Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Vegetable growers and processors throughout the region have purchased seeds for this crop year. Many of those spring planted seeds are already in the ground or are growing in greenhouses. However, seed for successive plantings through the summer must be stored. Much harm can be done to seed viability through the storage period and germination can be greatly reduced.
Many smaller growers buy larger quantities than they can use in a season to get volume discounts and then save the seed for upcoming years. Again, how that seed is stored can greatly affect germination in coming years. The most detrimental storage condition for seeds is high temperature coupled with very high humidity (think Delmarva in the summer). Seeds that have picked up moisture from the air will lose viability quickly. For each 1% increase in seed moisture, seeds lose half of their storage life. For each 9º F increase in temperature, seeds lose half of their storage life.
Uninsulated metal buildings make poor summer storage whereas older wooden sheds and barns or concrete block buildings are better. Seeds also should be kept in the dark. Most seed packaging excludes light but opened seed bags or containers can be at greater risk.
The ideal would be a well insulated structure that is shaded and kept dark. Air conditioning and refrigeration may be a good answer in the short term, especially for smaller lots.
As a general rule of thumb the combination of temperature with relative humidity in storage should be less than 100 ( 50°F + 50% RH, 40°F + 60 % RH, etc.) for seed storage. The colder the storage, the higher the allowable humidity, the hotter the temperature, the lower the allowable humidity. However, for longer term storage, the temperature and relative humidity should be kept somewhat lower (40°F; 30 % RH for most seeds).
How about freezing seeds (for example, long-term germplasm collections are stored at 0ºF)? Freezing will work very well if seeds are dry. If they have picked up significant moisture, they can be damaged in the freezing process. Also, freezing and thawing cycles can be damaging to seeds so remove seeds to be used and place the remainder back in the freezer quickly.
Vegetable seed that come in sealed containers or packaging should not be opened until just ready for use. Seeds in bags should also not be opened until being used. Open bags should be completely planted unless sealed and placed back in proper storage.
Vegetable seeds also vary by type in their ability to store for extended periods. For example, onion seed has less than 1 year storage potential and should be bought new each year. Sweet corn also stores poorly over 1 year, as does spinach. Beans and peas are intermediate with 2 year storage potential and peppers are also in the 2 year range. Melon, cucumber, squash, and pumpkin seed, as well as cole crop seeds (cabbage, broccoli, cauliflower, kale, collards), tomatoes, and eggplants can be stored for 3 years.