs and snails can still lurk, so check under pots and along the frame edges.

Cold frame care: air, water and insulation

Ventilation is the single most important winter job. Damp, still air encourages mould and rot, so open the lid a crack on mild days and wider on sunny ones. Close it again mid-afternoon to trap warmth for the night. Water sparingly in the morning, aiming at the soil rather than the leaves, and only when the surface is starting to dry.

Remove yellowing leaves promptly and keep the glass clean so every bit of low winter light gets through. Bubble wrap lining can add insulation, but it reduces light, so use it only during the coldest spells or on very exposed sites. A layer of straw or horticultural fleece over the plants at night is often enough. Brush snow off the frame so it does not block the light, and watch for mice nibbling seeds and young shoots.

Harvesting through to spring

Pick little and often rather than stripping plants bare. Cold weather often makes leaves sweeter, as plants convert starch to sugars to protect themselves from freezing. Use scissors for cut-and-come-again crops and take outer leaves from lettuces, spinach and leaf beet. Even in the depths of winter, a quick lift of the lid can reward you with a handful of peppery mizuna, a few tender spinach leaves and a crisp little lettuce heart.

By late winter, start sowing small batches again for spring. A cold frame is not just a shelter — it is a living larder by the back door, giving you fresh, home-grown pickings when the rest of the garden is sleeping.