How many plants should the seed saver grow?
The number of plants of each species that the seed-saver should grow varies depending on a number of factors. The most important is the motivation behind saving seed. This should be determined before embarking on this adventure. If the gardener/farmer simply wants enough seed to plant for the next year, then the minimum numbers of plants in order to avoid depressed vigor yet produce adequate quantities of seed should be grown. Therefore the one variable that cannot be avoided for the seed saver is whether the plant is an in-breeder or an out-breeder.
For in-breeders, fewer plants are needed for the home gardener than the plant breeder or farmer. Many sources recommend a minimum of 20 plants, however this is not always a possibility with small spaces. Therefore a few to ten plants are adequate for in-breeders, because of the existing depressed genetic diversity in these populations (unless they are a land race—more on that below). These plants evolved to be self-pollinating so they tolerate lower levels of genetic diversity as a payoff for having more reliable access to pollen. Cross-pollination can occur as a result of heavy pollinator activity, however it is not necessary for the survival of the species.
It is also commonly recommended that seed savers sow at least 100 plants for out-breeders, but this is clearly not possible for many home gardeners. For plants like corn, severe inbreeding depression occurs from a small population, so the seed saver has little choice. Others like the Cucurbitaceae family act like in-breeders and can tolerate a population as low as 5-8 plants. For more on population information, Carol Deppe’s book Breed Your Own Vegetable Varieties is invaluable for the home seed saver, and Suzanne Ashworth’s book Seed to Seed is another important resource. (It should be noted that some of the names of families have officially changed since the last printing of Ashworth’s book—for instance Leguminosae is now Fabaceae).
Whether an in-breeder or out-breeder, some cross-pollination can occur, and if the seed-saver is trying to preserve as much genetic diversity as possible, a larger population is needed. This is especially important with land races, which have greater variability in genetic material. According to Ashworth, the gardener saving from a land race should grow “double or even triple, with seed being saved from each plant”.
On the other hand, seeds saved from only one plant are preserving the qualities of that plant, not the diversity of the whole species. This may be desired if the seed-saver is attempting to create a new variety out of an existing one, however it is only possible with inbreeding plants like tomatoes.
When saving seed, not all plants will express ideal phenotypes. If all plants in the population have the same amount of space, sun, water, and nutrition, then only plants that show vigorous growth, are true to type, or flower in an appropriate time should be used for seed (ex. lettuce that flowers earlier than others in the population is not desirable). This is another reason that the seed-saver may want to grow more than the minimum number of plants—not all plants in the population will present the desired characteristics.
Some gardeners/farmers want to eat from the same plants that seed is harvested from. This is not always a possibility, and each population’s growth patterns should be considered, as this will also affect the number of plants to be grown. For instance, many people eat the outer leaves of loose-leaf lettuce (thus prolonging the “eating” stage of the plant) before it inevitably bolts in the heat of summer. This can be done to a certain extent without harming the quality of seed, so fewer plants may be grown than if it could not be harvested from at all. Eating from the same plants requires the seed saver to spend more time in the garden learning the lifecycle of the plants so as to ensure both food and seed.
The power of saving homegrown seed is being able to select for the characteristics most appropriate to the seed-saver’s environmental conditions.
Did the plant or population thrive or survive in a drought or heavy rains? Is this something to save for? Each seed-saver has unique pressures that will affect the plants ability to produce seed (fitness of the plant). It is these pressures that make home-saved seed better adapted to the environment of the gardener/farmer than store bought seed. The size of the population will obviously determine how much selection a gardener can do within a species.
