Vegetable Seed Saving Handbook
How you prepare and store your seeds can be as important to their eventual viability as how they were grown. Seeds must be carefully dried and then stored under the proper conditions in order to give them the best chances of germinating and producing healthy plants when they are planted. Luckily, all the needs of seeds are easily provided once you are aware of their requirements.
For purposes of storage, there are basically two types of seed: 'desiccation-tolerant' and 'desiccation-intolerant'. Most of the garden plants with which we are familiar produce desiccation-tolerant seeds, which means they can be safely dried for long-term storage. Exceptions include many aquatic plants, large-seeded plants, and some trees (such as oaks and buckeyes), many of which produce desiccation-intolerant seeds and will die if allowed to dry.
Desiccation-intolerant seeds do not enter dormancy after maturing. Instead, respiration and other physiological processes continue. Continued respiration after maturation causes desiccation-intolerant seeds to deteriorate rapidly once they have matured, so they must be planted while still fresh. Desiccation-intolerant seeds partially or completely lose viability if they are allowed to dry—usually they die.
Since desiccation-intolerant seeds must be stored moist, they can only be kept for short periods of time before they begin to succumb to fungal or bacterial rots, or run out of stored food reserves because of continued respiration.
Some seeds (such as citrus) are 'borderline' desiccation-intolerant. These can be dried and stored for some time, but lose viability quickly and germinate slowly once they've been dried. Slow germination rates and lowered viability make borderline desiccation-intolerant seeds susceptible to damping-off or other microbial damage during germination. For this reason even seeds which are only borderline desiccation-intolerant perform best if planted fresh.
Desiccation-intolerant seeds can be stored for a short period of time—sometimes months—if they are kept moist and cool (avoid freezing!). Moisture allows the seeds to continue respiration, while cool temperatures inhibit fungal and bacterial activity so that they do not rot during storage.
To prepare desiccation-intolerant seeds for storage, place them in a container filled with moist peat moss, sand or paper towels to keep them from drying out. Make sure the container admits a small amount of air (leave the lid loose or poke some holes), because desiccation-intolerant seeds will continue to respire or breathe and will suffocate if sealed into air-tight containers. Store in the refrigerator. Do not let the seeds freeze or dry out. Plant them as soon as practical, or they will begin to mildew or rot because of their high moisture content.
During ripening and drying, desiccation-tolerant seeds prepare for dormancy by greatly slowing or ceasing most physiological processes, and by converting food reserves from sugars to more stable fats and starches. After they have prepared for dormancy, and unlike desiccation-intolerant seeds, desiccation-tolerant seeds can be safely dried and stored for long periods of time without significant loss of viability (many years in some cases). Some desiccation-tolerant seeds even require drying to complete the ripening and dormancy process before they will germinate.
How desiccation-tolerant seeds are dried and stored is very important to maintaining their viability and vigor over the long term. Drying should be gradual and thorough, and desiccants used when drying seeds in air above 30% relative humidity or so. During storage, seeds must be kept at appropriate temperature and moisture levels for greatest longevity.
Even under ideal conditions, long-term storage lowers viability percentages (since some seeds die) and also lowers the vigor of seedlings produced by the stored seeds. Further, as length of time in storage increases, the number and percentage of seedlings with damaging mutations or tissue degeneration also increases. Roots in particular are adversely affected by long-term storage. The more frequently a batch of seeds is regrown (and therefore the shorter the storage times), the better will be the overall health of the plant populations produced.
Desiccation-tolerant seeds should be slowly dried in a shady spot. Spread seeds one or two thick in an airy, dry location—such as an air-conditioned environment or other place where relative humidity stays between 20% and 40%—for two or more weeks. Large seeds must be dried longer than smaller seeds. When dry, corn and beans will shatter when hit with a hammer, squash seeds will break instead of bending. Seeds which pass this test are dry enough for safe short-term storage.
Seeds air-dried during humid weather require additional drying with desiccants such as silica gel before final storage (but don't use heat!). Most seeds benefit from drying with silica gel if they are to be stored for very long. The longest storage life for desiccation-tolerant seeds is achieved by drying them to between 5% and 7% moisture content (by weight) and then storing them at several degrees below freezing. As storage temperature rises above freezing or moisture content rises above 5 to 7%, longevity in storage goes down and the incidence of mutation rises. Seeds dried to a low moisture content with silica gel and then stored in a freezer can usually retain viability for many years.
To use silica gels for drying seeds, place equal weights of dry silica gel and seeds to be dried in a well-sealed jar for 7 to 8 days. Then transfer the dried seeds quickly into airtight storage jars and place in a freezer, refrigerator or other cool, dark place.
Carefully follow the instructions which come with your silica gels for drying them after use (to avoid burning them and rendering them useless). Silica gels suitable for drying seeds can be purchased from Southern Exposure Seed Exchange, PO Box 460, Mineral, VA 23117.
Since desiccation-tolerant seeds stop almost all metabolic activity as they mature and dry, they can be stored for months or even years with only minor loss of viability and vigor. Desiccation-tolerant seeds which show high germination percentages when fresh—if properly dried and stored in a freezer—can typically retain their viability for years.
Seeds with low initial germination rates will begin to lose viability fairly quickly, however, even under ideal storage conditions. Seed lots with a low initial germination rate should be regrown as soon as possible. If a batch of seeds with poor germination is grown out and a healthy batch with good germination produced from them, the healthy batch can then be dried, frozen and stored for long periods successfully.
A common problem with stored seeds is mold or mildew resulting from incomplete drying before storage. Dry your seeds thoroughly before storing them (though drying them to 0% moisture will of course cause their death). If seeds sweat on insides of jars during storage, they are too wet and must be dried further in order to store successfully. At this point the use of a desiccant is a good idea. Don't tarry, because damp seeds will mildew quickly.
Fluctuation in temperature or moisture levels of stored seeds lowers the seeds' longevity significantly, causing loss of viability and vigor or even seed death. Rapid moisture fluctuations are particularly damaging to seeds. High moisture or temperatures encourage mutation of seed tissues—especially in root tips, which remain more active than other seed tissues. Cellular mutations affecting metabolism or root tissue structure are a common cause of seed failure upon germination.
Dry your seeds properly before placing them in cold storage. Keep your stored seeds at a constant temperature if possible and remove them from storage as seldom and as few times as possible. When seeds are removed from cold storage in order to retrieve samples, allow the entire container to come slowly to room temperature before opening the seal. This will help prevent condensation of atmospheric moisture onto the cold seeds which might otherwise occur.
Another common problem with stored seeds is infestation by weevils or other insects. In fact, it is best to assume that some insects are present in any sample of seeds. Seeds kept in frozen storage are safe from insect damage, since—though insects may survive the freezing—they will be rendered inactive while frozen by the low temperature. Even if your seeds will be stored frozen, however, it doesn't hurt to take precautionary measures against insects.
Insect infestations may be guarded against by the addition of a little diatomaceous earth (D.E.—available at organic gardening stores) to the stored seeds in their jars. Add just enough D.E. to cover the surfaces of the seeds (add a few pinches to the seeds in a bowl and gently stir to thoroughly cover each seed). Adding D.E. to jars of stored seeds helps insure the long-term safety of your seed stocks and is an inexpensive, safe, non-toxic and wise precaution.
Reprinted from Vegetable Seed Savers Handbook, Jack Rowe 1998, howtosaveseeds.com