No single isolation distance is correct for a crop, because published figures for the same crop differ several-fold: the distance needed depends on the purity you want, whether the crop is carried by wind or insects, and what lies between the plots. Decide the species boundary first, then choose a distance for your purity target, and where that distance cannot be reached use time, a cage or hand-pollination instead.
Part of Community Seed Bank Management
An isolation distance is the minimum separation between two varieties of one species that keeps cross-pollination below an accepted rate.
The Corpus Gives Three Different Sets Of Numbers
Three articles in this corpus give distances for the same crops, and they disagree. Distances in metres, with the source named:
| Crop | Seed bank carrier | Isolation Distance guide (minimum / ideal) | Cross-Pollinators guide (home / community) |
|---|---|---|---|
| Corn | 400 (800 preferred) | 400 / 1,600 | 400 / 800 |
| Beet, chard | 800 | 400 / 800 | 1,000 / 2,000 |
| Squash, one species | 800 | 400 / 800 | 500 / 1,000 |
| Brassicas, one species | 800 | 400 / 1,600 | 300 / 600 (cabbage, kale) |
| Carrot | 400 | 400 / 800 | 500 / 1,000 |
| Onion | 800 | 400 / 1,600 | 500 / 1,500 |
For beet the lowest figure is 400 m and the highest 2,000 m, a factor of five. The sources are the carrier, Isolation Distance and Cross-Pollinators. This article does not choose among them, because none of the three cites a field trial. It explains what the differences mean and how to decide.
Why The Numbers Differ: Purity Target
The Cross-Pollinators guide ties distance to purity: under 5% crossing for home use, under 1% for a community seed library, under 0.1% for certified seed. A stricter target needs a longer distance, or a barrier. If you cannot say what purity you need, you cannot choose a distance. For food-only use a few percent of crossed seed is usually tolerable; for a variety you will plant and replant for decades, aim at the community standard.
Species Boundaries Decide Who Needs Isolating
Distances apply between varieties of one species, not between crops of one family.
- Squash: Cucurbita pepo (zucchini, acorn, most pumpkins), C. maxima (Hubbard, buttercup) and C. moschata (butternut) do not cross with each other. Cucumber and melon are different species again.
- Brassicas: cabbage, broccoli, cauliflower, kale, Brussels sprouts and kohlrabi are all one species (Brassica oleracea) and cross freely. The carrier’s note that “all brassicas cross with each other” is too broad: turnip and Chinese cabbage (B. rapa) and rutabaga (B. napus) are different species that cross with B. oleracea only with difficulty, though turnip and rutabaga can cross with each other.
- Beets and chard are one species (Beta vulgaris) and will cross.
- Carrot crosses with wild carrot.
- Onion (Allium cepa) does not readily cross with garlic or with bunching onion (A. fistulosum).
Crops of one species grown for different purposes also count: sweet corn, field corn and popcorn are one species.
What Moves The Number
- Wind or insects. Wind carries pollen of corn, beet and spinach; insects carry pollen of squash, cucumber, brassicas, carrot and onion. Insect-carried crossing depends on how many pollinators there are and what else is flowering nearby.
- Barriers. The Cross-Pollinators guide credits buildings with reducing corn distance and hedgerows with halving brassica distance. Treat those as plausible but unmeasured here.
- Flower date. Two plots at the same distance cross far less if they flower weeks apart.
- The crossing rate of the crop. Tomato, bean, pea and lettuce cross at only a few percent even side by side.
Method: Choose A Distance You Can Defend
- Write down the species of each variety you plan to save seed from.
- Pick a purity target (home, community, strict).
- Take the largest figure in the range for that crop if you can and the purity matters.
- If it cannot be reached, use a second tool: stagger the planting, cage the plants, or hand-pollinate (Timing Isolation, Physical Barriers).
- Keep a record of the distance and method used with the seed lot (Seed Inventory and Catalog System).
- Check the following year: a plot of off-type plants shows that the isolation failed.
Failure Modes
- Neighbours. Another household’s squash plot 300 m away breaks the distance you thought you had. Agree on varieties across the whole community.
- Wild relatives of the crop flowering in hedges and ditches.
- Unchecked unit conversions. A quarter mile is 402 m and half a mile is 805 m; do not round both to 500 m.
What’s Next
- Learn next: Physical Barriers (Caging, Bagging) — when distance cannot be reached
- Combine with: Minimum Population Sizes to Avoid Inbreeding — the number of plants once the seed is pure