Vegetable Seed Saving Handbook

Seed Isolation Distances

Using Distance to Keep Seeds from Crossing

Perhaps the easiest way to keep similar varieties from crossing is simply to plant similar varieties far enough apart that their pollen cannot reach each other (called "distance isolation"). Distance isolation requires no equipment or special skills, but it does rely on your having a good idea of who is growing what in your immediate area (not always possible).

Which Distance Figures to Use?

Looking at the table below, you'll notice that there are two different columns of figures given for safe distance isolation. The first column shows the distances given by Suzanne Ashworth in her excellent book on seed saving, Seed to Seed. The second column gives the same figures as specified by the United States Department of Agriculture.

Purity vs Ease and Practicality

As you can see, Ashworth's distances are significantly larger than those given by the USDA... there are at least two reasons for this difference. The first reason is that Ashworth is concerned with keeping seed samples very pure, which involves preventing any possibility of crossing with different varieties. In fact, the USDA uses several 'grades' of quality of seeds' genetic purity, and does use much greater distances for the higher grades.

In general, the distances given by the USDA will be sufficient to prevent most crossing. The home gardener can easily weed out the occasional plant that does not grow true to type, or even take advantage of the crossing to possibly discover a new variety that they'll like even better than the original.

Size of Planting Affects Distances Needed

A second reason for the differences in recommended distances is that the USDA addresses mostly commercial seed growers growing extensive plantings of the same variety. In very large plantings, almost all undesired crossing happens around the edges of the planting—in fact the highest grades of certified genetically-pure seed are taken from the centers of large plantings (at specified distances from edges), and the outsides used for lesser, commercial grades of seed.

Ashworth's figures are aimed toward home gardeners, whose seed crops may be composed of just a few plants left to seed after the main harvest of food has been made.

How a Plant is Pollinated Matters

The way a given plant is pollinated makes a difference as to distances needed, too. Wind pollinated plants will need more room on an open plain than they would need in forested areas or in cities where trees and/or buildings help block winds. Bee pollinated plants may need more room in arid lands where bees must travel farther to forage for nectar than bees in more densely-vegetated areas. In actual practice, many factors can combine to change the distances needed for safe true-to-type seed saving.

Experiment and Observe

The most practical thing for most home gardeners will be to use a blend of the recommended distances, considering such factors as:

• level of purity desired;
• size of plantings involved;
• whether you've had problems in the past with similar crops;
• how pollination is effected (i.e., by wind, insects, etc.);

...and so on. After a few seasons' practice and experimentation you'll have a good feel for the different factors involved and for how picky you feel you need to be in order to get satisfactory results. Remember, the enormous variety of seed varieties that have been developed over the millennia relied to a great extent on accidental (or purposeful) crossing of different varieties, with the resulting plants selected for the traits that make them most successful in a given area or for a given purpose or taste. Experimenting can be a large part of the fun!

Plant Isolation Distances Table

Footnotes:

1. Green amaranths may need only ¼ mile, grain amaranths up to 2 miles.
2. See paragraph on Self Pollination in Distance Isolation in the article on Saving Seeds True-to-Type.
3. Potatoes are not commonly reproduced from seed.
4. See note on tomatoes and beans in the article on Saving Seeds True-to-Type.
5. "Distance adequate to prevent mechanical mixture is necessary".
6. Isolation distances for cotton vary from 100' between similar varieties, to ¼ mile between 'upland' and 'Egyptian' types ('foundation' or 'preservation' grade).
7. Extrapolated from similar species.