Sow an all-female F1 cucumber in April at about 21°C, plant it into an unheated greenhouse in late May once nights hold above 12°C, train the stem up a single string, keep the compost evenly moist, switch to a high-potash feed when the first flowers open, and cut fruit from July at 20 to 25 cm.
That, in one paragraph, is how to grow cucumbers in a greenhouse in the UK. The reason the greenhouse matters at all comes down to arithmetic. Cucumbers reach full production only once temperatures hold at 18°C or above, and the average July day in England peaks at 21.2°C (Met Office, 1991 to 2020). Outdoors the crop gets a few marginal weeks in a kind summer. Under cover it gets four months, from the first fruit in July to the last one in October.
Start with an all-female variety
Greenhouse seed racks hold two kinds of cucumber. All-female F1 hybrids such as 'Carmen' set fruit without any pollination, which is exactly what you want under glass: a pollinated greenhouse cucumber fills with seed, bulges at one end and turns bitter. Older open-pollinated varieties, 'Telegraph Improved' being the familiar name, carry male flowers as well, so through the whole summer you have to inspect the plant every few days and remove each male flower before an insect reaches it.
An F1 packet costs more per seed and repays you at picking time. The RHS trialled greenhouse varieties and gave these its Award of Garden Merit.
| Variety | Fruit | RHS trial verdict |
|---|---|---|
| 'Carmen' | Long, straight | Large crop, good disease resistance |
| 'Emilie' | Medium, smooth-skinned | A plentiful, steady supply |
| 'Mini Munch' | Snack size, picked at about 8 cm | Heavy cropper over a long season |
| 'Delistar' | Medium, thin translucent skin | Easy to digest, no peeling |
| 'Lili' | Large; happy under glass or outdoors | Predominantly female flowers, reliable crop |
Most modern greenhouse varieties are all-female, and the packet will say so. Check before you buy, because the male-flower job on an old variety runs from June to September. Grafted plants have also become easier to find. The graft buys extra vigour and a larger crop over a longer season, and it stands up to the soil-borne diseases that build up when cucumbers go into the same border year after year. Against that, grafted plants cost more and the variety choice stays narrow.
When to sow greenhouse cucumbers in the UK
Sowing time follows the heat you can offer the plant later, and for heated structures it runs earlier than most people expect.
| Where the plant will fruit | Sow | Plant into its final spot |
|---|---|---|
| Heated greenhouse | Mid-February to mid-March | From late March |
| Unheated greenhouse or polytunnel | April | Late May |
| Outdoors, warm and sheltered | Late April | Early June, after hardening off |
The sowing itself takes five minutes. Fill a 10 cm pot with peat-free seed compost, water it, then push one seed in on its side, 1 to 2 cm deep. Hold the pot at 21°C in a heated propagator, or under a clear plastic bag on a warm windowsill, and seedlings show within a week or two. Commercial nurseries run their germination rooms nearer 29°C and see emergence in two to three days (University of Florida), so extra warmth buys speed, though at home the slower timetable costs you nothing.
Cold compost is the commonest cause of failure. Cucumber seed refuses to germinate in cold soil. The University of Minnesota's extension service puts the working floor for direct sowing at a soil temperature of 21°C, below which seed sits in the wet and rots. If you only want one or two plants, garden centres sell young cucumber plants through spring and early summer, and buying them sidesteps the germination question entirely.
Planting out into the greenhouse
Move plants on about a month after sowing, once they grow strongly and before they turn leggy in the pot. For an unheated greenhouse that means late May across most of the UK, when the temperature inside stays above 12°C through the night. A heated house takes plants from late March.
Prepare a border a few weeks ahead by digging two bucketfuls of garden compost or well-rotted manure into each planting site, with sites 30 cm apart. In containers, give each plant a pot at least 30 cm wide and deep filled with peat-free multi-purpose compost. A standard growbag takes two plants. Water well before planting and keep the rootball intact, because cucumbers resent root disturbance and stall for a fortnight after a rough transplant.
One RHS habit worth copying from day one: sink an empty plant pot into the soil beside each cucumber and water into that instead of onto the surface. Moisture travels straight down to the roots while the neck of the stem stays dry, and a dry stem base spares you the rots that start where water pools.
Training a cordon up a string
Under glass, a cucumber grows as a cordon: one main stem trained vertically, with fruiting side shoots coming off it. Hang a string from the frame above each plant and secure it at ground level, or push in a sturdy cane that reaches the roof. Tie the young stem in loosely at the start; once its tendrils take hold the plant climbs mostly by itself, though a stem carrying half a kilo of fruit appreciates an extra tie. Vertical training earns its keep twice over, since a hanging fruit grows straight (the University of Minnesota notes trellised cucumbers come out "perfectly straight") and never touches damp soil.
Three pinches keep the cordon productive. When the growing point reaches the roof, pinch it out. When a side shoot carries a female flower, the one with a tiny cucumber behind it, pinch that shoot two leaves past the flower. And when a side shoot reaches 60 cm without flowering, pinch its tip as well.
Headroom decides how long the cordon runs before that first pinch. Every size of the NORDIC timber greenhouse is walk-in, from the £999 8 × 6 ft to the £1,549 8 × 20 ft, and a 45 × 45 mm pine frame takes a screw-in hook for a string at whatever point you decide the row should hang.
Watering and feeding rhythm
Steady moisture beats generosity in bursts. Check the compost daily from June and keep it evenly moist. Through hot spells, greenhouse and container plants need water every single day. As a benchmark, plants in open ground want around 25 mm of water a week (University of Minnesota), and a growbag under polycarbonate dries far faster than any open border. Water at the base in the morning, keep the leaves dry, and use rainwater from a butt when you have it.
Feeding runs to a calendar. Pots and growbags take a general liquid feed every 10 to 14 days, switched to a high-potash tomato feed at the same interval once the first flowers open. Border plants in well-prepared soil usually need nothing more: the compost dug in during May carries them through.
| Job | How often |
|---|---|
| Check moisture, water at the base | Daily, and never skip a day in a hot spell |
| Liquid feed (pots and growbags only) | Every 10 to 14 days (high-potash once flowering starts) |
| Tie in, pinch side shoots | Weekly |
| Damp down the path | Hot mornings |
| Cut fruit | Every 2 to 3 days at peak |
Two pointers for when the routine breaks. A July holiday is survivable with preparation, and our guide to watering a greenhouse while you're away sets out the options, from sunken reservoirs to a neighbour with a key. And if leaves start to yellow despite a steady rhythm, work through cucumber leaves turning yellow and come back: the causes and their fixes live on that page, not this one.
Heat and humidity: what cucumbers prefer
The working band is 18 to 25°C. Below a sustained 18°C, production slows, which is the floor the University of Florida sets for maximum output in commercial houses. At the other end, the RHS advises shading once summer pushes the greenhouse past 25°C, and prolonged spells above roughly 35°C cost you both fruit numbers and fruit quality. Between those limits, warmth plus light equals cucumbers.
Moist air suits them too, and raising it takes no equipment. On hot mornings, pour a full watering can over the path or concrete floor and let it evaporate through the day: the RHS recommends exactly this damping-down, and growth stays even while fruit swells smoothly. Humid air still has to move, though. Vents need to work from morning onwards, and our greenhouse ventilation guide covers how to hold humidity and airflow in balance through a UK summer. If a white coating ever appears on the leaves, go straight to powdery mildew on cucumbers. That subject has its own page and its own fixes.
Harvesting: little and often
Greenhouse cucumbers crop from mid-summer to mid-autumn. Cut rather than pull, taking the stem cleanly with a knife or secateurs, and pick at the size the variety was bred for: about 8 cm for minis, 15 cm for small-fruited sorts, 20 to 25 cm for full-sized ones. Check every couple of days at the peak of the season, because fruit swells fast, and an over-ripe cucumber turns yellowish, bulbous and soft at the tip.
Regular picking keeps the plant fruiting; leave one fruit to fatten past its point and the plant diverts its energy into ripening seed instead of opening new flowers. For scale, the long European type grown commercially is cut at 30 to 35 cm and weighs about 450 g (University of Florida). One more figure from the same handbook deserves a wider audience: cucumbers store best at about 13°C, and a fridge held at 4°C actually chills them into pitted skin and early softening, so a cool larder beats the salad drawer.
The whole season, compressed: seed into a 10 cm pot in April, into the border on the last weekend of May, first pinch at the roof around midsummer, then a fruit every two or three days from mid-July until October closes the house down for the year.
Greenhouse cucumber FAQs
When should I sow cucumber seeds in the UK?
Mid-February to mid-March for a heated greenhouse, April for an unheated one, late April for plants that will crop outside. All three assume sowing indoors at around 21°C. Direct sowing in the garden only works from late May or early June, once the soil itself has warmed to about 21°C.
Do greenhouse cucumbers need pollinating?
They don't, and under glass pollination actively harms the crop. Greenhouse varieties are mostly all-female and set fruit on their own, while a fruit that does get pollinated turns bitter and seedy. Remove any stray male flower, recognisable by the absence of a tiny fruit behind the petals. Outdoor ridge varieties work the opposite way: insects must carry pollen between their two flower types, so male flowers stay on those plants.
Why do greenhouse cucumbers turn bitter?
Pollination is the usual culprit under glass. One bee finding one overlooked male flower, or crossing in from an outdoor patch, is enough to spoil the fruit that follows. Grow an all-female F1 variety, remove any male flower it throws out, and keep the watering even; the bitterness question then rarely comes up again.
How often should you water cucumbers in a greenhouse?
Check daily and keep the compost evenly moist at all times. In hot spells that means watering every day, and sometimes generously. Open-ground plants get by on around 25 mm of water a week, while pots and growbags under cover dry much faster. Always water the soil at the base of the plant rather than the foliage.
Can cucumbers and tomatoes share a greenhouse?
They can, and in most home greenhouses they do. The compromise sits in the air: cucumbers prefer it more humid than tomatoes like, so damp down the path on hot mornings for the cucumbers' sake and keep vents open through the day for the tomatoes'. The end of the house furthest from the door holds its humidity best, which makes it the natural cucumber corner.
Sources & further reading
- RHS: how to grow cucumbers, the sowing months, training method, feed intervals and harvest sizes used throughout this guide.
- University of Minnesota Extension: growing cucumbers, the soil temperature floor for germination and the 25 mm weekly watering benchmark.
- University of Florida IFAS: cucumber chapter, Florida Greenhouse Vegetable Production Handbook, commercial temperature thresholds, germination speed and post-harvest storage figures.
- Met Office: temperature series for the UK and regions, the HadUK-Grid England data behind the July average quoted in the opening.
