Two varieties of the same species will cross if pollen moves from one to the other, and the seed from that cross grows into plants that mix both parents, so seed of cross-pollinating crops must be kept pure by isolation. The four tools are distance, timing, physical barriers and hand-pollination, and self-pollinating crops such as tomato, bean and pea need much less of any of them.
Part of Community Seed Bank Management
Isolation is the set of methods that stop pollen from a neighbouring variety reaching the flowers of a seed crop.
Crossing Happens Only Within A Species
Pollen from a squash cannot fertilise a cucumber, and pollen from a bean cannot fertilise a pea. Crossing is a question of species, not of family or of common name. The practical rule: you only need to separate varieties of the same species. Two varieties of the same species that you do not want mixed must be isolated; two different species of the same family often do not need to be.
This is why “squash” is not one crop for isolation purposes. Pumpkins, zucchini and acorn squash are one species (Cucurbita pepo); butternut is another (C. moschata); Hubbard and buttercup are a third (C. maxima). Each species crosses only with itself. Details by crop are in Isolation Distances by Crop Family.
Self-Pollinators And Cross-Pollinators Behave Differently
- Self-pollinating (tomato, pepper, bean, pea, lettuce, wheat, oats, barley): the flower pollinates itself, usually before it opens. Crossing is possible but rare, so a small gap between varieties is enough (Self-Pollinators).
- Cross-pollinating (corn, squash, cucumber, brassicas, beet, onion, carrot): wind or insects carry pollen between plants, and some of these crops cannot set seed without foreign pollen (Cross-Pollinators).
Self-pollination is a tendency, not a guarantee. Some crops, peppers among them, cross at a higher rate than tomatoes in the same garden. Treat “self-pollinating” as “low risk”.
Four Ways To Keep A Seed Crop Pure
| Method | How it works | Best for |
|---|---|---|
| Distance | Pollen and pollinators rarely cover the gap | Large plots, wind-pollinated crops |
| Time | Varieties flower at different dates (Timing Isolation) | Crops with a long planting window |
| Barrier | Cage or bag excludes pollen carriers (Physical Barriers) | Small plots, valuable varieties |
| Hand-pollination | You choose the pollen, then close the flower (Hand Pollination) | Squash, melon, cucumber |
The simplest tool of all is to grow only one variety of each cross-pollinating species in a given year and keep the other seed in storage.
Corn Shows Crossing In The Seed Itself
In corn, foreign pollen changes the colour or texture of the kernels it fertilises, in the same season. A yellow ear with scattered purple or starchy kernels among sweet ones has been crossed. For most crops the cross shows up only when the seed is planted the following year, so you cannot check in advance.
Common Mistakes
- Counting varieties instead of species. Four squash varieties from four species need no isolation from each other; four from one species do.
- Forgetting wild and weedy relatives. Wild carrot (Queen Anne’s lace) crosses with the garden carrot, and wild relatives of other crops can too.
- Trusting a hedge or fence. Pollen travels over low barriers; only dense, tall plantings reduce it.
- Trusting one method forever. Plan which of the four tools you will use before sowing.
What’s Next
- Learn next: Isolation Distances by Crop Family — how far, by crop
- Combine with: Isolation Distance — the Phase 2 reference table