Winter preparation is not a single job. It is a short sequence: clear out, clean thoroughly, repair what is broken, insulate, decide how much heat you actually need, and then restock with plants that will genuinely thrive in a cold greenhouse. Done in that order, each step makes the next one easier. Clean glass makes insulation more effective. Good insulation makes a small heater sufficient. A tidy interior makes it far easier to spot pests, water correctly and ventilate on mild days.
This guide walks through the whole sequence for an ordinary home greenhouse, whether it is glass or polycarbonate, heated or unheated. It covers what to clean and how, which insulation is worth the money, how to heat without wasting energy, and what to grow through the coldest months. It also covers the mistakes that undo good winter preparation, so you can avoid the most common ways gardeners lose plants between November and March.
Why October Is Greenhouse Month
October sits in a useful sweet spot. The main summer crops are finished or finishing, so you can clear benches without sacrificing a harvest. Days are still long enough and mild enough to work with the door open, wash glass without your hands going numb, and let everything dry before nightfall. At the same time, the first frosts are close enough to give the work urgency: tender plants still outside need to come under cover soon, and any repair you postpone now will be much harder to do in a cold snap.
Light is the other reason to act now. From late October onwards, daylight falls sharply, and a greenhouse loses light in three ways at once: shorter days, lower sun angle, and dirty glass. Algae, dust and limescale on panes can cut light transmission noticeably, and plants already struggling with short days do not need that extra handicap. A thorough autumn clean is therefore not cosmetic. It is one of the highest-impact things you can do for winter plant health.
October is also when garden centres and hardware shops stock winter greenhouse supplies: bubble wrap insulation, fleece, replacement clips and seals, paraffin and electric heaters, and cold-season seed. Buying now means you are not improvising with whatever is left when the first frost forecast appears.
Step 1: Clear Out Before You Clean
Start by emptying the greenhouse as far as you can. Take out everything that is finished: spent tomato and cucumber plants, dead annuals, empty pots, broken trays and the pile of canes in the corner. Compost healthy green material, but bin anything diseased. Fungal spores and virus-infected stems left in a corner over winter will reinfect next spring's seedlings, so this is not the time to be sentimental about a tired plant.
Wash empty pots and trays before you stack them. A bucket of warm water with a mild disinfectant, a stiff brush and ten minutes of scrubbing will remove soil, algae and pest eggs. Rinse well and let pots dry in the sun before stacking them upside down. Clean pots are also much easier to label and find in spring, when you will be glad you did this job properly.
Decide what stays. Tender plants that spent summer outside, such as pelargoniums, fuchsias, citrus in pots and agaves, should come under cover before the first frost, not after it. Check each one for pests before it crosses the threshold: look under leaves for whitefly and aphids, check stems for scale, and shake the plant gently over a sheet of paper to see what falls off. A single infested plant brought indoors in October can colonise a whole greenhouse by December, when biological controls are far less active.
Keep a clear path down the centre and clear access to vents, heaters and taps. Winter watering and ventilation are much easier when you are not climbing over stacked pots to reach a tap, and air circulates better around plants that are not crammed together.
Step 2: The Deep Clean, Inside and Out
Wash the glass or panels
Wash both sides of the glass if you can reach them safely. Outside, a long-handled soft brush and a bucket of warm water with a little washing-up liquid will shift most dirt. Inside, wipe panes with a sponge, paying attention to the overlaps and the glazing bars, where algae and moss collect. Rinse with clean water and, if your water is hard, finish with a wipe of diluted white vinegar on stubborn limescale marks. Avoid abrasive pads on polycarbonate panels, which scratch easily and then stay cloudy permanently.
Do not forget the roof. Roof panes collect the most algae and the most shade, and they are the panes that matter most in winter, when the sun is low and light comes at an angle. If you cannot reach the roof safely from a ladder, wash what you can from inside with an extendable pole and accept that as good enough. Safety matters more than a perfect pane.
Clean staging, floors and gutters
Scrub staging and benches with warm soapy water and a disinfectant suitable for greenhouses, then rinse and let them dry. Wooden staging benefits from a scrub with a stiff brush to remove algae from the slats, which otherwise stay damp and slippery all winter. Gravel floors can be raked and washed; concrete floors can be scrubbed and, if mossy, treated with a path cleaner that is safe to use around plants once rinsed.
Clear gutters and downpipes now, before autumn leaves fill them completely. A blocked gutter overflows down the greenhouse wall, keeps the base damp, and can rot wooden bases and encourage moss. While you are up there, check that downpipes drain away from the base, not into a puddle against it.
Disinfect to break the pest and disease cycle
After the physical clean, disinfect surfaces that held plants: benches, staging, pot stands and propagator lids. Use a greenhouse disinfectant at the dilution on the label, and allow surfaces to dry before plants go back. This step matters most if you had any disease this year: botrytis on tomatoes, mildew on cucumbers, or damping-off in seedlings. Disinfectant will not sterilise soil, but it does reduce the load of spores sitting on surfaces waiting for spring humidity.
Empty and clean water butts and watering cans too. Algae in a butt is harmless in itself, but a butt full of leaves and sludge smells, breeds mosquitoes in mild spells, and clogs watering-can roses. A quick scrub and rinse now saves a foul job in March.
Step 3: Repair What Winter Will Punish
A greenhouse is only as good as its weakest seal. Walk around it slowly and check every pane, clip, seal and hinge. Replace cracked panes before winter, not after a frost makes them worse. Cracked glass is also a safety risk in wind, and a missing pane turns a greenhouse into a wind tunnel overnight.
Check glazing clips and rubber seals. Clips work loose over a season of opening and closing vents, and a pane held by one tired clip can rattle out in the first winter gale. Replacement clips are cheap and take seconds to fit. Rubber seals around doors and vents compress and perish; if you can feel a draught with your hand on a breezy day, the seal needs replacing or the door needs adjusting.
Test every vent and louvre. Vents that stick shut are a winter problem in disguise: on a mild, sunny winter day, a closed greenhouse can overheat quickly, and plants that cannot be ventilated will suffer. Oil hinges, free sticking vents, and make sure automatic vent openers still move. If an automatic opener has failed, replace it now rather than discovering the failure on a warm February afternoon.
Check the base and frame. Timber bases rot from the bottom up, especially where soil or leaves pile against them. Clear debris from the base, and treat any soft timber with preservative while it is dry. Aluminium frames need less care, but check that bolts are tight and that the frame is still level; a frame that has shifted will make doors stick and panes sit badly.
Finally, check the door itself. It should close firmly with a single push and latch without lifting or shoving. A door that needs a shoulder to close will be left ajar on cold nights, and a door that does not latch will blow open. Adjust hinges, replace a worn latch, and fit a draught strip if there is a visible gap. This small job can save more heat than any heater upgrade.
Step 4: Insulate for Warmth Without Waste
Bubble wrap: the standard winter lining
The most cost-effective insulation for a home greenhouse is large-bubble horticultural bubble wrap, fixed to the inside of the frame. It traps a layer of still air against the glass, reduces heat loss, and still lets through most of the light. Use the large-bubble type, not small-bubble packaging wrap, which insulates less well and degrades faster.
Fix bubble wrap to the inside of the frame with clips, drawing pins or a staple gun on wooden frames, leaving it slightly loose rather than stretched drum-tight. A loose fit traps air better and is less likely to tear. Cover the roof and the north-facing side first, since those lose the most heat, then the other sides if you have enough material. Leave vents uncovered, or cut flaps over them so you can still open them for ventilation. A greenhouse sealed with bubble wrap and no ventilation will grow mould, not plants.
Bubble wrap does cut light a little, so do not put it up earlier than you need to. Late October or early November is right for most areas: after the autumn clean, before the first hard frost. Take it down in spring when nights warm up, clean it, dry it and store it rolled, not folded, so it lasts several seasons.
Other insulation options
For a small greenhouse, or for a particularly cold corner, horticultural fleece draped over plants on frosty nights adds a useful extra layer. It is cheap, light and easy to remove on mild days, but it must not stay on plants permanently, or they will become leggy and damp. Keep a roll by the door and use it as a nightly blanket for tender plants, not as a permanent cover.
Thermal screens and blinds, fitted inside the roof, work well in larger greenhouses but cost more and need a fixing system. They are worth considering if you heat your greenhouse all winter and want to cut heating bills significantly. For most home growers, bubble wrap plus fleece on the coldest nights gives most of the benefit for a fraction of the cost.
Insulate pots as well as the structure. Plants in pots lose heat from all sides, unlike plants in the ground. Group pots together, stand them on a bench rather than a cold floor, and wrap the outside of large pots with bubble wrap or hessian on frosty nights. A pot that freezes solid can kill roots even if the top growth looks fine.
Insulation compared
Bubble wrap lining: best all-round value; moderate cost, easy to fit, lasts several years, small light loss.
Horticultural fleece: cheapest; best for nightly frost protection of individual plants, not whole-structure insulation.
Thermal screens: most effective for heated greenhouses; highest cost, needs fixings, best light control.
Polystyrene or foam boards: good for solid walls and staging sides; not suitable for glass, blocks light completely.
Step 5: Heating — Only as Much as You Need
Not every greenhouse needs a heater. An unheated greenhouse still protects plants from wind, rain and frost to a useful degree, and many winter crops and hardy plants thrive in it. Before you buy or run a heater, decide what you are actually protecting. Tender exotics, citrus and overwintering pelargoniums need frost-free conditions, roughly above freezing. Winter salads and herbs need only protection from hard frost. Seedlings started in late winter need steady warmth, but that is a February job, not a November one.
If you do heat, a small electric fan heater with a thermostat is the most controllable option for a home greenhouse. Set the thermostat low, around 2 to 5 degrees Celsius for frost protection, rather than heating to room temperature. Every extra degree costs disproportionately more, and plants kept too warm in winter grow soft, leggy growth that is more vulnerable to pests and disease. A fan heater also circulates air, which reduces cold spots and damp patches.
Paraffin heaters are a traditional alternative and work without electricity, but they produce moisture and fumes, need ventilation, and require careful refuelling and maintenance. If you use one, keep it clean, use the correct fuel, and never run it in a completely sealed greenhouse. Electric tubular heaters, fitted low along a wall, give gentle background heat and are economical for small greenhouses, but they do not circulate air on their own.
Whatever heater you use, place a minimum-maximum thermometer at plant height, away from the heater and away from the door, and check it regularly. A thermometer tells you what your plants are actually experiencing, not what the thermostat claims. If the minimum is consistently well above your target, turn the thermostat down. If it is dropping below freezing on cold nights, add insulation or fleece before you turn the heat up.
Safety matters. Keep heaters away from staging, fleece and dry leaves. Do not run extension leads across wet floors. If you use electricity in a damp greenhouse, make sure sockets and connections are protected and that any heater is rated for greenhouse use. A heater is a tool, not a substitute for insulation and draught-proofing.
Step 6: Ventilation and Humidity in Winter
A winter greenhouse needs ventilation just as much as a summer one, only less of it. On mild, sunny days, open a vent or the door for an hour in the middle of the day, even in December. This lets out humid air, reduces condensation on cold glass, and prevents fungal diseases that thrive in still, damp air. Close vents again in the afternoon, before temperatures fall, so the greenhouse holds its warmth overnight.
Humidity is the hidden winter problem. Plants give off moisture, damp soil evaporates, and cold glass condenses water that then drips onto leaves. Botrytis, the grey mould that attacks soft growth, loves exactly these conditions. Reduce humidity by watering in the morning, not in the evening, so surfaces dry before night. Water the compost, not the leaves. Space plants so air moves between them, and remove dead leaves promptly rather than letting them rot on the bench.
Damp the floor down on mild days if the air feels very dry, but do not do this in cold weather, when extra moisture will simply condense on the glass. A greenhouse that is damp and cold is worse for plants than one that is dry and cold. Aim for air that feels fresh, not clammy, when you step inside on a winter morning.
Step 7: What to Grow and Keep in a Winter Greenhouse
Winter salads and herbs
An unheated greenhouse is ideal for winter salads. Sow or plant winter lettuce, rocket, spinach, chard, spring onions and land cress in October, and they will crop slowly through winter, picking up speed in February. These crops do not need heat, only protection from hard frost and wind, and they taste better for the slow growth. Water sparingly, pick outer leaves rather than whole plants, and you will have fresh leaves for months.
Herbs also do well under cover. Parsley, chives, mint in pots, thyme and winter savory will all keep going in a cold greenhouse long after outdoor herbs have died back. Keep herbs in pots so you can move them to the sunniest bench, and trim them lightly rather than cutting them hard in midwinter.
Overwintering tender plants
Use the greenhouse as a winter hotel for tender plants: pelargoniums, fuchsias, citrus, olive trees in pots, agaves and tender salvias. Keep them on the dry side, since most winter losses of tender plants are caused by wet roots, not cold air. Water only when the compost is dry an inch down, and do not feed them until growth resumes in spring. Check them weekly for pests, especially whitefly, which can build up unnoticed in a warm corner.
Citrus deserves a special note. Citrus trees in pots need light, cool conditions and careful watering in winter. They dislike central heating and dry indoor air, which is why a cool greenhouse suits them better than a living room. Keep them above freezing, give them the sunniest spot, and water only when the compost is nearly dry.
Early starts for spring
From late January onwards, a clean, insulated greenhouse becomes a propagation space again. Start sweet peas, broad beans and hardy annuals in pots, and sow early salads and herbs for a head start on spring. The work you did in October, cleaning and insulating, pays off here: seedlings started in a clean greenhouse with good light grow away far better than seedlings started in a dirty, draughty one.
The Winter Greenhouse Calendar
October: clear out, deep clean, repair, insulate, bring tender plants under cover.
November: finish insulation, set heaters and thermostats, plant winter salads, check fleece supply.
December: water sparingly, ventilate on mild days, check for pests weekly, protect against hard frosts.
January: keep watering minimal, watch for botrytis, start planning spring sowings.
February: increase ventilation on sunny days, start hardy seedlings, check insulation for damage.
March: remove bubble wrap gradually as nights warm, clean again before main spring sowings.
7 Mistakes That Undo Winter Preparation
1. Cleaning the glass but leaving the gutters blocked, so water still overflows and keeps the base damp all winter.
2. Insulating with bubble wrap but sealing every vent shut, creating a damp, airless box where mould thrives.
3. Heating to a warm temperature instead of frost-free, wasting money and growing soft, pest-prone plants.
4. Watering in the evening in winter, leaving leaves and compost wet overnight when fungal disease spreads fastest.
5. Cramming tender plants together so air cannot circulate, and one pest outbreak spreads to everything.
6. Bringing in infested plants without checking them, importing whitefly and aphids into a closed space.
7. Forgetting the thermometer, and discovering only in spring that the greenhouse froze on the coldest nights.
Frequently Asked Questions
Do I need to heat my greenhouse in winter?
Not necessarily. An unheated greenhouse still protects plants from wind and frost and suits winter salads, herbs and hardy plants. Heat only if you are overwintering tender plants that must stay frost-free, and then heat to just above freezing, not to room temperature.
When should I put bubble wrap up?
Late October to early November is right for most areas, after the autumn clean and before the first hard frost. Putting it up earlier cuts light unnecessarily, and putting it up later means you have already lost heat on cold nights.
How often should I water in winter?
Far less than in summer. Check compost before watering, and water only when it is dry an inch below the surface. Most winter losses come from overwatering, not underwatering, because cold, wet compost rots roots.
Why is there so much condensation on the glass?
Condensation forms when warm, humid air meets cold glass. Reduce it by ventilating on mild days, watering in the morning, spacing plants for airflow, and avoiding evening watering. Some condensation on very cold nights is normal and not a problem in itself.
Can I grow vegetables in an unheated greenhouse through winter?
Yes, for leafy crops. Winter lettuce, spinach, chard, rocket, spring onions and herbs all crop in an unheated greenhouse, slowly in midwinter and faster from February. Heat-loving crops such as tomatoes and peppers will not crop in winter without substantial heating and light.
Should I leave the greenhouse door open in winter?
Open it briefly on mild, sunny days for ventilation, but close it well before evening. Leaving it open overnight loses all the day's stored warmth and exposes plants to frost. A small roof vent is usually better than a wide-open door for winter ventilation.
The Bottom Line
Winter preparation is not glamorous work, but it is the work that decides whether your greenhouse is an asset or a liability from November to March. A clean, repaired, insulated greenhouse with a little well-chosen heat and sensible watering will carry tender plants, winter salads and early seedlings through the coldest months with far less fuss than a neglected one. Do the sequence in October, in order, and winter becomes a season of slow, steady growing rather than a season of rescuing plants.
Start this weekend if you can. Clear one bench, wash one side of glass, fix one draught, and you will already have a warmer, brighter greenhouse than you had last winter. The rest follows naturally, and by the time the first hard frost arrives, your greenhouse will be ready for it.
