Winterizing Your Greenhouse: Get Ready for the Cold Season

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Alright, let’s have a proper chat about getting your greenhouse ready for winter, because I see people making the same mistakes every year — including myself, if I’m being honest. Your greenhouse is probably the most valuable asset on your plot, and yet come October, most people just shut the door and forget about it until March. That’s like buying a car and only driving it in summer. Madness. A well-prepared greenhouse can keep you growing all winter, protect your tender plants, and give you a massive head start on spring. But only if you put a bit of work in now.

I remember my first winter with the greenhouse. I did absolutely nothing to prepare it. Just left whatever was in there and hoped for the best. Come January, the temperature inside was basically the same as outside, there was mould growing on everything, two of the glass panels had cracked from frost, and my supposedly overwintering geraniums looked like they’d been through a war. Ronny came over, took one look, shook his head, and said, “That’s not a greenhouse, man, that’s a walk-in fridge.” Fair point, to be honest.

The Big Clean — First Things First

Before you do anything else, your greenhouse needs a proper deep clean. I know, I know. Cleaning isn’t exactly the exciting part of gardening. But trust us on this — the grime, algae, and general muck that builds up over the growing season blocks an incredible amount of light. And in winter, when daylight hours are already short, every bit of light matters.

Start by clearing everything out. Every pot, tray, tool, and random bit of string you’ve been keeping “just in case.” Get it all out where you can see it. You’ll be amazed at what’s lurking in the corners. I found a cucumber plant behind my staging last year that I’d completely forgotten about. It had basically mummified. Not my proudest moment.

Wash all the glass or polycarbonate panels, inside and out. I use warm soapy water with a splash of Jeyes Fluid for the outside and plain soapy water for the inside. Pay special attention to the overlaps between panes — that’s where algae and grime love to hide, and it’s where most of the light gets blocked. A long-handled brush or an old car wash sponge on a stick works brilliantly for reaching the roof panels without acrobatics.

While you’re at it, wash down the frame, the staging, the floor — everything. If you’ve had any disease problems during the growing season (blight, botrytis, mildew), this clean-up is absolutely crucial for preventing problems carrying over into next year. I use a diluted disinfectant on the floor and staging, then rinse with clean water. Some people use a sulphur candle to fumigate, which works well but the smell is absolutely rotten and you can’t go near the greenhouse for 24 hours afterwards.

Insulation — The Single Most Important Thing You Can Do

Right, this is the big one. An uninsulated greenhouse is basically useless in proper cold weather. Glass is a terrible insulator — it lets heat escape almost as fast as it comes in. So you need to add some form of insulation to trap heat inside and keep the cold out.

Bubble wrap is the classic choice, and for good reason — it’s cheap, effective, and easy to install. But here’s the thing: you need to use horticultural bubble wrap, not the stuff that comes in Amazon parcels. Horticultural bubble wrap has larger bubbles and is UV-stabilised so it won’t go brittle in sunlight. The normal stuff will fall apart within a few weeks and leave you picking bits of plastic off everything.

Fix it to the inside of the glass using the special clips that push into the glazing bars, or use drawing pins if your greenhouse has a wooden frame. Cover everything — sides, ends, and especially the roof. Leave a gap of about 2-3cm between the bubble wrap and the glass. This air gap is actually where most of the insulation happens — it creates a dead air space that slows down heat transfer. Some people use two layers for really cold areas, and honestly, in Sunderland, I’d recommend it.

One downside of bubble wrap is that it does reduce light levels a bit — roughly 10-15%. In winter, that’s significant. So the compromise I’ve settled on is insulating the north-facing side and both ends with bubble wrap (where you don’t get direct sun anyway), and leaving the south-facing side clear to maximise light. It’s not perfect, but it’s a sensible middle ground.

If you want to go beyond bubble wrap, there are other options. Thermal screens that roll across under the roof at night are excellent but pricey. Some people hang old blankets or even duvet covers inside on the really cold nights and remove them during the day. Polystyrene sheets along the base walls (below staging height) work well because that area doesn’t need light. I lined the inside of my north wall with polystyrene from old packaging boxes and it made a noticeable difference to the temperature stability.

Heating — Do You Actually Need It?

This is the question everyone asks, and the honest answer is: it depends on what you’re trying to do. If you just want to keep hardy plants alive and protect them from the worst frosts, insulation alone might be enough. If you’re trying to overwinter tender plants like citrus, pelargoniums, or fuchsias, you’ll need some form of heating to keep temperatures above freezing.

The cheapest option is a paraffin heater. They’re effective and don’t need electricity, which is handy for allotment greenhouses with no power supply. But they produce moisture as a byproduct of burning paraffin, which increases humidity and can lead to botrytis (grey mould). You need to ventilate to manage this, which defeats the purpose of keeping the heat in. It’s a bit of a catch-22.

Electric fan heaters are better because they circulate dry warm air and most have thermostats so they only kick in when needed. But obviously you need electricity, and running a cable to an allotment greenhouse isn’t always practical. Plus the running costs can add up — leaving a heater on all winter will give you a nasty shock when the electricity bill comes in. I tried it one year and nearly fell over when I saw the bill. Never again.

My approach now is what I call “frost-free and no more.” I set my thermostat to come on at 2°C and go off at 5°C. That keeps the greenhouse just above freezing without costing a fortune. Combined with good insulation, the heater barely comes on most nights. On the really brutal nights — and we get a few up here — I supplement with a few tea light candles in terracotta pots. It sounds daft but a few of those can raise the temperature by 2-3°C in a small greenhouse, which can be the difference between survival and disaster.

Ventilation — Yes, Even in Winter

This catches a lot of people out. You’d think that in winter, you’d want your greenhouse sealed up tight as a drum. But actually, ventilation is critical even in cold weather. Without air circulation, humidity builds up, condensation drips on everything, and fungal diseases go absolutely wild. Botrytis is the main enemy — that grey fuzzy mould that appears on leaves and stems. It thrives in still, damp, cool conditions, which is basically what an unventilated greenhouse is in winter.

On any day when it’s above about 5°C and not raining sideways, crack open a vent for an hour or two around midday. Even a small opening makes a difference. I have a battery-operated fan in my greenhouse that runs on a timer — it comes on for 30 minutes every few hours just to keep the air moving. Cost about twelve quid from Amazon and it’s been running for three years.

The key is to ventilate during the warmest part of the day and then close everything up by mid-afternoon to trap whatever warmth you can for the night. It’s a bit like breathing — your greenhouse needs to exhale the damp air and inhale fresh dry air regularly to stay healthy.

What to Grow in Your Winterized Greenhouse

Here’s the fun part. A properly prepared greenhouse opens up loads of winter growing possibilities that you just can’t do outside. Hardy salad leaves like winter lettuce, lamb’s lettuce, mizuna, and winter purslane all thrive in an unheated but insulated greenhouse. I pick fresh salad leaves from my greenhouse right through until March, while the supermarket stuff costs a fortune and tastes of nothing.

Spring onions sown in September will be ready to pull by Christmas. Spinach keeps going all winter as long as it doesn’t get too wet. Pak choi and other oriental greens are brilliant greenhouse crops — they actually prefer the cooler conditions and shorter days compared to summer, and they’re much less likely to bolt.

If you’ve got a heated greenhouse, even at frost-free temperatures, you can overwinter citrus trees, tender herbs like lemon verbena and French tarragon, rooted cuttings of pelargoniums and fuchsias, and even start some early seeds in February. My greenhouse is always packed in winter — I sometimes think it’s more productive from November to March than it is in summer when everything is growing outside anyway.

Common Winter Greenhouse Problems and Solutions

Condensation dripping on plants: This is caused by warm moist air hitting cold glass. Improve ventilation and consider running a small dehumidifier if you have power. Alternatively, arrange plants so the most susceptible ones aren’t directly under drip points.

Grey mould (botrytis): Remove any dead or dying plant material immediately — it’s the main food source for the fungus. Improve ventilation and avoid overwatering. Space plants out so air can circulate between them. I lost an entire tray of overwintering pelargonium cuttings to botrytis because I’d packed them in too tightly. Gutting.

Mice: The greenhouse is warm, dry, and full of seeds. Of course mice want to live there. Set humane traps and check them daily, or get a cat. Ronny’s allotment cat, Mrs. Whiskers, patrols his greenhouse like a furry security guard. Works a treat.

Slugs: They don’t go dormant in a greenhouse because the conditions are mild enough for them to stay active all year. Check under pots and staging regularly. Beer traps work well. Or copper tape around your staging legs — slugs won’t cross it.

Look after yourselves. Take good care. 🌱