If you want salad leaves on the plate in January, the work happens in August. Sow your hardy leaves in August and September, while there is still light and warmth to bulk them up, and they hold through the dark months. Leave it to October and the same seed sits in cold compost, germinates slowly, and never reaches a size worth picking. The deadline is real, and it is now.
A productive winter salad greenhouse runs on a single idea: you grow the leaves in late summer, then harvest rather than grow once the days shorten. An unheated house in the UK will keep cropping corn salad, mizuna, mustard, winter purslane, land cress and hardy lettuce well past Christmas, provided the plants were near full size before the light gave out. This guide covers which leaves actually crop in low light, the ten-hour rule that governs the whole season, and the cut-and-come-again habit that keeps a single sowing going for weeks.
Why August, not October: the ten-hour rule
Plant growth all but stops once the day drops below ten hours of light. The American grower Eliot Coleman named this stretch the Persephone period in his Winter Harvest Handbook, and it explains the whole strategy. Below the threshold, photosynthesis slows to a crawl, so a small seedling stays small. Get the plant to roughly three-quarters grown before the light fails and it will sit, hardy and edible, waiting for you.
In the UK that threshold bites from early-to-mid November in the south and earlier the further north you live. On the shortest day, around 21 December, London gets about 7 hours 50 minutes of daylight, Edinburgh 6 hours 57 minutes, and Lerwick in Shetland just 5 hours 49 minutes. Every one of those sits well under ten hours, so a Scottish gardener has a longer dead window than a Cornish one. That is why the calendar matters more than the thermometer in autumn. You are racing the sun, not the frost.
The practical takeaway: sow in August and September so the leaves reach picking size while growth is still happening. A late-September sowing in the south can just make it; an October sowing usually cannot.
Which winter salad leaves actually crop in an unheated house
The reliable performers under cold glass are corn salad (lamb's lettuce), winter purslane, land cress, mizuna and the oriental mustards, plus a few hardy lettuce types. The RHS recommends sowing corn salad, land cress and oriental leaves such as komatsuna, mibuna, mizuna, mustard and rocket in late summer for cut-and-come-again picking through autumn, and through winter where they are covered. Sort them by cold-hardiness and the order tells you what to lean on.
The very hardy ones
- Corn salad / lamb's lettuce (Valerianella locusta): RHS hardiness H7, surviving below −20°C as a plant. Sow late summer or autumn; the leaves shrug off frost that flattens lettuce.
- Winter purslane / claytonia (Claytonia perfoliata): one of the hardiest salads going. Sow August to September outdoors, or under unheated cover from August right through to December. Few leaves grow when so little else will.
- Land cress (Barbarea verna): the peppery stand-in for watercress. It carries on growing through winter, germinates in 7 to 10 days, and a cloche over it allows continual picking.
The middle of the pack
Three crops occupy the middle ground: mustard greens, rocket, plus chard. Mustard leaves take frost to roughly −4°C, with the tougher red and green types holding lower; chard greens go to around −12°C. They crop well into autumn and recover on milder spells, though hard cold checks them.
What stalls first
Lettuce and mizuna are the least cold-hardy of the common winter leaves. Both suffer at around −4 to −6°C unprotected, so they slow and soften before the hardier crops even notice. Grow them by all means, but treat them as the early-winter treat and the corn salad and claytonia as your January insurance. The kill-temperatures here come from a market grower's hoophouse records rather than UK glass, so read them as a hardiness ranking rather than a forecast for your own border.
How do you sow winter lettuce and leaves for a cold house?
Sow into short drills about 1 cm (half an inch) deep, with rows roughly 30 cm (12 inches) apart in the border or a deep tray. Thin or space the plants 10 to 15 cm (4 to 6 inches) apart so air moves between them. Autumn-sown salads grown this way crop over winter in an unheated border.
Spacing earns its keep in winter. Crowded leaves stay damp, and damp cold leaves invite grey mould (botrytis). Give each plant elbow room and open a vent on any mild, dry day to move the air through.
For the oriental leaves, the RHS suggests sowing from mid-summer onwards, which fits the August window. Sow again two or three weeks later, and keep that relay going into September. Different-sized plants mean a hard frost that checks one batch will not empty the whole border at once. Stagger the seed and you stagger the risk.
When can you start picking?
Begin harvesting once plants reach about 10 cm (4 inches) tall. In summer that takes roughly a month from sowing; in the cooler, darker months it can take a couple of months, because low light slows everything. So an August sowing may give a first cut in September or October, then hold for repeat pickings through the cold.
This is the heart of the harvest-don't-grow approach. You are picking from leaves that already grew in warmer weeks and are now holding in cold storage on their own roots, rather than waiting for the plant to keep producing in December. A house with a door at one end and an opening at the other helps here too, letting you cross-ventilate on a mild day without chilling the border.
Cut-and-come-again: getting weeks from one sowing
The cut-and-come-again habit is what turns a single packet into a season of salad. Snip a few outer leaves from each plant at about 2.5 cm (1 inch) from the base, leaving the growing centre intact. The plant pushes out a fresh flush, and you come back a week or two later for more.
Expect about three or four cuts from each sowing before the plants are exhausted, and fewer in deep winter when regrowth is slow. Take a little from many plants rather than stripping a few, so every plant keeps a working crown. The lovely thing about this rhythm is that the bowl fills itself if you simply walk down the row with scissors twice a week.
- Cut at 2.5 cm above the base, never into the crown.
- Harvest outer leaves first; the heart keeps producing.
- Pick in the morning when leaves are turgid, and let any frost lift off them before you cut.
- Reckon on 3 to 4 cuts per sowing, then clear and refill the space.
What an unheated greenhouse can honestly give you
An unheated greenhouse sits roughly 2 to 5°C warmer than the open garden on a cold winter night. That margin keeps salad leaves frost-free through most British winters, though it will not save tender crops in a hard, still freeze. The glass buys you a degree or two and a dry roof, and the hardy leaves do the rest.
On the worst nights you can stack a second layer of protection. A sheet of horticultural fleece laid over the plants adds about 2°C of frost protection, which is often the difference between a sulking crop and a frozen one. Drape it loosely in the evening and lift it on milder days so light still reaches the leaves. A cloche over a row of land cress or corn salad does the same job in a smaller space.
Be realistic about pace. Growth crawls under ten hours of daylight, so the border will not refill quickly once you have cut it. Treat winter leaves as a steady trickle and plant enough in August to see you through. You are harvesting a standing larder you built in late summer, topped up by slow winter regrowth.
The growing advice here works in whatever greenhouse you already own, from a second-hand aluminium frame to a lean-to. If you are choosing a new house with winter cropping in mind, ventilation matters as much as warmth: the NORDIC greenhouse in Swedish pine and polycarbonate has a rear opening window opposite the door, so you can cross-ventilate on mild winter days and keep grey mould off damp salad leaves without chilling the border. A greenhouse helps, but the leaves above will crop under any cover that holds a couple of degrees and lets you open a vent.
A simple August-to-March plan
Pull it together and the season looks like this. Through August and early September, sow in succession at 1 cm deep and 10 to 15 cm apart: corn salad, claytonia, land cress, mizuna, mustard, plus a hardy lettuce or two. Through autumn, take your first cuts at 10 cm tall and keep the air moving on dry days. From November the leaves hold rather than grow, so you pick lightly and lean on the hardiest crops. By February the light returns above ten hours and growth picks up again, carrying you to a March finish before spring sowings take over.
One more thing worth doing before frost: while you are clearing summer crops, read up on ripening the last of your tomatoes so the border is free for salad by late August, and if grey mould has been a problem under your glass, the same airflow habits that beat powdery mildew on cucumbers will keep winter leaves clean. Sow your first tray of corn salad this week, space the plants a hand's width apart, and you will be cutting leaves while the seed catalogues are still arriving.
A bigger span like the SteelRoot steel-arch greenhouse gives room for staggered winter sowings, so you can cut from one bed while the next fills in.
The hardy six: a sow-by and picking guide for an unheated house
These are the leaves that genuinely earn their place under cold glass in a British winter. Sow them in module trays or straight into a border bed from August, water them in, and let them bulk up before the short days slow everything down. The window below assumes a south-of-England unheated greenhouse; gardeners in Scotland and the north should sow a week or two earlier and expect the picking season to start later.
| Leaf | Sow by | Picking window (under glass) | Notes |
|---|---|---|---|
| Corn salad (lamb's lettuce) | Mid-September | October to March | The most dependable of the lot, hardy to roughly -15°C under cover. First cut about 8 to 10 weeks from a summer sowing, slower as the days shorten. |
| Mizuna | Mid-September | October to February | Fast under cover; 'Kyoto' is the variety to grow through winter. Baby leaves in as little as a month from sowing. |
| Mustard greens | Late September | October to January | Quickest of the six and peppery in the cold. Crop young before frost toughens the leaf. |
| Winter-hardy lettuce | Early October | From late autumn, with the main cut in spring | 'Valdor', 'Arctic King' and 'Winter Density' overwinter as young plants and head up for a cut from late April into May. |
| Land cress | Mid-September | October to late winter | Hardy to about -12°C, ready 7 to 8 weeks from sowing and regrows in roughly three weeks after cutting. |
| Winter purslane (claytonia) | Mid-October | November to March | Germinates best at 8 to 12°C, so it suits later sowing. First leaves about 6 to 8 weeks from sowing. |
The verdict from a cold house: corn salad, land cress and winter purslane are the three to lean on if you want a leaf in the bowl every week from November, as they shrug off the hardest frosts. Sow all six in two batches a fortnight apart, keep a sheet of fleece to hand for the coldest nights, and vent on dry days to keep grey mould off the lower leaves.
