Growing Cucumbers in a Greenhouse

Greenhouse cucumbers are one of the most rewarding crops you can grow undercover. Given warmth, moisture and a bit of regular attention, a couple of plants will give you smooth, sweet, supermarket-length cucumbers right through summer — far better than anything you'll buy in a packet. The trade-off is that they're thirsty, hungry and they hate the cold, so they're best treated as a warm-greenhouse crop rather than something you can neglect.

This guide is part of our wider greenhouse growing guide, and sits alongside our companion piece on greenhouse tomatoes — the two crops are often grown side by side, though as you'll see they don't want quite the same conditions.

Greenhouse vs outdoor (ridge) cucumbers

The first thing to get right is the type of cucumber, because greenhouse and outdoor varieties are bred for completely different conditions.

Greenhouse (or "indoor") cucumbers produce long, smooth, thin-skinned fruit — the classic salad cucumber. They need the warmth, shelter and humidity of an undercover space and will sulk outdoors in a typical British summer.

Ridge (or "outdoor") cucumbers are tougher, shorter and often knobbly or spiny-skinned. They're bred to crop outside in our climate and are pollinated by insects, so they carry both male and female flowers quite happily.

For a greenhouse, choose a greenhouse variety. Trying to grow a ridge type undercover, or a greenhouse type outdoors, is the most common reason a first crop disappoints. If you only have a cold, unheated greenhouse, you can still succeed — just sow a little later and pick a vigorous modern variety.

Best varieties (all-female F1)

Modern greenhouse cucumbers are almost all all-female F1 hybrid varieties. That word "all-female" matters more than it sounds.

Older cucumber varieties produce both male and female flowers. If a male flower pollinates a female one, the resulting fruit turns bitter, swells at one end and fills with seeds. All-female types sidestep the problem entirely — they produce only female (fruiting) flowers, so there's no pollination to go wrong and no male flowers to remove. For a beginner, this is the single biggest reason to spend a little more on good seed.

Reliable all-female varieties for UK greenhouses include:

• 'Carmen' F1 — the standard recommendation. Heavy cropping, disease-resistant and very forgiving. A brilliant first cucumber.
• 'Femspot' F1 — long, dark, smooth fruit; excellent in a warmer greenhouse.
• 'Bella' F1 — vigorous and resistant to mildew and leaf spot, good for an unheated house.
• 'Mini Munch' F1 — small, snack-sized fruit; crops prolifically and is great if you'd rather pick little and often.

Sowing and planting

Cucumbers germinate fast but demand warmth — they will rot in cold, wet compost. Aim for 20–25°C to get them going, which in practice means a heated propagator or a warm windowsill indoors rather than the greenhouse bench in early spring.

Sowing. Sow seeds individually, on their edge (this helps shed water and prevents rotting), about 1.5cm deep in 7–9cm pots of multipurpose or seed compost. Sow from late March if you have heat, or April–May for an unheated greenhouse. Keep the compost just moist, never soggy. Seedlings appear in 7–10 days.

Growing on. Once up, give them good light and steady warmth, and pot on before they become pot-bound. Like all tender crops they need hardening off only if moving to a colder spot — but most greenhouse cucumbers go straight from a warm propagator into a warm greenhouse, so the shift is gentle.

Planting out. Move plants into their final position from mid-May to early June, once you have two or three true leaves and night temperatures inside are reliably above 12°C. You have three good options:

• Greenhouse border soil — the most vigorous, lowest-maintenance choice if your border is healthy. Enrich it well with garden compost first.
• Large pots (at least 30cm / 10 litres) — one plant per pot, in good multipurpose compost. Easy to manage but needs more watering.
• Growbags — two plants per standard bag. Convenient, but growbags dry out fast, so many growers cut the bottom out of a bottomless pot ("a ring") and sit it on the bag to give a deeper root run.

Space plants about 45cm apart. Don't be tempted to cram in more — cucumbers need air moving around them to stay healthy.

Training and supporting

Greenhouse cucumbers are climbers and will quickly sprawl if left to their own devices. Training them upward keeps fruit clean, improves airflow and makes picking easy.

Up a string or cane. The simplest method is to run a vertical string from a wire above the plant down to the base, tying it loosely at soil level. As the main stem grows, wind it gently around the string (or tie it to a tall cane every 20cm or so). The plant climbs the support naturally.

Pinching out. Let the main stem grow up to the top wire, then pinch out the growing tip. Fruit forms on side shoots, so train these along horizontal wires or let them trail, and pinch each side shoot back to two leaves beyond a developing fruit. This sounds fiddly but becomes second nature after a week — it keeps the plant productive rather than turning into a leafy jungle.

Remove the lowest 2–3 side shoots entirely so the bottom of the plant stays open and airy; this cuts down on disease near the soil.

Warmth, humidity, watering and feeding

This is where greenhouse cucumbers reward attention. They are a humidity-loving, thirsty crop — quite the opposite of tomatoes.

Warmth. Cucumbers want 18–25°C by day and ideally no colder than 12°C at night. They stop growing below about 10°C. In a hot spell, though, an enclosed greenhouse can shoot past 30°C, which causes stress, wilting and bitter fruit — so cooling matters as much as warmth.

Humidity. Unlike tomatoes, cucumbers thrive in a humid, "jungly" atmosphere. Damping down — splashing water on the floor and staging on warm mornings — raises humidity and helps keep red spider mite at bay. Our guide to keeping a greenhouse cool and watered explains damping down and ventilation in detail, and it's well worth reading before a heatwave.

Watering. Keep the compost consistently moist — never bone-dry, never waterlogged. In high summer a potted plant may need watering daily, sometimes twice. Irregular watering is the leading cause of bitterness and misshapen fruit, so steadiness matters more than volume. Water in the morning where you can.

Feeding. Once the first fruit starts to swell, feed weekly with a high-potash liquid feed (a tomato feed is perfect). Heavy croppers in pots may need feeding twice a week by midsummer.

Once you've got the watering and feeding rhythm going, a good idea is to invest in a few bits that make the daily routine easier. The list below is the kit most greenhouse cucumber growers reach for.

Problems

Most cucumber troubles trace back to either stress or the warm, humid conditions the plants otherwise love. The three you're most likely to meet:

Bitter fruit. The classic complaint. On all-female F1 varieties, bitterness comes from stress — irregular watering, sudden heat, or letting plants dry out and then drowning them. The fix is steady watering and cooling the greenhouse in hot weather. On older varieties, bitterness comes from male flowers pollinating the females, so remove every male flower (or simply switch to an all-female type next year).

Red spider mite. A tiny sap-sucking mite that thrives in hot, dry air. Look for fine pale speckling on leaves and, in bad cases, faint webbing. The single best defence is humidity — regular damping down makes the greenhouse far less hospitable to them. Keep plants well watered and ventilated, and remove badly affected leaves.

Powdery mildew. A white, dusty coating on the leaves, common late in the season and when plants are dry at the roots. Improve airflow, never let plants go short of water, and remove affected leaves promptly. Choosing a mildew-resistant variety like 'Bella' or 'Carmen' heads off most of the trouble before it starts.

Get the basics right — an all-female variety, steady warmth, humid air and never-dry roots — and greenhouse cucumbers are among the most generous things you can grow. Two well-tended plants will keep a household in salads all summer, and once you've tasted one straight off the vine, the shop-bought kind never quite measures up. For the wider picture on managing your space through the season, head back to the greenhouse growing guide.