Why a Small Greenhouse Earns Its Keep in Winter

There is a particular pleasure in stepping into a little glasshouse on a cold February morning and finding a tray of green. A small greenhouse — even a modest 6x4 or a lean-to against a shed wall — does something remarkable for the kitchen garden: it lifts the temperature by a few precious degrees, shields leaves from wind and driving rain, and keeps slugs and pigeons largely at bay. That is often all you need to grow salad leaves right through the lean months, long before the outdoor beds are workable.

The trick is not to try to force summer crops. Tomatoes and cucumbers in January are a recipe for disappointment. Instead, work with hardy, cold-tolerant salad varieties that actually prefer cool conditions, and treat them as cut-and-come-again crops. Sow them thickly, harvest them young, and you can be picking fresh leaves for weeks from a tray no bigger than a seed catalogue.

Choosing Varieties That Genuinely Cope With Cold

Not all salad is created equal when the mercury dips. Lettuce will germinate at low temperatures, but it sulks and bolts if it gets too chilly for too long. Better to lean on the tough stuff.

• Winter purslane (claytonia) — wonderfully hardy, with fleshy, mild leaves that keep producing through frost.
• Corn salad (lamb's lettuce) — the classic winter leaf, nutty and soft, happy in an unheated greenhouse.
• Land cress — peppery, quick, and remarkably cold-proof.
• Mizuna and mibuna — mustard family leaves that germinate fast and add a gentle bite.
• Pak choi — sown for baby leaves, it grows quickly and tolerates cool glass.
• Winter-hardy lettuce — look for named winter types rather than summer butterheads.
• Rocket — wild rocket is far tougher than the salad type and will self-sow if you let it.

Mixing three or four of these in a single tray gives you a cut-and-come-again salad with variety in both flavour and texture. A pinch of each, scattered thinly, is plenty.

Sowing in Trays: Compost, Depth and Spacing

Use a fresh, peat-free multipurpose compost, ideally with a little added grit or perlite for drainage. Old compost from last year's pots can harbour damping-off fungi, and young seedlings are vulnerable — start clean and you will avoid a lot of heartache.

Fill your tray or shallow pot to within a centimetre of the rim and firm it gently with your fingers, not your boot. Water the compost before sowing, using a fine rose or a mister, until it is evenly damp but not sodden. Scatter seed thinly across the surface — aim for roughly one seed every centimetre for larger seeds like corn salad, closer for tiny mustard seeds. Then dust over with a fine layer of compost, no more than the depth of the seed itself. Larger seeds can be covered by about 5mm; the smallest barely need any covering at all.

Label the tray. This matters more than people admit. A tray of unlabelled seedlings in March all looks suspiciously similar.

Watering, Ventilation and the Art of Just Moist

The single most common mistake with greenhouse salad is overwatering. Cold, wet compost in a cool greenhouse is a breeding ground for rot, and seedlings will collapse overnight. Aim for just moist: damp to the touch, never glistening, never dry enough to shrink from the sides of the tray.

Check daily, but water only when needed — often every second or third day in winter, more frequently as spring warms up. Water in the morning so any surface splashes dry before the temperature drops at night. A small watering can with a fine rose, or a mister, is far kinder than a jug.

Ventilation matters just as much. On any mild, still day, open a vent or the door for an hour or two to let humid air escape. Damping-off, grey mould and mildewed leaves all thrive in stagnant conditions. Conversely, on a hard frost night, close everything up and consider throwing an old fleece or a sheet of bubble wrap over the tray for extra insurance. A greenhouse heater is not necessary for these crops; a bit of thermal mass, such as a full water butt inside the glass, helps smooth out the night-time dips.

Harvesting Young Leaves for a Long Season

This is where the reward comes. Once seedlings have two or three true leaves — usually four to six weeks from sowing in cool conditions — you can start cutting. Use sharp scissors and take the outer leaves, leaving the growing centre intact. Snip a centimetre or so above the compost.

Cut little and often. A light trim every week keeps plants productive and prevents them running to seed. If a tray does start to bolt, don't despair — the flowers are edible and the bees will appreciate them. You can also sow a fresh tray every three weeks for a continuous supply, which is far more reliable than expecting one sowing to last all season.

Feed nothing at first. The compost has enough nutrients for a few weeks of leaf growth. If you are cropping the same tray for more than two months, a very dilute liquid seaweed feed once a fortnight will keep leaves tender.

Making the Most of Every Inch

Space in a small greenhouse is precious, so think upwards. A shelf at waist height takes trays off the floor and uses the warmer air that rises. Stacked seed trays on a simple frame work well, as long as light reaches every level. If you have a heated propagator, use it only for germination, then move trays onto open staging — seedlings kept too warm become pale and leggy.

Combine your salad trays with a few pots of herbs such as parsley or chives, which will also appreciate the shelter. And as spring arrives and the greenhouse fills with young tomato plants, don't abandon the salad habit: a tray of rocket or mizuna tucked under the staging will carry on cropping in the shade. Few things taste better in March than a handful of peppery leaves picked minutes before they hit the plate.