Proper storage of seed is vital to conserving its vigor and vitality. Seeds can last from a few years to centuries, depending on the species and the storage conditions. In general, most seeds stored in cool, dry conditions will survive longer than seeds stored in a wet, warm environment. In many parts of the world, agricultural seeds are stored in bins that are open to the ambient conditions, often resulting in short storage life and poor seed quality in hot, humid regions, as well as losses due to insects and rodents. As the viability period for seeds decreases by half for every 1% increase in seed moisture content or 10°F (~5°C) increase in temperature, sealed bins and controlled environments are used to maintain seed viability for longer periods. A common rule of thumb is that the temperature (in Fahrenheit) plus the relative humidity in the air (in percent) should total less than 100 for satisfactory seed storage.
At seed banks that store seeds to preserve genetic diversity, seeds are dried to optimum moisture content, evaluated for quality and genetic purity and sealed in moisture-proof containers. For short-term storage, seeds are dried and placed in sealed containers at 5°C. They are stored at temperatures below freezing for long-term preservation (0°F or -20°C), including the use of cryopreservation, or freezing in or over liquid nitrogen at -180°C, for extremely long-term storage. But when it comes to seed storage, one size does not fit all. Some species have short-lived seeds that do not tolerate dehydration and are therefore difficult to store. Plants producing these recalcitrant seeds must be maintained as living populations, making them vulnerable to loss due to changes in land use or weather patterns.
The vigor and viability of seeds stored even at low temperatures declines over time. The continued maintenance of specific seed lines requires that they periodically be removed from storage and used to produce a new crop of seeds. Seed storage facilities therefore need not only modern storage equipment but also the land, personnel and expertise to periodically grow the stored seeds under conditions that maintain their genetic diversity and purity in order to replenish the original seed stock. Thus, core seed repositories such as the U.S. National Center for Genetic Resources Preservation in Ft. Collins, Colorado, are complemented by a number of branch stations in different growing environments to store and replenish seed collections of diverse species.
Recalcitrant seeds are seeds that do not survive drying and freezing and therefore cannot be stored for long periods of time. Some plants that produce recalcitrant seeds include avocado, mango, lychee and some horticultural trees.