Using a Greenhouse to Extend Your Growing Season

What a Greenhouse Really Buys You

A greenhouse does far more than keep the frost off. On a typical UK allotment or back garden, an unheated greenhouse effectively buys you four to six weeks at the start of the season and another four to six at the end. That is the difference between a few decent tomatoes and a proper glut, and between bare beds in November and a steady supply of salad leaves.

The shelter matters as much as the warmth. Seedlings raised under glass avoid the wind, the driving rain and the slug patrol that mow down direct sowings. Foliage stays dry, so damping-off and blight pressure are lower. Soil in raised pots or a border inside warms up earlier in spring, and that warmth is what actually triggers germination and root growth.

What a greenhouse will not do is work miracles in the dark. From November to January, light levels are the limiting factor, not temperature. Sow too early and you get pale, leggy seedlings that never catch up. The trick is to use the structure for what it does best at each point in the year.

Getting an Early Start in Late Winter and Early Spring

January and February are for the slow, heat-loving crops. Chillies, sweet peppers and aubergines need 18–21°C to germinate reliably, so a heated propagator on the greenhouse bench, or a sunny windowsill indoors, is the sensible route. Once germinated, move them somewhere bright and cooler to grow on steadily rather than stretching.

From late February into March, the greenhouse comes into its own:

• Sow tomatoes in modules, one seed per cell, and keep them at around 15–18°C.
• Start lettuces, radishes, spinach, spring onions and rocket for early salads.
• Sow hardy brassicas — cabbages, kale, sprouting broccoli — in modules for planting out in April.
• Start broad beans and early peas in deep root trainers to beat the mice.

Use modules rather than a big seed tray wherever you can. Roots suffer far less disturbance at planting time, and you avoid pricking out twenty tiny seedlings on a cold afternoon. Keep a maximum–minimum thermometer on the bench; it tells you far more than a glance out of the kitchen window.

Hardening Off Without Setbacks

The commonest mistake is to raise beautiful plants under glass and then plant them straight out into a cold wind. Give them ten to fourteen days of transition instead. Move them into a cold frame, or outside in a sheltered spot, opening the lid or removing fleece by day and closing it at night.

Reduce watering slightly during this period and let the plants toughen. Then plant out when the soil has genuinely warmed, usually late May for tender crops in most of the UK, and keep fleece or a cloche to hand for late frosts. Even a hardy brassica benefits from a week of protection after planting.

Stretching the Harvest into Autumn and Winter

Late summer is when an empty greenhouse earns its keep. As tomatoes finish, clear a bed or a few pots and sow winter salads: corn salad, land cress, mizuna, winter lettuce and claytonia all crop through the coldest months with minimal fuss. A row of overwintering spring onions or a parsley plant or two is a genuine luxury in February.

To push tomatoes to the finish line, stop sideshooting in late summer, remove the growing tip in late August so the plant concentrates on ripening what is already set, and strip lower leaves to improve airflow. Any green fruit can be ripened on a sunny windowsill or turned into chutney.

From October, line the greenhouse with bubble wrap. It costs a little light — roughly a tenth — but it lifts night temperatures by two or three degrees, which is often the difference between survival and mush. Fleece over plants inside the house adds another layer on the coldest nights.

Watering, Ventilation and the Daily Routine

A greenhouse can overheat in March as easily as it can freeze in November. Open vents once the temperature inside reaches about 15°C, and on sunny days open them early rather than waiting until the plants are wilting. Close them mid-afternoon to trap warmth for the evening.

Water in the morning, and water thoroughly rather than little and often — half-hearted watering encourages shallow roots and weak plants. Damp down the path on hot days to raise humidity and help pollination. In winter, do the opposite: water sparingly, because cold, wet compost is the fastest way to rot off overwintering plants.

Making the Most of a Small Greenhouse or Plot

Space is always the pinch point. Use staging with shelves beneath, growbags on the floor under the bench, and hanging baskets for trailing tomatoes or strawberries. One beefsteak tomato per growbag is plenty; two smaller bush varieties will manage.

Treat the greenhouse primarily as a nursery. Raise your transplants there, move them out onto a hardening-off bench, then use the freed space for the crops that genuinely need it — tomatoes, cucumbers, peppers, melons. Sow little and often in modules so a gap never stays empty for long, and keep a simple notebook of sowing dates and results. After a season or two you will have a rhythm that fits your own plot, your own soil and your own weather.