Isolation
distances

What isolation distance actually means

Pollen moves. Wind carries it for grasses and some brassicas; insects carry it for squash, cucumbers, alliums and most herbs. If two varieties of the same species flower within that range, the seed you save is a cross, and next year's plants will not be the variety you planted.

Isolation distance is the separation at which cross-pollination is considered acceptably rare. Self-pollinating crops (tomato, lettuce, bean, pea) set seed before flowers open fully and need very little. Insect-pollinated crops need hundreds of metres, which almost no garden has.

What to do when you cannot manage the distance

Three techniques substitute for distance, and for most gardeners they are the only practical option:

• Grow one variety per species per year. Crude, reliable, and what most home seed savers actually do.
• Bag the flowers. Fine mesh or tulle over the flower head, then hand-pollinate with a brush. Standard practice for squash and brassicas.
• Separate in time. Two varieties of the same species that flower at different times cannot cross. Works best with biennials and with early versus late sowings.

Isolation is not the same as seed life: the far-right column is how long seed stays viable in dry, cool storage, not how far apart to plant it.

By crop

Lettuce — 3–6 m (9.8–20 ft)

Lettuce is where everyone should start. It is self-pollinating, so isolation is a formality. It is annual, so there is no overwintering. And a single mature plant throws a metre-high spike carrying a thousand or more seed, which means one plant is genuinely enough for years.

The only part that needs care is timing. Lettuce seed does not wait for you — the heads shatter over about a fortnight and the seed blows away on its own schedule, not yours.

Full seed-saving guide for Lettuce →

Tomato — 3–10 m (9.8–33 ft)

Tomato seed is enclosed in a gelatinous sac that inhibits germination and can carry fungal and bacterial disease from the fruit. In nature the fruit rots, the gel breaks down, and the seed survives. You are copying that process in a jar, which takes about three days and is the only fiddly part of saving tomato seed.

Tomatoes are self-pollinating — the flower releases pollen inside itself before it opens — so crossing is rare. Isolation of 3–10 m (9.8–33 ft) handles the occasional bumblebee.

Full seed-saving guide for Tomato →

Pepper — 30–60 m (98–197 ft)

Pepper seed is easy to extract and dry. There is no gel to ferment and the seed comes away cleanly from the central core. The difficulty is not the method, it is the genetics.

All Capsicum annuum varieties cross with each other — bell peppers, jalapeños, cayennes, padróns. They are self-pollinating, so crossing is less frequent than in squash, but insects do it and the consequence is delayed by one generation.

Full seed-saving guide for Pepper →

Eggplant — 30–50 m (98–164 ft)

Aubergine seed is embedded in flesh rather than gel, but it still benefits from a short fermentation and the fruit must be genuinely mature. A glossy eating aubergine is nowhere near ready.

They are self-pollinating but insects cross them readily, so isolation of 30–50 m (98–164 ft) is worth observing if you grow more than one variety.

Full seed-saving guide for Eggplant →

Beans — 3–5 m (9.8–16 ft) (often unnecessary)

Beans are the ideal first seed crop if you want volume. The flower fertilises itself before it opens — crossing is genuinely rare, not just unlikely — so you can grow several varieties and still get seed that comes true. One plant yields enough seed to plant a row next year.

The process is almost entirely passive: leave the pods on the plant until they are brown and rattling, then shell them.

Full seed-saving guide for Beans →

Peas — 3–5 m (9.8–16 ft) (often unnecessary)

Peas are cleistogamous: the flower self-pollinates and never opens properly, so cross-pollination is close to impossible. You can grow several varieties side by side and still save true seed.

They mature earlier than beans, which is useful — you get the seed crop finished before autumn damp sets in.

Full seed-saving guide for Peas →

Cucumber — 500 m–1 km (1,640–3,281 ft), or bagging

Cucumber seed keeps longer than almost anything else here — properly dried and stored, five to ten years is normal. The processing is a two-day fermentation, the same idea as tomato.

The complication is variety type. Most modern indoor cucumbers are parthenocarpic: they set fruit without pollination. That is excellent for growing under cover, and useless for seed saving unless you deliberately hand-pollinate.

Full seed-saving guide for Cucumber →

Melon — 500 m–1 km (1,640–3,281 ft), or bagging

Melon seed saving is mechanically identical to cucumber: scoop, ferment, rinse, dry. The difference is ripeness. A melon picked at peak eating quality is not necessarily mature enough for viable seed.

All Cucumis melo — cantaloupe, honeydew, charentais, galia — cross with each other. Watermelon is a different genus and does not cross.

Full seed-saving guide for Melon →

Squash — 500 m–1 km (1,640–3,281 ft), or bagging

Squash have separate male and female flowers on the same plant, and insects move pollen between them and between plants. Within a species they cross freely, which means a courgette and a pumpkin of the same species grown together will produce seed that is neither.

The good news is that the species boundary is real: Cucurbita pepo and Cucurbita maxima do not cross. If you grow one of each, both come true.

Full seed-saving guide for Squash →

Brassicas — 800 m–1.6 km (2,625–5,249 ft), or time separation

Brassicas are the group that teaches you why seed saving is a skill rather than a chore. Brassica oleracea — cabbage, kale, broccoli, cauliflower, kohlrabi — is biennial. You grow the plant in year one, it survives the winter as a stump, and in spring it sends up a two-metre spike of yellow flowers.

Brassica rapa — pak choi, mizuna, tatsoi, turnip — is faster and often sets seed in the same season, because it bolts so willingly.

Full seed-saving guide for Brassicas →

Root vegetables — 500 m–1 km (1,640–3,281 ft)

Root crops are saved by a method called steckling: you grow the root, harvest it, select the best ones, store them over winter, and replant them in spring. The plant then flowers and sets seed.

This is more work than an annual, but it has an advantage — you can assess the root before deciding to keep it. You cannot do that with a tomato.

Full seed-saving guide for Root vegetables →

Alliums — 800 m–1.6 km (2,625–5,249 ft)

An onion in flower is one of the best sights in a garden: a globe of small white or purple flowers on a tall hollow stem, covered in bees. The seed that follows is plentiful but short-lived.

Onions and leeks are biennial, as with brassicas and root crops. Chives are perennial and set seed readily every year.

Full seed-saving guide for Alliums →

Herbs — 150 m–800 m (492–2,625 ft)

Herbs are not one group when it comes to seed. Basil and coriander are annual and set seed in the same season. Dill is annual and shatters violently. Parsley is biennial and needs a winter. Mint is perennial and is propagated by division because seed-grown mint is rarely true to type.

All are insect-pollinated and cross readily within their genus — and dill crosses with fennel, which is a different genus in the same family.

Full seed-saving guide for Herbs →

Microgreens — n/a

A microgreen is a seedling cut at the cotyledon stage, before it has flowered or even produced true leaves. There is no seed to save, because you have eaten the plant before it could make any.

This is not a gap in the catalogue — it is the nature of the crop. If you want to save seed from a species, grow it to maturity as a full plant instead.

Full seed-saving guide for Microgreens →

Questions

Cite this page: DarkGrow Seed Bank, “Isolation distances for 14 vegetable crops — seed saving reference”, https://seedbank.dirvenbank.com/pages/isolation.html (accessed ). Figures compiled for 891 catalogue varieties.