What can I grow in an unheated greenhouse over winter?
The crops that genuinely produce in a cold greenhouse from November to March, why light rather than temperature sets the limit, and how to stop everything rotting.
- Difficulty
- intermediate
- Time
- 45 min
- Read
- 5 min
- Safety
- caution
Short answer
Hardy salads, oriental leaves, spinach, chard, winter lettuce, spring onions, garlic, broad beans and overwintering onions all crop or establish in an unheated greenhouse. The limit is light, not cold: from mid-November to late January there is too little light for real growth, so everything must be sown early enough to be well grown by then. Ventilation on mild days is what prevents rot.
An unheated greenhouse in a British winter is typically a few degrees above outside, dry, and sheltered from wind — and that combination is worth a great deal. What it cannot do is manufacture light. The practical consequence is that winter greenhouse growing is really autumn sowing, with the winter spent harvesting from plants that are already established.
Safety
What you'll need
- An unheated greenhouse or polytunnel
- Horticultural fleece
- Seed of hardy winter varieties
- Bubble insulation for the coldest section (optional)
- Max-min thermometer (optional)
Step by step
- Sow in time, which is earlier than feels right.September to early October for most winter salads and leaves in the south, and late August to mid-September in the north. Sown in November they germinate and then sit doing nothing until February.
- Grow the reliable winter crops.Winter-hardy lettuce, mizuna, mibuna, mustards, land cress, corn salad, rocket, spinach, chard, pak choi, spring onions, parsley and coriander. These are the ones that genuinely produce.
- Use it to establish spring crops.Garlic, overwintering onion sets, broad beans and autumn-sown sweet peas all sit through winter under cover and come away weeks earlier than outdoor sowings.
- Understand the light limit.Between about mid-November and late January, day length and light intensity are too low for meaningful growth almost anywhere in the UK. Plants hold rather than grow. Harvest sparingly through that window and expect regrowth from February.
- Ventilate on every mild dry day.This is the difference between a productive winter greenhouse and a mouldy one. Still, damp, cold air causes botrytis and downy mildew. Open a vent or the door whenever it is mild and dry, and close it well before dusk.
- Water in the morning and sparingly.Just enough, at the base of the plants, early in the day so foliage dries before evening. Winter overwatering kills more greenhouse plants than cold does.
- Add fleece rather than heat for cold snaps.A layer or two of fleece draped over the crop on frosty nights gains several degrees at the plant, which is where it matters, and costs nothing to run. Remove or lift it during the day.
- Insulate selectively if you want to.Bubble insulation on the north-facing side and the ends cuts heat loss with less light penalty than lining the whole structure. Anything you line reduces the light, which is the scarce resource.
- Keep it clean through autumn.Clear out old crops, dead leaves and fallen debris before winter. Botrytis overwinters on dead material and spreads to living plants in still damp air.
- Use the floor and the border, not just the staging.The soil border holds warmth better than pots on staging, and roots in the ground are far less vulnerable to freezing than roots in a container.
- Protect containers from freezing solid.Pots in an unheated greenhouse can freeze through, which kills roots that would be fine in the ground. Group them, wrap them in fleece or bubble insulation, or stand them on the border soil.
- Watch for the winter pests.Mice eat pea, bean and sweetcorn seed and are a serious nuisance in a winter greenhouse. Slugs and snails shelter under staging and pots. Whitefly and red spider mite can survive the winter on overwintering plants and start early in spring.
Tips
- Sowing date is the whole game. A winter greenhouse rewards planning in September rather than effort in December.
- Cut-and-come-again leaves harvested lightly through winter regrow strongly in February, but cut hard in December and they may not recover at all.
- A max-min thermometer tells you what your greenhouse actually does overnight, which is usually milder than people assume and occasionally much colder.
Common mistakes
- Sowing winter crops in November — There is not enough light for them to establish. They germinate, sit, and produce nothing until spring. September is the month that matters.
- Keeping the greenhouse shut all winter to keep it warm — Still damp air causes botrytis and mildew, which does far more damage than the cold. Ventilate on every mild dry day.
- Watering generously in December — Plants are barely growing and the compost stays wet for days. Overwatering is the main cause of winter losses under cover.
- Lining the whole greenhouse with bubble insulation — It cuts the light substantially at exactly the time of year when light is the limiting factor. Insulate the north side and the ends only.
If it doesn't work
Grey fuzzy mould on leaves and stems
Cause: Botrytis in still, damp air — Fix: Remove affected material, ventilate on every mild day, water only in the morning and space plants further apart.
Crops sitting doing nothing from December
Cause: Normal — insufficient light — Fix: Harvest lightly, keep them healthy, and expect growth to resume in February.
Pots frozen solid after a cold snap
Cause: Container roots exposed on all sides — Fix: Group pots on the border soil and wrap them in fleece or bubble insulation.
Seedlings disappearing overnight
Cause: Mice or slugs — Fix: Store seed securely, check under staging and pots, and use control appropriate to the site and the current label.
Everything leggy and pale by February
Cause: Low light plus mild temperature — Fix: Clean the glass thoroughly, remove any unnecessary lining, and thin crowded plants.
Questions people ask
How much warmer is an unheated greenhouse?
Typically a few degrees above the outside overnight, and much more on a sunny day. It will still go below freezing in a hard frost, which is why fleece over the crop matters more than the structure alone.
Is it worth heating a greenhouse over winter?
Only if you need to keep something frost-free, and then only to a couple of degrees above freezing rather than to a comfortable temperature. Heating a greenhouse to grow through midwinter is expensive and still limited by light. Fleece plus a frost-free setting is the economical approach.
Can I grow tomatoes over winter?
No. They need light and warmth that a British winter greenhouse cannot supply at any sensible cost. The greenhouse year for tomatoes starts with sowing in late winter under cover indoors.