The UK greenhouse growing calendar: what to do every month of the year

A polycarbonate greenhouse does not have an off-season. From the first propagation sowings in January to the final overwintering preparations in December, every month of the year offers something worth doing — and the gardener who understands what is possible in each month gets dramatically more from their greenhouse than the one who treats it as a summer-only facility.

This calendar is written specifically for owners of KLASIKA and BALTIC LT polycarbonate greenhouses in the UK. The timing reflects the insulation advantage of twin-wall Brett Martin polycarbonate over glass and unheated outdoor conditions — the 3–5°C overnight temperature advantage that makes earlier starts and later finishes possible compared to outdoor growing. Tasks are differentiated between heated and unheated greenhouses where the distinction matters.

Work through the calendar from the month you are in, not from January. Each month stands alone as a practical guide to what is worth doing right now.

A note on raised seedbeds: the growing foundation

Before the monthly calendar begins, a word on the growing infrastructure that makes greenhouse cultivation most productive.

Raised seedbed sets designed to fit the internal widths of KLASIKA and BALTIC LT greenhouses transform the growing environment from a patch of ground into a managed, defined, and productive growing space. They are available in dimensions matched to the principal greenhouse widths in the range:

For 3m-wide greenhouses (KLASIKA ARCHED, BALTIC LT, KLASIKA TUBE, KLASIKA EASY), seedbed sets fit the internal width precisely, sitting on each side of the central path with no awkward gaps and no wasted ground. The soil within the raised beds warms faster in spring than open ground — a meaningful early-season advantage — and remains more workable through wet periods when open soil would compact underfoot.

For 2.35m and 2.5m-wide greenhouses (KLASIKA HOUSE, KLASIKA BERNARD, STANDART KLASIKA, KLASIKA DROP), equivalent sets fit the narrower widths with the same precision. The slightly smaller footprint of these greenhouses makes the defined bed structure even more valuable — every usable centimetre is accounted for.

Raised seedbeds also eliminate the edge weeding that open beds require, keep soil contained within defined areas, and give the greenhouse interior a neat, purposeful appearance year-round.

The nursery/seedbed with polycarbonate cover

The nursery/seedbed with polycarbonate cover (100×93×38cm, covered with 4mm polycarbonate) is the most versatile single piece of growing equipment you can add to a greenhouse. Used inside a polycarbonate greenhouse, it creates a microclimate within a microclimate — the 4mm polycarbonate cover holds warmth and humidity around germinating seeds and emerging seedlings, maintaining higher temperatures than the surrounding greenhouse air without any additional heating.

In practice: seeds sown in the nursery seedbed in January or February, in an unheated greenhouse where overnight temperatures might drop to 4–6°C, germinate in the warmth held under the cover as if they were in a minimally heated space. The cover lifts for ventilation and watering and replaces easily. When seedlings are large enough to be potted on or planted into the main greenhouse beds, the nursery seedbed becomes a propagation bay for the next batch of sowings.

This is the piece of equipment that makes true year-round growing possible without heating infrastructure — used in combination with a polycarbonate greenhouse, it starts seeds weeks earlier than any other unheated method.

January: the year begins underground

January is the month most gardeners consider the greenhouse dormant. It is not — it is the month when the decisions that determine the success of the entire growing season are made.

Heated greenhouse (frost-free minimum, 5°C+)

Sow: Pelargoniums — one of the longest-lead-time crops, needing 16–20 weeks from seed to flowering. January sowing produces bedding-ready plants by May. Sow thinly in modules or a seed tray in the nursery/seedbed, cover lightly, and keep the cover closed until germination.

Sow: Sweet peppers and chilli peppers. These are slow from seed — they need time and warmth to establish, and an early January start gives them the best possible growing season. Germinate in the nursery/seedbed at the warmest point of the greenhouse.

Start: Forced bulbs brought into the greenhouse in December will be producing their first shoots. Water sparingly and ensure they receive maximum available light.

Prepare: Clear any remaining crop debris from the beds. If the autumn disinfection was not completed, carry it out now before any new sowings begin.

Unheated greenhouse

Sow: Hardy annual flowers for early spring colour — calendula, cornflowers, nigella — in the nursery/seedbed under its polycarbonate cover. The cover creates sufficient warmth for cold-tolerant germination in most UK regions.

Prepare beds: Turn over the soil in raised seedbed sets if they were not cleared in autumn. Add well-composted material to the top layer and allow it to settle over the remaining cold weeks.

Inspect: Walk through the greenhouse and check all panels, end caps, and glazing bar seating after any heavy frost. Cold can cause expansion at panel edges — any panel that has shifted slightly in its channel should be re-seated before the growing season begins.

February: the propagation engine starts

February is when the greenhouse earns its keep for the whole year. A polycarbonate greenhouse in February is 5–8°C warmer than outdoors — enough to germinate many of the season’s most important crops weeks ahead of outdoor conditions.

Heated greenhouse (5°C+ minimum)

Sow: Tomatoes — the flagship greenhouse crop. For a heated greenhouse, mid-February is the ideal sowing time. Sow two seeds per module or small pot, germinate in the warmest location available (ideally on a heat mat in the nursery/seedbed), and thin to the stronger seedling when the first true leaves appear. February-sown tomatoes will be planting-ready by mid-April in a heated greenhouse.

Sow: Aubergines. These need even more warmth to germinate than tomatoes — use the nursery/seedbed or heat mat and do not expect rapid germination at temperatures below 20°C. An early February start compensates for their slow pace.

Pot on: January-sown peppers and chillies that have their first true leaves should be potted on into individual 7cm or 9cm pots. Return to the nursery/seedbed or a warm shelf close to the glass.

Unheated greenhouse

Sow: Broad beans — these are genuinely cold-tolerant and will germinate in an unheated greenhouse when outdoor sowing would fail. Sow direct into the raised seedbed or in root trainers for transplanting out in late March.

Sow: Onion sets and salad leaves. Winter-hardy salad varieties — landcress, corn salad, hardy winter lettuce — will germinate slowly but reliably in the warmth of the unheated greenhouse and provide pickings from early April.

Sow: Spinach and chard direct into the raised seedbeds. These will establish slowly in February’s low light but will be producing useful quantities from late March.

Use the nursery/seedbed: Tomato seeds can be started in mid-to-late February in an unheated greenhouse in the nursery/seedbed in most UK regions south of Scotland. The cover holds enough warmth for germination to occur, though it will take longer than in a heated space — allow 10–14 days rather than the 7–10 days of a heated greenhouse.

Force: Chicory roots potted up and covered in the nursery/seedbed produce forced chicons — the pale, tight-headed forced leaves that are a February delicacy and would otherwise require a heated cellar.

March: the greenhouse fills up

March is the busiest month in the greenhouse calendar. The increasing day length and rising sun angle mean more light and more warmth, and the growing list expands significantly.

All greenhouses

Sow: Cucumbers — for a May planting date in the greenhouse, March is the ideal sowing time. Cucumber seeds germinate quickly in warmth (ideally 24°C) — use the nursery/seedbed for consistent germination. Sow one seed per 9cm pot.

Sow: Summer squash, courgettes, and pumpkins intended for early outdoor planting — sowing in mid-to-late March in the greenhouse gives a four-week advantage over outdoor sowing and produces transplantable plants ready for the last frost date.

Sow: Celery and celeriac — both slow from seed and benefiting from an early start. Sow in seed trays under the nursery/seedbed cover.

Pot on: February-sown tomatoes should now be at the stage of having their first or second set of true leaves. Pot on into 9cm pots and give them the best-lit position in the greenhouse.

Plant: Onion sets into the raised seedbeds for the earliest possible outdoor-quality onion harvest.

First beds: Begin warming the raised seedbeds in earnest. If you have not already done so, cover the beds with black polythene or a layer of horticultural fleece laid directly on the soil surface for two to three weeks before planting — this accelerates soil warming by several degrees and makes a measurable difference to early plantings.

Unheated greenhouse check: By late March, overnight temperatures in the greenhouse should be consistently above freezing in all but the most exposed UK locations. The propagation window using the nursery/seedbed expands significantly.

April: the first plantings

April is transition month — the point at which the greenhouse begins to function as a full growing environment rather than just a propagation space.

All greenhouses

Plant out: Tomatoes into the greenhouse raised seedbeds or growing bags. For a 3m × 6m greenhouse, plant two rows of cordon tomatoes — one down each side — with plants at 45–50cm spacing. Train the main stem vertically and tie to a string or cane attached to the ridge or an overhead wire. Remove all side shoots as they appear to maintain the cordon habit.

Plant out: The first cucumber plants — late April for most UK greenhouses. Cucumbers need warmth at the root and will sulk if night temperatures drop below 10°C. If late frosts are still possible, keep a sheet of fleece to hand for overnight protection in the first week or two after planting.

Ventilate: April sun can raise greenhouse temperatures very quickly on bright days. Roof vents should be open by mid-morning on any sunny day from now on, and the door open in the afternoon on warm days. This is the month when automatic vent openers earn their keep — without them, a sunny April morning followed by a cloud and cold afternoon requires manual vent management throughout the day.

Harden off: Brassica plants and other hardier vegetables that were started in the greenhouse in February and March should begin the hardening-off process in April — progressively exposing them to outdoor conditions to prepare them for planting out in May.

Sow: French beans, runner beans, and sweet corn in pots in the greenhouse for planting out in mid-to-late May after the last frost.

May: full production begins

May is when the greenhouse transforms into the productive growing environment the whole year has been building toward.

All greenhouses

The greenhouse is full. By mid-May, a well-planted greenhouse should have tomatoes establishing on both sides, cucumbers beginning to climb, possibly peppers and aubergines in the prime positions, and shelving carrying pots of basil, herbs, and any overflow plants awaiting outdoor bed space.

Training: Tomato side shoots should be removed every seven to ten days throughout May. The main stem should be tied in regularly as it extends — to strings or canes attached to the frame at the ridge. The galvanised steel frame takes this load without concern; a plant tie or string looped around an arch section or tied to a ridge bar is a firm anchor point that will hold a full season’s growth comfortably.

Cucumber training: Cucumbers grow fast in May. Train the main stem upward and remove all the lateral shoots for the first 45–50cm from the ground to encourage upward growth. Above this point, allow laterals to develop and tie them horizontally to wires run between the arch sections.

Shade: From mid-May onwards, apply the greenhouse shading solution to the roof panels and south-facing sides when daytime temperatures are regularly above 25°C inside the greenhouse. Early application prevents the heat stress on establishing young plants that can set back growth significantly.

Outside sowing: The greenhouse’s role in May is also to produce plants for the outdoor garden. Runner beans, sweet corn, squashes, French beans, and basil are all sown in the greenhouse in May for outdoor planting in late May or early June.

June: peak management

June is the month of maximum growing pace and maximum management demand.

All greenhouses

Ventilation daily. The greenhouse needs active ventilation management throughout June — roof vents open, door open on warm afternoons, any available through-draught maximised. Temperatures inside an unventilated greenhouse in June sun can reach 45°C and above — lethal to plants within an hour.

Watering. In June, greenhouse crops — particularly tomatoes and cucumbers — may need watering daily in warm conditions. The raised seedbed structure within the greenhouse helps water retention and reduces the frequency of watering compared to open beds or bags, but daily checking of soil moisture is essential in warm weather. If you have an irrigation system connected to the greenhouse, June is when its value is most apparent.

Feed: Begin regular feeding of tomatoes once the first truss of fruit is set — not before. Use a high-potash tomato fertiliser weekly. Cucumbers benefit from a balanced general-purpose liquid feed every ten to fourteen days once they are in active production.

Pest watch. The first red spider mite populations of the year typically establish in June on the undersides of cucumber and tomato leaves. Dry, warm greenhouse conditions favour them — maintaining adequate humidity by misting foliage or wetting the floor is a simple cultural preventive measure. Check leaf undersides weekly.

July: the harvest begins

July is the first serious harvest month — and often the point at which gardeners realise they needed a bigger greenhouse.

All greenhouses

Harvest regularly. Tomatoes, cucumbers, and peppers should be picked as they ripen — not left on the plant past peak ripeness. Regular harvesting encourages the plant to continue setting new fruit rather than diverting all its energy into maturing the existing crop.

Train and tie. Tomatoes in a 3m × 8m greenhouse may have their leading stems at roof height by mid-July if they have grown well. When the plant reaches the ridge, the stem can be lowered by removing the lowest training string and allowing the plant to hang slightly, effectively giving it another 30–40cm of growing height before pinching out becomes necessary.

Side shoots: Without fail, every seven to ten days. A missed week of de-shooting in July can set a tomato plant back significantly as energy diverts into unwanted laterals.

Sow: Autumn and winter salad crops — winter lettuce varieties, land cress, lamb’s lettuce, spinach — can be sown in late July for transplanting into the raised seedbeds in late August after the summer crops are clearing. July-sown winter lettuce will be providing pickings from October.

Begin propagation for overwintering: Take cuttings of tender perennials — pelargoniums, fuchsias, begonias — in mid-to-late July. Root them in small pots on the staging and they will be well-established plants ready for overwintering in the greenhouse by October.

August: production peak, autumn eye

August is the height of greenhouse productivity — and the month to start thinking about autumn.

All greenhouses

Maximum harvest: Tomatoes, cucumbers, peppers, aubergines, and chillies should all be in full production. Pick every two to three days.

Clear cucumber plants that are showing signs of collapse — yellowing heavily from the base up, with foliage looking tired and production reduced. A cucumber plant past its productive peak occupies valuable space that could be cleared and replanted with autumn crops. If you have a second batch of cucumber plants that were started later, this is the transition point.

Sow: Oriental leaves, rocket, winter purslane, and hardy annual herbs for autumn and winter production. August-sown oriental leaves produce abundantly in the warmth of late summer and continue through autumn.

Plant: Winter brassica transplants — kale, chard, winter spinach — into the raised seedbeds as summer crops clear. In a 3m × 6m greenhouse, one end of the greenhouse can be cleared and planted with winter crops in late August while the main summer crops continue at the other end.

Shading: Begin to assess whether the shading solution is still needed. In late August, as the sun angle drops, too much shading can start to restrict light to the extent that it slows ripening of late tomato trusses. Remove the shading from the roof panels if overnight temperatures are dropping and ripening is becoming a concern.

September: the season extends

September is where the investment in a quality polycarbonate greenhouse pays its most tangible dividend — the growing season continues when outdoor production has effectively ended.

All greenhouses

Tomatoes: The final push. Any remaining green tomatoes above the highest formed truss should be removed — the plant cannot ripen more fruit than it currently has set. Remove any diseased or yellowing foliage to improve air circulation and slow the botrytis risk that cool, humid September conditions create.

Close vents earlier. From September, begin closing roof vents earlier in the afternoon — by 3–4pm rather than at dusk. Trapping the afternoon heat in the greenhouse extends the warm period into the evening and makes a measurable difference to the overnight temperature retention that the insulating panels provide.

Salad crops peak. The August and late July sowings of winter salad are now producing reliably. Lamb’s lettuce, land cress, winter spinach, and hardy Asian leaves are at their best in September and October greenhouse conditions.

Clean the nursery/seedbed. With the summer’s propagation work complete, clean the nursery/seedbed thoroughly with the universal surface cleaner and allow it to dry. It is ready for its autumn and winter roles — forcing chicory, starting overwintering herb divisions, and hosting January–February sowings.

Harvest chillies: Chillies ripen in the warmth of September. Those that have not coloured can be brought inside to ripen on a warm windowsill, or left on the plant until the first frost risk concentrates their heat further.

October: the handover

October is the month the greenhouse transitions from summer production to autumn and winter mode.

All greenhouses

Clear summer crops. Once the first outdoor frost occurs (or when tomato plants are clearly finished — all trusses harvested, foliage yellowing comprehensively), clear the greenhouse of summer crops completely. Remove plants root and all, clean growing bags or raised seedbed soil of old roots, and remove all crop debris.

Pot up tender perennials for overwintering — pelargoniums, fuchsias, cannas, dahlias, begonias. Cut back, pot into fresh compost, and water in. These will overwinter successfully in a frost-free greenhouse with minimal attention. If the greenhouse is unheated, a fleece cover over the pots on the coldest nights provides the marginal frost protection they need.

Plant overwintering crops into cleared raised seedbeds: hardy spinach varieties, over-wintering onion sets, winter salads. In a 3m × 6m or larger greenhouse, a full planting of overwintering crops can supply useful quantities of greens through the quiet months.

Autumn external clean. Carry out the full external panel wash with the universal surface cleaner — removing the summer’s algae accumulation and the shading solution residue. The clean greenhouse going into autumn and winter is both more attractive and more productive, transmitting maximum light when days are short and light is the primary limiting factor on growth.

Start autumn disinfection. After crops are cleared and before overwintering plants are introduced, carry out the full internal disinfection with the concentrated greenhouse disinfection solution. This is the most important annual maintenance task.

November: quiet preparation

November is the quietest month in the greenhouse calendar — a natural pause between the close of the summer season and the beginning of the new growing year.

All greenhouses

Overwintering plants settled in. The greenhouse is now populated with overwintering tender perennials, late salad crops, and possibly forced bulbs in pots. Watering frequency drops dramatically — overwintering pelargoniums and fuchsias want very little water in November and will rot if kept wet.

Check the heater. If you use a frost-stat heater, confirm it is operating before the coldest weather arrives. A five-minute test on a mild day — setting the thermostat above the current temperature to confirm the heater fires — is the maintenance action that prevents catastrophic plant loss on a cold night.

Cover tender plants. A sheet of horticultural fleece draped over overwintering pot plants on the coldest nights provides meaningful frost protection in an unheated greenhouse. The polycarbonate greenhouse envelope does the heavy lifting — the fleece provides the additional degree or two of protection that makes the difference on a -3°C night.

Sow: Broad beans into deep pots or root trainers for an extra-early crop. November-sown broad beans in an unheated greenhouse will have established root systems that give them a head start when February arrives.

Rest the raised seedbeds. Beds that are not being used for overwintering crops should be covered with a layer of well-rotted compost or manure laid on the surface. This can be incorporated in spring and will have broken down to a workable condition by February.

December: winter maintenance and planning

December is a month of minimal active growing and maximum planning — the ideal time to review the season that has just ended and prepare for the one about to begin.

All greenhouses

Minimum intervention. Water overwintering plants only when the compost has dried out completely — this may mean watering once per fortnight or less in December. Overwatering in winter is the primary cause of loss for overwintering tender perennials. When in doubt, do not water.

Light is limiting. The short days of December mean even a clean, well-positioned greenhouse is receiving the lowest light levels of the year. This is not the time to start sowings — germination in low light produces leggy, weak seedlings that are never as productive as those started in February when day length is increasing. Wait.

Plan next year’s growing list. December is the planning month. Review what worked and what did not in the season just ended. Which tomato varieties produced best? Did the cucumbers run out of headroom? Was the greenhouse large enough? The answers to these questions inform the seed order for next year and, if the greenhouse proved too small, the case for an extension module.

Order seeds. The best seed varieties sell out. Ordering in December ensures access to the full range from quality seed suppliers and gives any slow-germinating crops (pelargoniums, aubergines, peppers) the maximum number of weeks before their January or February sowing date arrives.

Physical inspection. Carry out a full structural check in December — ideally before the worst winter weather arrives. Panel end caps, glazing bar seating, aluminium tape on top panel edges, frame condition at ground level, door and vent operation. Any issues found now can be addressed before the spring growing season begins.

The nursery/seedbed is ready. Cleaned in September, the nursery/seedbed is sitting ready for its January first use. Check that the 4mm polycarbonate cover is intact and clean — this is the piece of equipment that makes January propagation possible, and it should be in good order before the first sowings of the new year.

The growing year in a single view

Accessories that make this calendar possible

The growing tasks in this calendar are most achievable with the right growing infrastructure. The products most directly relevant to year-round greenhouse growing:

Raised seedbed sets — sized to fit 3m and 2.35/2.5m greenhouse widths. The foundation of the productive growing environment for all crops from March onwards.

Nursery/seedbed with polycarbonate cover (100×93×38cm) — the essential tool for January and February propagation. Makes seed starting in an unheated greenhouse possible months earlier than any other unheated method.

Automatic wax-cylinder vent openers (THERMOVENT, VENTOMAX, UNIVENT) — Danish-made, no electricity required. The single most important accessory for active-season temperature management from April through September.

Hanging shelves (45cm wide, 75/105/200cm lengths) — for staging seedling trays, pot plants, and herbs above the main beds, maximising the productive use of the greenhouse volume.

Plant binding sets — for tying tomatoes, cucumbers, and climbing crops to the steel frame. Ordered by greenhouse length to provide the full support system from base to ridge.

Irrigation systems — drip or capillary, connected to mains or gravity-fed from a water tank. The June and July watering demand in a productive greenhouse is substantial; an irrigation system removes the daily hand-watering obligation and ensures consistent moisture levels that uniform fruit development requires.

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