Vegetable Seed Saving Handbook
Plants which can be cross-pollinated by similar varieties are still easy to save if only one variety of the same species is grown at a time (e.g., one eggplant or one pepper). If using this method, however, make sure your neighbors aren't growing different varieties of the same plant, and that wild varieties of your plants do not grow in the local area. Queen Anne's Lace is the same species as the common carrot, for instance, and will cross-pollinate your carrots—carrots you grow from the resulting seeds will not grow true to the variety you started with.
If you want to grow more than one crossable variety of a species at a time and save genetically pure seed from any of them, you can prevent crossing by 'isolating' your heirloom varieties from each other. Isolation means to prevent pollen from different varieties being able to reach each other and cross-pollinating (i.e., mixing) the varieties.
Each of these methods is discussed in more detail below.
An excellent resource for learning more about techniques for protecting seed crops is Suzanne Ashworth's excellent book Seed to Seed, available from Seed Savers Exchange, an excellent resource for all seed savers.
The simplest way to isolate varieties is to plant them far enough away from each other so that cross-pollination is not possible (see Plant Isolation Distances). The amount of space needed for safe distance isolation varies for different plants—and there is always the risk that someone nearby may grow a cross-pollinating variety, so get to know your gardening neighbors!
Some plants can be protected from crossing by just a small distance, such as lettuces (25 feet between varieties) and eggplants (50 feet). Others, such as peppers, stretch the possibility of distance-isolation in the average garden by requiring 500 feet for safe isolation between varieties. Bee-pollinated plants such as the cabbage family (collards, broccoli, etc.), squashes and okra require from ¼ to 1 mile for complete safety—often difficult to accomplish or verify, due to gardening neighbors. Corn, a wind-pollinated plant, can require a mile or more for safe distance isolation and members of the Beet Family may need as many as 5 miles!
The exact distance you'll need to safely isolate a particular crop depends on a number of factors. Central among these are the type of plant and how it is pollinated (i.e., wind, insects, self—or a combination of these). Also important are the particulars of your growing situation including exposure, climate, prevailing wind patterns and surrounding terrain and vegetation features.
Relative size of plantings has an impact, too. Small plantings are more vulnerable to foreign pollen (whether carried by wind or insects) than large plantings, since in a large planting foreign pollen is spread among many more plants. Further, plants on the edge of a group are more vulnerable to crossing than plants in the middle. For this reason, whenever a variety is important, collect seed from plants nearest the center of a planting for greatest purity.
These and many other factors help account for differences between the distance-isolation figures given by Suzanne Ashworth in Seed to Seed and those given by the USDA or other authorities (see the Plant Isolation Distances Table). While the USDA figures are based on large plantings, Ms. Ashworth's figures are better-adapted to the scale of the home gardener.
Actual distances which will work for you may have to be determined experimentally. Until you know that smaller distances are safe for your conditions and plant varieties, use the larger distances. Caution is especially important with rare or heirloom plants. If you experiment with smaller isolation distances, use common varieties instead of rare ones in case of accidental cross-pollination. Remember that once two varieties have crossed, their genes are inextricably mixed.
Distances needed for isolation of wind-pollinated plants varies with your conditions. Wind patterns and strength, existence of windbreaks, etc., all have an effect. Wind-pollinated plants grown in wooded, low-wind areas will need much less distance than the same plants grown in vast, open, windy areas, for instance.
Pollen from corn, which is relatively heavy, falls to the ground within a few paces in a quiet protected spot, but can travel half a mile or more in open, windy areas. Windbreaks are more effective for plants with heavy pollen like corn, than for plants with light pollen like beets or lamb's quarters.
Isolation distances needed by insect-pollinated plants varies according to what animal or insect is the pollinator, and on how plentiful local nectar and pollen sources are. In areas where nectar and pollen sources are plentiful, bees and other pollinators do not have to travel as far in their foraging as in sparsely-vegetated areas.
If bees are the pollinators, distances needed also depend on which hives collect from where within your local area. Bee hives are territorial and do not overlap foraging areas—plants in one hive's territory won't be bee-pollinated by plants in a neighboring hive's territory. For this reason, distances needed to isolate bee-pollinated plants in areas with plentiful, diverse vegetation may be a quarter-mile or less, whereas distances for the same plants in sparsely-vegetated areas might be a mile or more.
Self-pollinated garden plants' flowers include both male and female parts and are receptive to their own pollen. This allows them to fertilize themselves. Among self-pollinating plants, some are exclusively self-pollinated, whereas others can be fertilized either by themselves or by other plants of the same species.
Self-pollinating plants whose flowers are open during the time pollination takes place (such as okra or peppers) can either fertilize themselves or be pollinated by other plants (and so they do need some protection from crossing). Since they will self-pollinate, however, they can be caged without worrying about letting insects into their cages to pollinate them.
If these types of plants are grown out of the wind where bumblebees can't reach them (i.e., in a greenhouse or coldframe, etc.), it will help to gently jiggle the plants a time or two per day to mimic the wind and bees. This helps to move the pollen around within their flowers.
Self-pollinating plants which fertilize themselves before their flowers open (such as tomatoes and beans) are exclusively self-pollinating. These do not need to be isolated under normal circumstances, since by the time their flowers open they will have already pollinated themselves. These plants also benefit from being shaken if they are grown where wind and/or bumblebees cannot shake their flowers naturally.
Even plants whose flowers are closed during pollination, however, might cross-pollinate under certain circumstances. Self-pollinating flowers can be forced open by hungry insects in areas with few nectar or pollen sources, for instance. For these reasons even plants which do not normally cross-pollinate may benefit from protection from nearby varieties of the same species in deserts or other sparsely-vegetated regions, or if their purity is particularly important.
When distance isolation is a problem, caging is too costly or troublesome, and you're only growing a couple varieties in a season, you can use 'time isolation' to prevent crossing. Time isolation works with any two varieties that shed pollen over a limited time and have sufficiently different rates of maturation (as usual, don't forget your gardening neighbors, who may be growing varieties that will cross with yours).
To use time isolation, plant two similar varieties at staggered times. If properly times, by the time the later of the two varieties is flowering, the earlier variety will already have finished flowering and will no longer be producing or receptive to pollen. Plant the earlier, faster-maturing crop a couple or three weeks before the later, slower-maturing one. If they are plants which flower over a short time, the two flowering periods may not overlap at all.
Even if the flowering periods of two crossable varieties overlap a little, time isolation may still be an option. The secret in this case is to manually prevent flowers from blooming on both varieties at once. Simply remove late flowers from the earlier-flowering variety (after seeds have been produced) so that the later crop can pollinate and set seed without being crossed. Remove flower buds well before they open, since some varieties open early in the morning. This method can work well with plants which produce seed over a long season, such as okra.
If safe isolation distances are not available and time isolation is not suited to the plant (as for basil with its long flowering period and numerous, small flowers), 'caging' or 'bagging' techniques can also be used to insure against crossing. In caging, plants are protected by cages covered with mesh or fabric. The weave of the covering must be small enough to prevent passage of insects or pollen (depending on whether the plant is insect or wind-pollinated, respectively). Bags over individual flowers or flower heads can be used for self or wind-pollinating plants. In this case the bags simply act as tiny 'cages'.
Cages covered with regular window screen are fine for keeping insects safely away from insect-pollinated plants. Use large frames which hold the mesh away from the plants inside them, so that insects cannot reach flowers that might otherwise press against the inside of close-fitting cages. Build the cages large enough for the plants at maturity, and place them over the plants as soon as they start to flower (remove any flowers which open before the cages are put into place).
Caging procedures vary according to whether your plants can self-pollinate (such as peppers), or whether they are insect-dependent and so actually require insects for pollination (such as squash or basil). If they are dependent on insects for pollination, 'alternate day caging' will have to be used.
Insect-dependent plants—which actually require insects for pollination—must have their cages removed periodically so that insects can reach their flowers and pollinate them.
Plant two or three varieties and cover them with separate lightweight cages. Once every other day (for two varieties) or every three days (for three varieties), remove the cage from one variety (don't remove cages from any two varieties on the same day, and replace the removed cage in the evening).
This exposes a different variety to pollinating insects each day, and gives each variety a chance to be pollinated every two or three days. Remember, however, that even removing just one cage at a time can still allow crossing if your neighbors are growing plants that will cross yours.
Insect-pollinated plants which are also self pollinating can be left in their cages full time and will produce seed.
When using cages to protect wind-pollinated plants from crossing, cover the cages with a fine-weave fabric (such as spun polyester) that keeps pollen out but allows air and sunlight to pass. Grow as many plants as possible inside each cage for a large genetic base. Shake plants or cages daily to help distribute pollen inside the cages.
If the plants you're protecting are small and closely-planted, spun polyester or like material can simply be wrapped around the group and tied at the base to form a frameless 'cage'. Be sure to thoroughly seal the bottom of the bag with cotton, leaves or mulch, etc.
For certainty with the very light-pollened plants such as chard and beets, or other wind-pollinated plants if they're growing in close quarters, plastic or other wind-impermeable material may need to be used.
If none of the above isolation techniques are practical for you, or you want to maintain a high degree of control over which plants pollinate each other, you can individually hand pollinate flowers or flower heads and enclose them in cloth or paper bags. This technique works well with large-flowered plants such as squash or daylilies.
For more detail on any and all of these techniques, see Suzanne Ashworth's book Seed to Seed.
Reprinted from Vegetable Seed Savers Handbook, Jack Rowe 1998, howtosaveseeds.com