Vegetable Seed Saving Handbook
What counts as 'good' seed saving practice is very different depending on whether you want to:
This chapter will tell you how to preserve an heirloom variety in its original genetic makeup. This is very different from growing seeds for adaptation to your personal gardening climate, tastes and/or habits.
If you want to save seeds to plant in your own garden, changes can be desirable. Many creative factors come into play as you choose which plants to allow to produce seeds, and which of those to harvest from when their seeds are mature.
For more on how to save seeds that adapt to your own garden, see also Breeding New Varieties.
When you grow seeds to preserve a variety, on the other hand, much greater care is required since your goal is to ensure that all the variety's genes are transmitted from each generation to the next—any changes to an heirloom variety are undesirable from a preservation standpoint.
The goal when you save heirloom seeds for preservation is to prevent any type of loss or change due to outside influence (such as crossing by other varieties, or adapting to your local gardening conditions). You want to preserve both the entire range of genetic traits, as well as the original balance of traits. Any selection for or against any of the variety's traits would result in a change in the variety's overall genetic composition.
In practical terms, this means that for conservation purposes you want to:
These two practices together help to minimize genetic loss due to plants that perform poorly in your garden—they might be another gardener's best performers!
There are many ways you can accidentally change an heirloom variety—some obvious, others more subtle. To successfully grow an heirloom variety for preservation, it helps to grow, harvest, process and store your seeds with care and a good understanding of their health needs.
Of course, you must also keep them from mixing or 'crossing' with other varieties—which would ruin the batch of seeds for preservation purposes.
Here are some of the ways you can accidentally change a variety as a result of the way it is grown:
Most vegetable (and flower) varieties must be kept protected, or 'isolated,' from other similar varieties (i.e., same species) during flowering to avoid cross-pollinating the plants and mixing their genes. Seeds saved from plants that have been cross-pollinated by other varieties do not reproduce true-to-type. Instead, cross-pollinated seeds produce plants with an unpredictable mix of traits from both varieties.
Crossing is not always undesirable if you enjoy experimenting (see Genetics and Plant Breeding). Once crossed, however, the original set of genes is permanently altered, and the original variety lost (unless there are other seeds of the variety growing elsewhere). For preservation purposes, cross-pollination between crossable varieties must be prevented by physically isolating the varieties from each other.
Carefully plan your garden at planting time to avoid cross-pollinating heirloom varieties. Before planning your garden's layout, study the isolation requirements and means of pollination of the plants you will grow for seed (see Details for Saving Seeds from Common Vegetables and Table of Plant Isolation Distances).
Plan your garden to give plants the room they need to avoid crossing, to allow room for caging, etc. Remember—even if you are not saving seed from a particular variety, it may still pollinate other plants that you are growing for seed. A little time spent laying out your garden before planting can save time and effort later—and can make the difference between producing pure seeds or crossed ones.
Happily, some vegetable varieties are exclusively self-pollinating and can be grown with virtually no danger of crossing. These plants include beans, cowpeas and tomatoes. Producing genetically pure seeds from these plants is as easy as growing the plants and harvesting their seeds at the appropriate time (in the case of tomatoes, see Why Ferment Some Seeds?).
Plant different varieties of these plants far enough apart to avoid mixing them up at harvest time, however (an especially good idea with vining plants).
Collecting seeds from too few parents can damage a variety's genetic base, weakening the variety seriously (called "inbreeding depression"). Since no individual plant can contain all the genes belonging to a variety (except in the case of clones), some minimum number of plants is needed in order to maintain the diversity and vigor of the variety. In general, the larger the number of plants grown together, the healthier the population will be in the long term.
The exact number of plants you have to grow to keep a variety healthy depends on the plant. One important question is: How variable are the individual plants within the variety? Highly variable varieties need larger populations to preserve their genetic base than are needed by more uniform varieties.
Another factor governing healthy population size is how pollination is accomplished by the variety. For self pollinated plants such as tomatoes or beans, just a few plants will suffice since these plants do not have to be cross-pollinated to produce seeds. Even self-pollinating plants, however, rely on a small amount of crossing (typically 2% to 5% per year) to reinvigorate seed populations, and can lose vigor if too small a number of plants are grown for too many seasons in a row.
Plants that are both self-pollinated and cross-pollinated, such as peppers and lettuces, also do not rely completely on cross-pollination—these varieties can maintain their vigor with a relatively small number of parent plants (maybe 5 or 10). Larger plantings grown every second or third year help to reintroduce genetic variability and keep the population healthy. Again though, the best practice is simply to allow as many plants to produce seed each year as is practical for your circumstances.
A plant known to need a particularly large parent population for long-term breeding health is corn. Corn needs at least 100 parent plants to produce healthy see batches (200 are better). This means that at least 100 ears must be left on their stalks to mature. If you're growing corn to eat fresh, you'll have to let an ear on each plant dry on the stalk, or plant extra plants to save seeds from (see How to Avoid Inbreeding Depression in Corn).
Even after careful planning and adequate growing conditions, loss of genetic material can result if proper harvesting, cleaning and drying techniques are not followed.
As discussed earlier, the goal of seed preservation (as opposed to seed breeding) is to conserve a variety's entire range and balance of genetic characteristics. In practice, this is easier said than done. If you're not careful, it is relatively easy to reduce or alter a variety's genetic makeup without even noticing that you're doing so until after permanent changes have already occurred.
Allowing unintended or unnoticed influences to change the genetic composition of a variety—a common danger in seed preservation efforts—is called 'unconscious selection.' There are a number of ways this can happen. Accidentally crossing your variety with another variety is common and fairly obvious. Less obvious are changes due to unconsciously favoring some plants over others because of personal gardening practices or local climatic conditions.
Unconscious selection can be caused by failure to nurture less-adapted plants into producing seeds, or by selective seed collection at harvest time (such as taking a larger number of seeds from plants that perform better in your garden).
Even if unintended, selection for locally-adapted traits will automatically occur if you save more seed from plants which do better for you (a good idea if you are trying to change the variety to adapt it to your growing conditions, but not a good idea when you're trying to preserve a variety).
If you save proportionally more seed from more successful plants, poorer performers will start to become under-represented in the seed batches you produce. The variety begins to adapt to its circumstances, losing characteristics not suited to your particular conditions (though these characteristics are part of the variety's genome and may make it better suited to growing under other conditions).
Even though choosing traits adapted to your local conditions is part of producing new, locally adapted strains (see Selecting New Strains from Existing Varieties), it is not acceptable for preservation purposes. The reason is that plants that don't do well when grown under your conditions might do fine if grown by another gardener. They might even be her best performers! It would be a mistake if some of a variety's genes were lost forever simply because of some peculiarity in your own climate or gardening habits. Allow selection to take place only when it is your specific goal to produce locally adapted seeds—but not when you are growing a variety for genetic conservation purposes.
For these reasons, when growing seeds for preservation always save equal amounts of seed from each undiseased plant. Saving seeds from each plant helps ensure that all the traits within the variety are preserved; saving equal amounts of seed from each plant helps ensure that the original balance and range of traits is preserved.
Rather than wasting extra seeds produced by plants that do well for you, they can be saved and treated separately as a locally adapted strain. Keep two seed batches—one with equal amounts of seed from each plant and another made up mostly of seeds from your best adapted-plants. This is a good idea, since it results in both preserving the original variety (all plants) and in producing a new strain adapted to your own growing conditions (most-productive plants). After beginning this process, the two strains would need to be treated as separate varieties and isolated from each other.
Even after seeds have been properly grown, collected and processed, their genetic material can still change or be lost in storage. Changes can occur due to the death of some of the seeds if they are stored for too long a time, or if they are stored under unfavorable storage conditions (see Storing Your Seeds for Long Life).
Even seeds that survive storage and sprout after planting can suffer loss of genetic material because of mutations that occur during long storage. Genetic mutations increase in frequency as storage time increases. Roots, especially, are susceptible to loss of vigor in storage due to cellular mutations. Properly dried and kept in frozen storage, your seeds should remain vigorous in storage for the lengths of time given in Details for Saving Seeds from Common Vegetables.
Reprinted from Vegetable Seed Savers Handbook, Jack Rowe 1998, howtosaveseeds.com