Short answer
Seed comes true only if the flower was pollinated by the same variety. So the question for every crop is: will it cross with something else in or near your garden?
Tomatoes, peas and beans mostly pollinate themselves, and a short gap between varieties is enough for home use. Peppers, squash and cucumbers are crossed by insects and need real distance, or a bag or cage over the flowers. Corn is crossed by wind and pollen travels far, so in a garden the dependable method is to bag and hand-pollinate. Whatever the crop, save from as many plants as you can.
This guide covers the annual vegetables we sell seed for: corn, peppers, tomatoes, squash, cucumbers, peas, beans and eggplant. Biennials and perennials, which flower in their second year or later, need a guide of their own. Where sources disagree on a distance, we give the range and say why.
1. The one question: will it cross?
Varieties of the same species can cross; that is the working rule. All corn, whether sweet, flour or popcorn, is one species, Zea mays, and crosses freely. A jalapeño and a bell pepper are both Capsicum annuum and cross just as readily.
How easily a crop crosses depends on how it is pollinated. Self-pollinating crops such as tomato, pea and bean set most of their seed from their own pollen, often before the flower opens. Insect-pollinated crops such as squash and cucumber depend on bees moving pollen between flowers. Corn sheds pollen into the wind. The further along that list, the more distance or protection you need.
2. Isolation at a glance
| Crop | Crossed by | Home seed-saving distance | If you lack the space |
|---|---|---|---|
| Corn | Wind | At least 1 mile (University of Arizona); seed-saving guides call distance unreliable in a garden | Bag and hand-pollinate |
| Squash, cucumber | Insects | Half a mile | Tape or bag blossoms and hand-pollinate |
| Peppers, eggplant | Mostly self, often insects | Disputed: 50 ft for home use to 500 ft (University of Arizona); commercial growers use far more | Bag flowers or cage plants |
| Tomato, modern | Mostly self | About 10 ft | A tall crop between varieties |
| Tomato, older varieties (we include our wild cherries) | Self, more insect crossing | 20 to 25 ft | Bag flower clusters |
| Peas, beans | Self | A short gap, around 10 ft for beans | Rarely needed |
Why doubling the distance helps so much
Crossing falls away quickly with distance, not in a straight line. The isolation charts published by Saving Our Seeds give a working rule: every doubling of distance cuts cross-pollination by about a factor of four. That is why a modest separation removes most crossing, and why the last fraction of a percent needs very large distances. Commercial seed growers, who must deliver near-total purity, isolate peppers by a quarter mile to a mile. A home grower keeping a line for their own garden can accept far less.
The same charts advise doubling the recommended distances when a large planting of the same crop is nearby, because a field sheds far more pollen than a garden row.
3. Corn
Corn is the hardest crop to keep true and the one most worth the effort, because many of our corns are old lines with a long human history. The flour and flint corns of the upper Missouri, for example, were developed over centuries by Mandan, Hidatsa and Arikara farmers. Keeping them true is part of respecting that work.
Corn pollen rides the wind. The University of Arizona gives at least a mile between varieties; seed-saving guides note that pollen travels a quarter mile or more and that, with corn grown almost everywhere, isolation by distance is unreliable. In a garden the dependable method is hand-pollination: cover the young ear shoots with bags before the silks appear, collect pollen from tassels of the same variety, and apply it to the silks, keeping the ears bagged until the silks dry.
A second rule matters just as much: save from many plants. Corn is naturally cross-pollinating, and New Mexico State University Extension notes that a population that is too small loses genetic variation and suffers inbreeding depression: weaker plants and smaller ears in later generations. Take seed from ears across the whole planting, never from one or two favorite plants. Time isolation, planting two varieties so they tassel weeks apart, can also help where your season allows it.
4. Peppers
Peppers mostly pollinate themselves, but their open flowers are visited by insects, so varieties grown close together can cross. How far apart to grow them is disputed. The University of Arizona recommends 500 feet. Seed-saving guides report home-garden recommendations from a few feet to about 50, and commercial isolation of a quarter mile to a mile. For home use, keeping varieties at opposite ends of the garden reduces crossing a great deal; for seed you intend to share, bag the flowers or cage the plants.
Species matter, and here the sources disagree too. Our peppers come from five species:
- C. annuum, C. chinense and C. frutescens: closely related. Plant breeders use C. chinense as a bridge to cross the others, so these crosses clearly can produce seed. How often it happens by insects in a garden is disputed: New Mexico State University calls a jalapeño crossing a habanero highly unlikely, while the Bauta Family Initiative on Canadian Seed Security describes the three as crossing prolifically. If purity matters, separate them as you would two annuum varieties.
- C. baccatum (our aji types): very low crossability with C. annuum, according to the breeding literature. A safe neighbor.
- C. pubescens (rocoto): crosses only with its own species.
Eggplant behaves much like pepper: largely self-pollinating, but visited by insects. Treat it the same way.
5. Tomatoes
Most modern tomatoes hold the stigma inside the cone of anthers, so they pollinate themselves and rarely cross. Published seed-saving guidance gives about 10 feet between modern varieties. Older varieties, and those whose stigma sticks out beyond the anther cone, cross more readily and need 20 to 25 feet. New Mexico State University notes the same effect of an exserted stigma.
Our two wild cherry tomatoes are Solanum lycopersicum var. cerasiforme. We treat them like older, cross-prone varieties and give them the full 20 to 25 feet.
6. Squash and cucumbers
Bees cross squash and cucumbers readily, and a cross will not show until the next generation: the fruit you eat this year looks normal, and next year's plants surprise you. Seed-saving guides recommend half a mile for squash varieties that cross. Squash crosses within its species, so know which species each variety is before you plan.
Hand-pollination is practical in a garden: tape shut a male and a female flower of the same variety the evening before they open, transfer the pollen the next morning, and tape the female flower closed again. Mark the fruit so you know which one to save.
7. Peas and beans
Both largely pollinate themselves, often before the flower opens. A short gap between varieties is enough for home use; about 10 feet is a common figure for beans. Save from several plants rather than one.
8. Seed saved from hybrid parents
Three of our pepper lines began as seed saved from F1 hybrids. Provenance Dark Jalapeno and Provenance Warren County Jalapeno are still experimental: plants vary, some resemble the parent and some do not, and seed you save from them will vary again. Provenance Red Horn has been selected here over many seasons and is now sold as open-pollinated seed. None of them is a hybrid, and none carries its parent's name.
Planning a seed-saving garden
- List the varieties you want seed from, and note each one's species
- Choose one variety per species to save from this year, or plan bags and cages
- Grow corn for seed in a block, and bag and hand-pollinate the ears you will keep
- Put the widest possible distance between varieties of the same species
- Save from many plants across the planting, not from one favorite
- Label seed with variety and year the day you harvest it
- Dry seed thoroughly, then store it cool and dry in a sealed container
References
- University of Arizona Cooperative Extension, Seed Saving
- New Mexico State University Extension, Vegetable Seed Saving for Home Gardeners and Small-scale Farmers, Guide H-262
- Saving Our Seeds, Isolation Distances for Seed Crops: Principles and Practices
- Bauta Family Initiative on Canadian Seed Security, Seed Saving and Selection for Peppers
- Manzur, J. P. et al. (2015), wide hybridization of common pepper (Capsicum annuum) with cultivated ají (C. baccatum), PLOS ONE
Related: Seed for sale · Corn · All growing guides