What to Grow in an Unheated Greenhouse Year-Round

An unheated greenhouse is one of the most useful things you can have in a UK garden, and it earns its keep in every season — not just summer. Glass traps warmth, keeps off rain and wind, and lifts the temperature inside by a few crucial degrees. That's often the difference between a seedling that sulks and one that romps away.

The trick is to think of the space as working all year, with a different job each season. This is a planning guide to keep your greenhouse busy from January to December. For the full how-to on getting set up, ventilation, watering and the rest, see the main greenhouse growing guide — this piece is about what goes where, and when.

Spring: propagation central

Spring is when an unheated greenhouse really comes alive, and for most growers it is the busiest season of all. From late February the lengthening days and the shelter of the glass make it the perfect nursery for raising seedlings — a head start of several weeks on anything sown straight outside.

This is where you do your sowing and pricking out. Fill modules or small pots with seed compost, sow, and stand them on the staging out of the wind. As seedlings fill their pots, prick them out (move each one gently into its own larger module) so the roots have room. A cold, bright greenhouse in March beats a warm, dim windowsill for sturdy, stocky plants.

The greenhouse is also the launch pad for hardening off — gradually acclimatising indoor-raised plants to outdoor conditions before they're planted out. A cold-frame just outside the door is the ideal halfway house: pop trays in it by day, close the lid at night, and your plants toughen up without a check. On frosty nights, throw fleece over tender seedlings still inside the glass.

While all that propagation is going on, get an early salad crop growing in the border soil or in deep trays. Sow cut-and-come-again lettuce like 'Lobjoits Green Cos' or a mixed leaf blend, plus rocket and spring onions, from late February. Under glass they'll be ready weeks before anything outdoors. Early radishes ('French Breakfast') and a first sowing of carrots in a deep container are also worth a go.

By late April and May, the staging starts to empty as plants head out to the garden, leaving room for the main summer act. Just keep half an eye on the forecast — in a cold spring, hold tender plants under cover a little longer rather than rushing them out. The frost-date checker will tell you when your area is usually clear.

Summer: the main act

Summer is what most people picture when they think of a greenhouse, and for good reason. The warmth and shelter let you grow heat-loving crops that struggle outdoors in much of the UK — and the harvest is a genuine cut above the supermarket version.

Tomatoes are the headline crop. Planted into the border soil or large pots in May, they'll crop from July until the first frosts. Cordon varieties like 'Shirley' and 'Sungold' do brilliantly under glass; pinch out the side shoots and tie the main stem to a cane or string. For the full method, see growing greenhouse tomatoes, and for the wider crop, the main tomato guide.

Cucumbers love the humid warmth even more. An all-female F1 variety such as 'Carmen' or 'Femspot' will give you a steady supply of crisp, ridge-free fruit all summer — growing greenhouse cucumbers walks through training and watering them. Peppers and chillies are the other classic glasshouse summer crop: 'Hungarian Hot Wax' and 'Apache' chillies, or sweet peppers like 'Gypsy', ripen far more reliably under cover than out in a British summer.

Don't forget the herbs. A few pots of basil thrive in the warmth alongside the tomatoes — handy, since they're natural kitchen partners. Tuck them between the larger plants where they'll get the heat they crave.

Keeping the air moving and the plants watered is the whole game in high summer — there's a dedicated guide on keeping a greenhouse cool and watered if your space tends to bake.

Autumn: clear, sow and overwinter

As the summer crops wind down through September and October, autumn is about the changeover. Pick the last tomatoes and peppers (green tomatoes will ripen on a windowsill or in a drawer with a banana), then pull the spent plants out. Sweep up fallen leaves and debris, which harbour pests and disease over winter, and give the glass a wash inside to let in every scrap of the weakening light.

The cleared border and bench space is then prime real estate for hardy winter crops. From late August into October, sow winter salads and hardy leaves that will sit through the cold and give you pickings when the garden outside has nothing. Mizuna, mustards, winter purslane, land cress, lamb's lettuce (corn salad) and hardy lettuces like 'Winter Density' all do well — see winter salad leaves for a full list and the cut-and-come-again technique.

Autumn is also the moment to bring tender perennial herbs in out of the weather. Lift a clump of basil, or pot up rosemary, tender mint and any half-hardy plants you want to keep, and stand them under the glass. They won't grow much, but they'll survive the wet and cold far better than they would outside.

Winter: protection, hardy salads and planning

A winter greenhouse won't be lush, but it is far from idle. Its main job now is protection: keeping the frost off plants you've overwintered, and giving you a frost-free spot to store pots of tender plants, dahlia tubers and the like.

For the salads sown in autumn, the cold slows them to a crawl rather than stopping them — pick little and often on milder days and they'll keep ticking over. On the coldest nights, a layer of horticultural fleece draped inside the greenhouse over the crops adds a few extra degrees of protection, effectively a greenhouse within a greenhouse. It's cheap, quick to throw on, and makes a real difference on a hard frost.

Winter is also a good time to get garlic going in pots under cover. Autumn-planted garlic actually needs a cold spell to bulb up well, but starting cloves in deep pots or trays in the greenhouse protects them from soggy soil and hungry birds before you plant them out — see planting garlic in autumn vs spring for the timing. Broad beans and early peas can be started the same way for an extra-early crop.

Beyond the few live crops, winter is planning season. Use the quiet weeks to wash pots, order seed, and map out next year's sowings. The planting calendar is the easy way to plot what goes where, and it's worth checking your local last-frost timing on the frost-date checker before you commit to a sowing schedule. A little planning now is what keeps the greenhouse full all of next year.

Once you're confident with the rhythm, it's worth exploring the wider getting started guides to fit the greenhouse into your whole growing year.

A simple month-by-month summary

Use this as a quick at-a-glance plan. Timings suit most of the UK; nudge them a week or two later in the colder north and earlier in the mild south-west.

The pattern is the same every year: spring fills the bench with seedlings, summer fills the borders with fruit, autumn changes the guard, and winter ticks over while you plan the next round. Keep something growing in every season and an unheated greenhouse will pay for itself many times over.