A cucumber plant is one of the fastest returns on a seed packet in the whole vegetable garden: three seeds sown on a warm windowsill in late April can hand you a cucumber every other day from July until the first cold nights of autumn. It is also one of the easiest crops to get wrong, and almost always in the same three ways — sown too early, planted out too cold, and then watered too erratically to stop the fruit turning bitter.
Cucumis sativus is a tender annual from the foothills of the Himalayas, and it behaves like one. It wants warmth, steady moisture and something to climb, and it sulks at any check to its growth. Get those right and it is close to unstoppable. This guide covers the three types worth knowing, when to sow for your set-up, how to train a cordon in a greenhouse or sprawl a ridge cucumber outdoors, and what to do about bitterness, mildew and male flowers.
- Sow February–March with heat, April–May for most gardens, up to early July for a late crop
- Germination 3–7 days at 20–25 °C; below 16 °C the seed simply rots
- Spacing 45 cm for cordons up a string, 90 cm for sprawling ridge types
- Harvest July–October, every two to three days — picking is what keeps it fruiting
Ridge, greenhouse or snack: which cucumber to grow
Everything in the Cucumis sativus range falls into one of three camps, and the choice decides how you grow it. Ridge or outdoor cucumbers — Marketmore, Telegraph Improved, White Wonder, Gele Tros, Chinese Slangen, the yellow Lemon Apple — are tougher, mostly open-pollinated, and happy in a sheltered bed or a cold greenhouse. They carry separate male and female flowers and need insects to set fruit. Greenhouse types such as Laureen F1 are all-female and parthenocarpic: they set long, smooth, seedless fruit without pollination at all, but they want real warmth. Snack cucumbers — Iznik F1, Kaikura F1, Kalimero F1 and organic Picolino F1 — are compact all-female plants carrying dozens of 10–15 cm fruits, which makes them the best choice for a pot or a small greenhouse.
Then there are the gherkins: National, Parisian Pickling, Hokus, Beth Alpha, Levina F1 and organic Profi F1, bred to be picked at five to eight centimetres for pickling. They are the same species, grown the same way, simply harvested far younger and far more often. If you would rather keep the whole thing organic, the organic cucumber range covers ridge, snack and pickling types.
When and how to sow cucumber seed
The single most common mistake is sowing in February without a heated propagator. Cucumber seed needs 20–25 °C to germinate and the seedlings need to stay above about 16 °C afterwards; in a cool spring a plant sown in mid-May will overtake one sown in March, because the early one spends six weeks standing still and going yellow. For most gardens, late April to mid-May is the sweet spot — check the April and May sowing lists for what else goes in alongside.
Sow one seed per 9 cm pot of moist, peat-free compost, about 2 cm deep, and stand the seed on its edge rather than flat: water drains off a vertical seed instead of sitting on it, which noticeably cuts the number that rot. Two seeds per pot is fine insurance — pull the weaker seedling rather than trying to separate the roots. Cucumbers loathe root disturbance, so never prick out a seedling bare-rooted, and never sow into a seed tray you plan to split up. Our guides to seed starting indoors and germinating seeds cover the basics of both.
Planting out and training
Cucumbers go outside only once the nights are reliably above 10 °C — late May or early June in most of northern Europe, a fortnight after the last frost date rather than on it. Harden the plants off over a week or ten days first; a cucumber moved straight from a windowsill to an open bed can lose a month. Plant with the rootball proud of the soil surface, never buried up to the seed leaves, because a stem sitting in damp soil is an invitation to collar rot.
Under glass, grow them as cordons: one stem tied or twisted up a vertical string, side shoots pinched out at one leaf beyond a developing fruit, and the growing tip stopped when it reaches the roof wire. Outdoors you can either let ridge types sprawl over warm soil with a mulch underneath, or train them up netting or a wigwam — the vertical version takes a quarter of the space, keeps the fruit clean and dramatically reduces mildew and slug damage. Our guide to supports and trellising covers the structures, and container growing the pot sizes: a snack cucumber needs at least a 30-litre pot to keep its feet wet.
Sowing and growing, step by step
- Sow warm, sow on edge One seed per 9 cm pot, 2 cm deep, set on its side. Propagator or warm windowsill at 20–25 °C; up in three to seven days.
- Grow on bright and above 16 °C Move into full light the moment the seedlings appear, or you will get leggy seedlings. Pot on into a 1-litre pot once the roots reach the sides.
- Enrich the planting position A barrowful of compost or well-rotted manure per plant. Cucumbers are hungry and thirsty; poor soil shows up as bitter, tapering fruit. See preparing your soil.
- Plant out after hardening off Late May to early June outdoors, 45 cm apart up a support or 90 cm sprawling. Fleece or a cloche for the first fortnight buys a week of growth.
- Water steadily, feed weekly, pick constantly Never let the roots dry out, mulch to hold moisture, and switch to a high-potash tomato feed once the first fruits set. Cut, do not pull, every second day.
All-female F1 or open-pollinated?
All-female F1 hybrids
Iznik, Kaikura, Kalimero, Picolino, Laureen, Profi, Levina.
- Set fruit without pollination — ideal under glass or on a balcony
- Heavy, even crops of smooth, seedless, never-bitter fruit
- Saved seed will not come true, so you buy fresh each year
Open-pollinated ridge types
Marketmore, Telegraph Improved, White Wonder, Gele Tros, Beth Alpha, Hokus.
- Need bees, so they belong outdoors or in an open greenhouse
- Tougher in a cool summer, with far more flavour in the skin
- You can save your own seed and keep the variety going
Older ridge varieties do produce male flowers, and there is a persistent instruction to remove them. For modern varieties it is unnecessary: bitterness in today's cultivars is driven by stress, not by pollination. What does still matter is keeping pollinators around the outdoor plants, since every female flower needs a bee to become a cucumber.
Common mistakes
- Sowing in February without heat. Cold, wet compost rots the seed. Wait for warmth, or use a propagator.
- Planting out too early. One cold night at 5 °C checks a cucumber for weeks; frost kills it outright.
- Letting the pot dry out, then drowning it. Irregular watering is the single biggest cause of bitter, misshapen fruit.
- Burying the stem. Plant level with the rootball; a buried collar rots at the base and the whole plant wilts.
- Leaving a fruit to grow huge. One over-mature cucumber tells the plant its job is done and production stops. Pick young.
- Wetting the leaves in the evening. Water the soil, in the morning, or you invite powdery mildew.
Frequently asked questions
Can I sow cucumbers in September?
No — there is neither the warmth nor the daylight left to bring a plant to fruit, and the first frost would finish it in any case. Cucumber is a February-to-July sowing; for what does still go in this month, see what to sow in September and our guide to the ten-hour daylight rule, which explains exactly why autumn sowings run out of road.
Why are my cucumbers bitter?
Cucurbitacin, a natural compound concentrated in the skin and stem end. It builds up when the plant is stressed — drought followed by a flood of water, temperatures above 30 °C, cold nights, or starved soil. Mulch, water consistently and ventilate the greenhouse on hot days, and it rarely appears. A bitter fruit is not dangerous in small amounts, but it is not worth eating.
How many plants do I need?
Two or three for a household. A single healthy greenhouse cordon can produce twenty-five to forty fruits in a season, and a snack variety considerably more. Sow a second plant three weeks after the first if you want to spread the glut — the same logic as succession sowing salad.
Why do the tiny fruits shrivel and drop off?
Usually a fertilisation or stress problem. On outdoor varieties it means the female flowers were not pollinated, so encourage bees and avoid spraying. On all-female types it points to cold, drought or a plant carrying more fruit than it can feed — remove a few of the smallest and the rest will fill out.
Cucumbers reward attention more than skill: warmth at the start, water in the middle, and a knife in your hand every other day at the end. When the vines finally give up, let one last fruit ripen to gold for next year's seed, clear the bed and move the cucurbits along the rotation — their relatives in pumpkin, squash and Cucurbita pepo share the same pests. Browse the full cucumber, gherkin and organic vegetable ranges, everything else in vegetable seeds, or read on through our growing guides and the beginner's guide to growing vegetables from seed.
