UK greenhouse growing calendar

How to use this calendar

The table below is a planning framework for a typical UK unheated greenhouse, not a precise instruction for every postcode or crop. A heated building, a mild coastal garden and an exposed inland site will run differently. Seed packets, measured night temperatures and short-range weather forecasts take priority. The RHS likewise treats greenhouses as season-extending structures while noting that year-round heated production is rarely cost effective for home growers.

Late winter: prepare before accelerating

January and February are useful for inspecting glazing, cleaning reachable surfaces and checking doors, vents and staging. Remove diseased residue and wash reused seed trays. A greenhouse can be warmer than outdoors at lunchtime yet offer little protection from a hard overnight frost, so put a max/min thermometer at plant height and record several nights.

The RHS recommends sowing hardy vegetables such as lettuce, onions, peas and brassicas for later planting out. Tender tomatoes or peppers can begin early only when germination warmth and a light, frost-free growing-on area are available. A heated propagator warms a small zone but cannot make the entire greenhouse suitable for newly emerged plants. See how to start seeds in a greenhouse.

Spring: two competing uses of space

March and April bring more light but not an end to frost risk. Sow, prick out and pot on in batches. Fast-growing cucumbers, squash and courgettes should not be started so early that they become root-bound before final conditions are safe. A tray-heavy bench now may need to disappear when mature summer crops require the same light and headroom.

Ventilate as the sun strengthens. Closed glazing can overheat seed trays even when outdoor air is cool. Keep watering proportional to compost need and ensure covered propagators do not stay permanently saturated. As outdoor-bound plants develop, harden them off gradually rather than moving them from a warm greenhouse to exposed beds in one step.

Early summer: commit the final positions

Plant tender crops into greenhouse beds, suitably sized containers or growing bags when night conditions reliably support them. Tomatoes, cucumbers, peppers and aubergines have different space and warmth needs, so choose a realistic mix. Fit supports before roots fill the ground and leave a usable route to every watering point and vent.

Our what-to-grow guide compares crops by job and space. For the most common greenhouse crop, the tomato guide covers cordons, containers, watering and feeding.

High summer: maintain rather than just harvest

In June through August, check water daily in hot spells, especially in containers. A timed drip system may help but still needs inspection for blocked emitters and leaks. Open suitable vents and doors, use shade when ventilation alone cannot keep leaves comfortable and train climbing crops before they block airflow.

Look under leaves as well as at fruit. Whitefly, red spider mite and aphids can multiply quickly in warm conditions. Sticky cards can give early warning of some flying insects, but identification and an appropriate response matter more than a card covered with pests. Our UK greenhouse pests guide explains the first checks.

Late summer and autumn: sow into the gap

As crops finish, remove them rather than leaving exhausted foliage in place. The RHS suggests late sowings of lettuces, spicy salad leaves and baby carrots where space allows, then hardy lettuces, pea shoots and overwintering peas or broad beans in autumn. The key is to establish plants while light remains; seed sown very late in an unheated house may sit rather than grow.

Reduce irrigation as temperature and day length fall. A winter salad plant in cool compost needs less water than a July tomato. Clean empty staging and wash pots before storage or reuse. This is also a good moment to note which summer crops justified their footprint.

Winter: protect established plants and reset

Expect slow growth and harvest conservatively. Ventilate during suitable mild periods to reduce stagnant moisture, then close up before temperatures fall. Check frost protection and remove damaged leaves promptly. Inspect gutters, glazing and anchors after severe weather without climbing on the greenhouse or leaning on panels.

Use the quiet period for a greenhouse maintenance checklist and hygiene reset. Record which sowings were too early or too late for your garden. That local record will improve this general calendar far more than a rigid national date.

Sources and further reading

The sequence follows RHS advice on growing vegetables in greenhouses, including its late-winter, mid-spring, summer and autumn crop suggestions. Consult each crop's RHS guide and the seed packet for specific sowing and planting needs.