A Month-by-Month Sustainable Greenhouse Plan for UK Growers

A greenhouse does not cancel the British seasons, it softens them. The same structure that gives you ripe tomatoes in July can become a damp, airless box by November if nobody is paying attention. The trick is to work with the year rather than against it, adjusting what you sow, how much you water and how wide the vents stay open as light and temperature shift.

What follows is a working plan, month by month. Treat it as a framework rather than a rulebook. A greenhouse in Cornwall and one in Aberdeenshire behave very differently, and a small lean-to holds heat in a way that a large freestanding glasshouse never will.

Late winter: waking the greenhouse up (January–February)

January is quiet and that is a good thing. Light levels are still low, so anything sown now tends to grow slow and leggy unless you have a propagator or supplementary lighting. Use the month for maintenance instead: wash the inside of the glass with a mild soapy solution, check glazing clips and seals, and service vents and automatic openers so they are ready before the first warm day catches you out.

By late February, start the slow, sturdy crops — onions and leeks, early brassicas, sweet peas, and hardy salad leaves such as winter lettuce and mizuna. Most of these germinate happily at around 18–21°C, so move seedlings somewhere cooler and brighter the moment they are up.

Water sparingly. Damping-off fungus thrives in cold, wet compost, so water in the morning, use tepid water rather than icy tap water, and let the surface dry a little between waterings. On a sunny February day the greenhouse can climb well above outside temperatures by noon and then drop below freezing at night. Open the vents for an hour mid-morning and close them by mid-afternoon.

Spring: the busiest stretch (March–May)

March is for sowing tomatoes, peppers, chillies and aubergines under cover. April is for hardening off, potting on, and planting tomatoes into grow bags or border soil once night temperatures stay reliably above about 10°C, which usually means late April in the south and mid-to-late May further north. By May the greenhouse is full: pinch out the side shoots of cordon tomatoes, tie in the stems, and begin feeding once the first truss has set.

Watering in spring is an exercise in restraint. Plants want consistent moisture, not a daily flood, so aim for a deep watering every couple of days rather than a light sprinkle every morning. A drip line on a timer earns its keep if you are away during the week.

Ventilation matters more than most growers expect. Automatic roof vents pay for themselves in spring, opening on any bright day and closing before evening so you keep the warmth overnight. Fork over border soil, work in a couple of inches of well-rotted compost, and avoid planting tomatoes in the same soil year after year. Rotating to beans, cucumbers or salad crops reduces the build-up of soil-borne problems.

Summer: heat, water and airflow (June–August)

This is when the greenhouse earns its keep, and when things can go wrong quickly. On a still, sunny day the internal temperature can climb far above the outside air. Shade the sunniest side with netting, blinds or a wash-on shade, damp down the path in the morning to raise humidity, and keep vents open from early morning.

Check watering twice a day in a heatwave. Water early, never let tomatoes or cucumbers wilt in the middle of the afternoon, and mulch with compost or grass clippings to slow evaporation. Feed tomatoes and peppers weekly once flowering, switching to a high-potassium feed as the fruit begins to swell.

Keep an eye out for red spider mite in hot, dry spells — fine webbing and pale speckling on the leaves — and raise humidity if you spot it. Whitefly and botrytis are the other regular visitors. Stripping the lower leaves of tomatoes improves airflow and slows the spread of spores. Harvest courgettes and cucumbers young and often, and sow a late crop of salad leaves and radish in the shade of taller plants.

Autumn: sowing for winter and cleaning up (September–November)

September gives you a second season. Clear spent crops, top up the compost, and sow winter salad — rocket, mizuna, corn salad, winter lettuce, spinach — plus spring onions under cover. These will crop through mild spells if you give them a little protection later on.

October is harvest and clean-up time. Ripen the last green tomatoes on a windowsill, or turn them into chutney. Wash pots and trays, sweep the floor, and empty saucers so they do not hold stagnant water. This is also the best moment for the big clean: wash the glass inside and out to let in maximum winter light, remove shade netting and wash-on shading, and check for loose panes before the gales arrive.

Do not leave border soil bare over winter. Sow a green manure such as field beans or grazing rye if the space will be empty, or spread a layer of compost and let the worms work it in.

Winter: protection and planning (December)

Growth slows to a crawl. Ventilate on mild, dry days to keep mould at bay, but keep the door shut otherwise. Water very little — once every week or two for most plants, and only when the compost is dry below the surface. Never water on a frosty day. Bubble wrap fixed to the inside of the glazing holds a few degrees of frost off tender plants, and a layer of fleece over a simple frame will carry winter salads through the coldest nights. Use the quiet weeks to note what worked, order seeds, and clean your tools.

A five-minute monthly routine

Whatever the month, the same four questions keep a greenhouse in good order. Run through them on the first day of each month:

1. What can I sow now? Check light levels and heat before you sow anything tender.
2. How much water does this need? Less in winter, more in summer, always in the morning.
3. How is the air moving? Open vents early, close them in good time, and watch for still, damp corners.
4. What is happening to the soil? Feed, mulch, rotate, and never leave it bare for long.

Adjusting the plan to your own plot

Keep a small notebook in the greenhouse and jot down the date you sowed, the date seedlings appeared, and the first harvest from each crop. After two or three years you will have a schedule tailored to your own garden, which is worth more than any generic calendar. If you garden in the north, push everything back by a fortnight; in a sheltered southern spot, you can often bring it forward. The plan above is a starting point, not a straitjacket — the best greenhouse growers are simply the ones who notice what their plants are telling them and adjust accordingly.

Photo: scholty1970 / Pixabay