Cold Greenhouse Growing: What Actually Survives an Unheated Winter

Let me start with a confession. For the first two winters I owned a greenhouse, I treated it like a very expensive way to store empty pots and a leaking watering can from October to March. Everyone told me an unheated greenhouse was "useless in winter", and I believed them. It turns out they were spectacularly wrong — and I've spent the years since proving it, one frosted-over pane at a time.

Here's the thing nobody tells beginners: a cold greenhouse isn't about keeping plants warm. It's about keeping them alive, dry-ish, and sheltered from the wind and driving rain that actually does most of the killing outdoors. The temperature inside your unheated glasshouse might only be a couple of degrees above outside on the coldest night, but that shelter — plus the protection from waterlogging and wind chill — is the difference between a bed of mush and a bed of thriving winter salad.

In this guide I'm going to walk you through exactly what survives, what limps along, and what will absolutely turn to slime the moment the mercury drops. No heater, no fleece heroics unless you fancy it, no fantasy. Just the crops that have genuinely earned their place in my cold house across more than a few brutal British winters — the sort where the water butt froze solid and the kale looked like it had been dipped in glass.

Why an Unheated Greenhouse Works (When It Really Shouldn't)

The physics here is delightfully simple, and understanding it changes everything about how you'll approach the season. Glass and polycarbonate don't generate heat, but they trap it — and crucially, they block wind and rain. Whilst a hard frost outdoors is bad, it's the combination of frost plus wet roots plus howling wind that turns tender leaves to jelly. Take away two of those three factors and suddenly a surprising number of plants shrug off temperatures that would flatten them in the open ground.

The other quiet hero is what I call the "buffer effect". Soil and the mass of plants inside hold onto warmth, so your greenhouse cools more slowly at dusk and warms faster at dawn. On a sunny February day the temperature inside can leap 15°C or more above the outside air, even with the door shut. Your plants get little windows of active growth that outdoor crops simply never see.

The single biggest mistake I made early on was expecting winter greenhouse crops to grow the way they do in summer. They don't. Once daylight drops below roughly ten hours — which for most of the UK means from early November to early February — plants essentially press pause. This is what growers charmingly call the "Persephone period". Your job in winter isn't to grow crops; it's to have crops already grown and standing, so you can harvest them slowly like a living larder. Anything you want to eat in January needs to be nearly full-sized by the end of October.

The Cold-Hardy Champions: What Genuinely Thrives

Right, let's get to the stars of the show. These are the crops I'd stake my reputation on. Given a September or October start, they'll stand through a proper unheated winter and give you something worth eating whilst everyone else is buying limp supermarket bags.

Mâche (Corn Salad / Lamb's Lettuce)

If I could only grow one thing in my cold greenhouse over winter, it would be mâche — and it wouldn't be a close call. This unassuming little salad thrives in low temperatures, producing small, tender rosettes with a mild, nutty flavour that's genuinely lovely in a winter salad. Where lettuce sulks and rots, mâche just gets on with it. It's arguably the most cold-tolerant salad leaf you can grow, and it seems to positively enjoy the miserable conditions that finish everything else off.

Sow it thickly in September and you'll be picking rosettes from December right through to March. It doesn't mind if it barely grows during the darkest weeks — it simply sits there, perfectly edible, waiting for you. I treat mine as a cut-and-come-again patch and it rewards me for months.

Kale — Winterbor and Redbor Especially

Kale is the tank of the winter garden. Varieties like Winterbor and Redbor bring extreme cold tolerance to the table, laughing off frosts that would flatten softer greens. Under cold glass, kale becomes almost bomb-proof — and there's a well-known bonus: a touch of frost actually sweetens the leaves, converting starches to sugars so the winter harvest tastes better than the summer one.

Redbor also happens to be gorgeous, with deep purple-red frilly leaves that look wonderful glinting under a light frost. Winterbor gives you that classic curly blue-green kale in serious quantity. Both will stand for months and let you pick a few leaves at a time without ever committing to a full harvest.

Chard and the Wider Brassica Crew

Beyond the salads, a whole squad of cold-tolerant vegetables will happily overwinter under glass. Chard, broccoli, brussels sprouts, cabbage, turnips and radishes all belong to the group that typically thrives down to Zone 4 conditions. Under a cold greenhouse's shelter, they get an easier ride than their outdoor cousins and often come through in noticeably better condition — cleaner leaves, no wind damage, and none of that mud-splashed, slug-chewed look.

Chard in particular is a brilliant multitasker. It'll slow right down in deep winter, but the moment light returns in February it bolts back into growth and gives you an early flush of leaves weeks before anything outdoors stirs.

The Reliable Middle Rank: Solid Performers with a Bit of Help

Not every crop is quite as fearless as mâche and kale, but plenty sit in a comfortable middle tier — they'll survive a cold greenhouse winter reliably, provided you've sown them in good time and don't ask too much of them during the darkest weeks. This is where the bulk of your winter harvest actually comes from.

Carrots

Carrots are a genuinely brilliant cold-greenhouse crop and one that surprises people. As part of the Zone 4-hardy group, they cope beautifully with cold, and stored in the ground under glass they stay firm, sweet and protected from the worst of the wet. I sow mine in a deep bed and simply leave them there, pulling as needed through winter like a natural refrigerator. There's no woody, stored-in-a-shed texture — they come up crisp.

Winter Onions and Overwintering Alliums

Onions belong firmly in the cold-tolerant camp, and autumn-sown or autumn-planted alliums are made for this. They'll sit quietly through winter, barely moving, then surge into growth as the light returns — giving you a harvest weeks ahead of anything spring-planted. Spring onions in particular are a superb cut-and-come-again option under cold glass.

Peas for Shoots

Peas make the cold-hardy list too, and whilst you won't get pods in midwinter, they're wonderful grown for their shoots. A tray sown in autumn gives you sweet, tender pea tips for salads and stir-fries right through the cold months — a fresh, green, distinctly summery flavour when you least expect it.

The Risky Business: What Might Make It (And Often Doesn't)

Now for the crops I'd file under "worth a punt, but don't be heartbroken". These are the ones that can survive a mild unheated winter but will likely turn to mush in a hard one. I still grow some of them because when they work, they're lovely — but I never rely on them.

Winter Lettuce

Certain hardy lettuce types — the tough, loose-leaf winter varieties — can scrape through under cold glass, but they hate two things above all: cold combined with damp, and low light. In a mild winter you'll get away with it. In a cold, wet one, botrytis and downy mildew will spread through them like wildfire. If you grow winter lettuce, keep it dry at the crown and give it maximum ventilation on milder days.

Coriander and Parsley

Hardy herbs are a mixed bag. Parsley is genuinely tough and will often stand all winter under glass, giving you fresh leaves when the outdoor plants have collapsed. Coriander is fussier — it'll survive mild spells but resents both hard frost and the damp, stagnant air of a closed winter greenhouse. Grow them, by all means, but treat any harvest as a bonus rather than a guarantee.

Cold Hardiness Compared: How the Crops Stack Up

People always ask me to rank the crops, so here's my honest take based on years of watching what survives and what surrenders. I've scored each on cold survival — how likely it is to still be alive and edible after a genuinely hard unheated winter.

The pattern is clear as a frozen pane: mâche and kale are in a league of their own, the brassicas and roots form a dependable middle, and the salad-and-soft-herb brigade sits firmly in "cross your fingers" territory. If you're new to this and want guaranteed wins, plant heavily from the top of that chart and treat everything below parsley as an experiment.

Cold Greenhouse vs Cold Frame vs Open Ground

An unheated greenhouse isn't your only option for winter growing, and it's worth being honest about where it sits against the alternatives. A cold frame is cheaper and cosier for a small crop; open ground with cloches or fleece is the budget route; and the cold greenhouse is the deluxe walk-in version. Here's how they compare for the crops we've discussed.

My verdict? If you already own a greenhouse, using it in winter is close to free and delivers by far the most versatile, comfortable growing. A cold frame is a brilliant complement for extra salad space. Open ground with fleece works fine for the truly tough crops like kale, but it's a grim way to garden in January.

Getting the Timing and Setup Right

I've hammered the point about timing, but it genuinely is the whole ballgame. Let me lay out the practical rhythm of a cold greenhouse year so you can actually pull this off rather than just reading about it wistfully.

The Sowing Calendar

For most of the UK, September is prime sowing month for winter crops, with early October as the last realistic window for faster growers like mâche and salad leaves. Kale, chard and brassicas ideally want an even earlier start — think mid-to-late summer — so they're already substantial plants by autumn. Overwintering onions go in during autumn. Miss these windows and your plants simply won't have the size to coast through the dark months.

Watering: The Silent Killer

If frost gets the blame for winter losses, water is the true culprit more often than not. Cold, wet roots in stagnant air are a recipe for rot. In winter I water rarely — maybe once every week or two — and always in the morning so the surface can dry before the cold night. Never leave plants sitting in saucers of water, and never splash the leaves if you can help it.

Light and Cleanliness

Every scrap of light matters when days are eight hours long. Clean your glass or polycarbonate inside and out in autumn — a grubby pane can rob a shocking amount of light. Keep dead leaves cleared away too, because they harbour the botrytis spores that spread through a damp greenhouse like gossip.

Rating the Cold Greenhouse Approach

So how do I score the whole enterprise of growing in an unheated greenhouse through winter? Here's my honest breakdown across the factors that actually matter to a real gardener juggling weather, time and expectations.

Running cost scores a perfect ten for the obvious reason — there isn't one. Flavour ranks nearly as high because frost-sweetened winter kale and nutty midwinter mâche genuinely taste better than anything you'll buy. It only loses marks on yield, because the dark months cap how much you can realistically harvest, and on reliability, because a truly savage winter can still take out your borderline crops.

Who Should Give This a Go?

Cold greenhouse growing isn't for everyone, and I'd rather be straight with you than oversell it. Here's who I think gets the most out of it.

Who should think twice? Anyone dreaming of winter tomatoes, cucumbers or basil — those need real heat and there's no honest way around it. And if you cannot commit to sowing in late summer and autumn, the whole thing falls apart, because you can't plant your way out of trouble once the light has gone.

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