Mid-September is the one moment in the year when saving cucumber seed costs you nothing. The vines are winding down, the last few fruits will not amount to much anyway, and the plant is about to be pulled out regardless. Leave one cucumber on the plant now and by mid-October you will have a packet of seed worth five or ten years of sowings — provided you pick the right fruit, from the right plant, and are willing to let it get truly, alarmingly ripe.
That last point is where most attempts fail. A cucumber is one of the very few crops we eat while the fruit is still immature, so the seed inside the glossy green one you slice for a salad is white, soft and completely unviable. Saving cucumber seed means abandoning a fruit for another month or more and letting it turn into something you would never put on a plate. This guide covers choosing the fruit and the plant, why some varieties will never come true, the fermentation step that most people skip, and how to dry and store the seed so that it still germinates in 2033.
- Ripen 4–6 weeks past eating size — until the skin is yellow-brown and hard
- Ferment 2–4 days at room temperature, until a pale film forms on top
- Dry 2–3 weeks on a plate, airy, out of sun, never above 35 °C
- Store Cool, dark and dry — well-saved cucumber seed lasts 5–10 years
The cucumber you eat is not a ripe cucumber
A seed cucumber looks wrong, and it is supposed to. Over four to six weeks past normal picking size the fruit swells, the skin loses its shine and turns yellow, then orange-brown, often with a corky netting or fine cracks across it. The stalk dries and shrivels. Inside, the seeds fill out, harden and develop the cream coat that means a live embryo. A green cucumber picked at the usual moment contains none of that.
There is a cost to this, which is why the timing matters. A cucumber plant is programmed to stop once it has successfully set mature seed, so a fruit left to ripen will noticeably slow or halt the production of new ones. In July that would wreck your crop. In the middle of September, with the nights drawing in and the greenhouse due a clean, it costs you nothing at all. Pick a fruit that is already well-grown, mark it with string so nobody harvests it by accident, and leave the plant to get on with it.
Once the fruit is fully coloured, or once frost threatens, cut it with a short piece of stalk and bring it indoors. Another week or two on a cool windowsill finishes the job — the seed continues to mature inside the picked fruit, and this after-ripening measurably improves germination. It is the same principle as ripening green tomatoes indoors, and much the same window.
Which plants are worth saving from
Two things decide whether your seed will come true. The first is the variety. Open-pollinated cucumbers — Marketmore, Telegraph Improved, White Wonder, Lemon, Gele Tros, Chinese Slangen, Beth Alpha and the traditional pickling gherkins such as National, Parisian Pickling and Hokus — breed true, and their seedlings will be recognisably the same plant. F1 hybrids will not. Save from Iznik, Kaikura, Picolino, Kalimero, Laureen, Profi or Levina and the next generation splits into a lottery of shapes, sizes and vigour, most of them worse than either parent. Heirloom, F1 and organic seed explains the genetics, and the packet itself will usually tell you which you have — see understanding seed packets.
The second is who else was flowering nearby. Every cucumber and gherkin we sell is Cucumis sativus, they are pollinated by bees, and they cross with each other freely — so a Marketmore grown next to a gherkin may well give you seed that produces neither. For genuinely pure seed you want a good half-kilometre between varieties, which almost nobody has; in practice, grow just one open-pollinated variety for seed in that year, or hand-pollinate a female flower with a male from the same plant and tape it shut. And save from five or six plants rather than one, so the variety keeps its vigour.
One persistent myth is worth dispatching. Cucumbers do not cross with courgettes, pumpkins or squash — those are Cucurbita, a different genus — and they do not cross with melons either. Whatever made your cucumber bitter, it was not the courgette next door. Bitterness comes from cucurbitacin, which is partly inherited and partly a stress response to heat or drought, so simply do not save seed from a fruit that tastes bitter.
Finally, check what kind of plant you have. Many modern greenhouse varieties are all-female and parthenocarpic: they set fruit without pollination at all, and an unpollinated cucumber contains no viable seed whatsoever. If you want seed from under glass, you need a plant that carries male flowers too, which in practice means an older ridge or outdoor type from the Cucumis sativus range.
Saving the seed, step by step
- Choose the fruit, then leave it alone One well-formed, typical cucumber on a healthy plant of an open-pollinated variety. Tie a strip of cloth to the stem so it survives the next harvest round, and let it ripen for four to six weeks until it is yellow-brown, hard and dull.
- Cut it open and scoop out the pulp Slice lengthways. Run a spoon down each seed cavity and tip seeds, jelly and all into a jam jar. Add a splash of water — roughly as much water as pulp — and do not rinse anything yet.
- Ferment for two to four days Cover loosely, leave at normal room temperature and stir once a day. A pale, slightly mouldy film on the surface means it is working. It will smell; put it somewhere you are not eating.
- Wash, and let the water sort them Fill the jar to the top and stir hard. Good, full seed sinks; pulp, skins and hollow seed float. Pour the floaters away, refill and repeat until the water runs clear, then tip the sinkers into a sieve.
- Dry slowly, then store cold Spread in a single layer on a china plate or a glass dish — never kitchen paper, which they glue themselves to — somewhere airy and out of the sun, for two to three weeks. When a seed snaps rather than bends, it is dry. Then follow storing and preserving seed.
Why the fermentation step is not optional
Fermented
Seed left in its own pulp and water for two to four days.
- The gel coat is digested away — that coat contains germination inhibitors
- Seed-borne bacterial diseases such as angular leaf spot are largely destroyed
- Clean, free-running seed that dries evenly and stores without sticking together
Rinsed only
Seed washed straight out of the fruit and dried.
- Residual gel dries to a hard varnish and slows or blocks germination
- Any disease carried in the pulp is dried and stored along with the seed
- Seeds clump, dry unevenly and are far more likely to go mouldy in the packet
It is the same process used for tomato seed, and for the same reasons. Do not let it run much past four days, though: once the pulp is gone the seed itself will start to sprout in the jar, and a seed that has begun to germinate cannot be dried and kept. Drier-seeded crops need none of this — beans and peas simply dry in the pod.
Common mistakes
- Saving seed from an eating cucumber. The seed is immature and white. It will not grow, however carefully you dry it.
- Saving from an F1 hybrid. It will germinate perfectly and then give you a row of disappointing strangers.
- Drying on kitchen paper. The seed sticks fast and tears when you lift it. Use china or glass.
- Drying in the airing cupboard or a sunny window. Anything over about 35 °C kills the embryo. Slow and airy beats warm.
- Bagging it up too soon. Seed that feels dry but bends is still damp inside, and a sealed jar of damp seed turns mouldy by Christmas.
Frequently asked questions
How do I know when the cucumber is ripe enough?
The skin is the giveaway: it goes from glossy green to matt yellow, then to a golden or russet brown, and it becomes hard enough that a thumbnail will not mark it. Many varieties develop a corky net of fine cracks. The stalk withers, and the fruit feels heavy. If in doubt, leave it another week — over-ripe is not a problem, under-ripe is fatal.
How many seeds will one cucumber give me?
Between one and three hundred from a full-sized fruit, which is ten to thirty packets' worth. One fruit from one plant is plenty for a household; the reason to save from several plants is genetic breadth, not quantity.
Do I need to test the seed before sowing next spring?
It is ten minutes well spent. Roll ten seeds in damp kitchen paper, keep them at about 20 °C and count how many sprout within a week — the method is set out in how long do seeds last, and how to germinate seeds covers what the sprouts should look like. Cucumber seed is long-lived and often germinates best in its second and third years.
Which crops should I not bother saving seed from?
Anything you would have to keep for two years before it even flowers. Biennials such as carrots, leeks, beetroot and celery and celeriac only set seed in their second summer, which means overwintering the plants and giving up the harvest. Start with the crops that ripen seed in the same season — cucumbers, tomatoes, beans, peas and lettuce — and add the harder ones later.
Seed saving is the one garden job that pays forward rather than back, and cucumbers are among the kindest crops to learn it on: one fruit, one jar, three days of smell and a fortnight of drying. Label the packet with the variety and the year before you forget both, and keep it in the coolest dry corner you have. While the vines are coming out, it is worth deciding what follows them — what to sow in September and the September sowing list have the options, and crop rotation will tell you where the cucurbits should go next year. Browse the cucumber, gherkin and organic cucumber ranges, everything else in vegetable seeds, or read on through our growing guides and the beginner's guide to growing vegetables from seed.
