Seed Saving Bulletin No. 9

Isolation techniques for the seed saver

There are a few ways to isolate varieties of plants to prevent unwanted crossing. Some are more suited to the farmer or gardener with ample space, and some work for anyone no matter the square footage. The important thing for the seed saver to keep in mind is that within a family of plants, all members of the same species can cross-pollinate (including wild relatives). Therefore it is important to know the name of the family, genus, and species to be a successful seed saver!
After all, if you just know the species of something, you won’t get very far. Broccoli and spinach are in two different families, but they are both species oleracea. (Latin for “used as food”)

The first way to isolate is distance. This is the most difficult for people with small spaces. Each crop has different distance requirements based on the pollination technique. For wind-pollinated plants such as carrots or corn, recommended isolation distance is 1-3 miles depending on other barriers present such as trees or buildings. For self-pollinated plants such as tomatoes or peas, 10-25 feet are recommended. It should be noted that the more “organic” the farmer, the more isolation needed. This is because of higher pollinator activity found in biodiverse fields. Each plant’s isolation distance should be researched before planting to insure purity of seed, noting that much of the recommended distance information is based on chemically treated fields (so an organic gardener needs more space). To get around this, another technique some gardeners use is to plant crops in blocks instead of rows. This strategy seems to produce a higher percentage of progeny that are true to type in the middle of the planting, with the border plants used as food. This is not necessarily foolproof with the out-breeders, but may cut down on distance requirements.

Time is another isolation technique that works for both large scale and small-scale growers. Time can be used in many ways. One way is to save seed from different varieties of the same crop that mature at different times, thus ensuring the flowers will not be receptive on both varieties at the same time. Another possibility is to take advantage of the shelf life of seeds. For instance, if lettuce seeds typically last three years, each year of the expected seed life the gardener should grow a different variety, rotating through the cycle so as to always have seed. In other words, each of the three years the seed-saver should save a different variety, starting over in the fourth year.

The third technique is mechanical isolation. This is done with “cages” or bags. Cages are simply wooden box frames with fabric made out of spun polyester cloth (Reemay) tacked on to form the walls and ceiling. This fabric can be purchased from many garden retailers. The cages are placed over a population in order to isolate it from another variety. The cages can be removed from each population on alternate days in order to ensure pollination. The more varieties being isolated, the less seeds will result. More than four varieties is not recommended for this technique.
The second mechanical isolation option is using bagging, which requires the seed saver to hand pollinate. For this, the seed saver must have bags made of the same spun polyester and knowledge of the plant flower parts. Brightly colored markers are then used to indicate which flowers have been pollinated after the bags have been removed. It is recommended to pollinate the same flower at least three days in a row.
Many seed savers also grow what is called a “barrier crop”. This is a crop like sunflowers that are highly attractive to pollinators, and is grown between varieties of plants that are being isolated.

Whatever the isolation technique used, it is essential the seed-saver get to know the varieties of plants grown so that the most effective method is employed.