Saving Seeds for the Future - the Seed Bank at Berry Botanic Garden
Volume 2 Number 9 - December 1997
Edward Guerrant and Linda McMahan
A seed bank is perhaps the most useful technique for off-site or ex situ conservation of wild plants. Seeds are compact, dormant germplasm packages that lend themselves to storage.
Nature's gift of seed dormancy makes it possible to maintain genetically representative samples of endangered plant populations in 'suspended animation' for long periods. Storing seeds is a relatively inexpensive conservation measure, and takes so little space that millions of seeds can be stored in a small freezer. Anyone can set up a seed bank, even at home.
This seeming simplicity, however, masks some important concerns and scientific questions. The science behind seed banking is still emerging, not always with complete agreement amongst experts. Also, saving seeds has its hidden dangers. Storing seed can create a false sense of security, leading some people to conclude that they have "saved" a species. Finally, the stored seed sample may turn out to be woefully inadequate for the intended purpose.
Despite these and other cautions, we believe that seed banking, done correctly, has a central role in plant conservation. However, it should never be the only action taken to conserve a plant species. Rather, seed banking is one component of the comprehensive approach of integrated conservation strategies, which are rooted in conserving habitat but use all available means to conserve a species. Seed banks are also good sources of plant material for research, when taking seeds from the wild is difficult or undesirable.
In 1983 we set up at the Berry Botanic Garden the first seed bank in the United States for the purpose of conserving the rare flora of an entire region - in this case the Pacific Northwest (Oregon, Washington, Idaho and N California). The first Curator of the Seed Bank, Julie Kierstead Nelson, was hired under a small grant from a local foundation. She purchased the basic references available at the time, a small chest freezer, a desiccator, and boxes of screw-cap glass vials. Soliciting seeds from botanists in conservation non governmental offices and government agencies, she soon had a respectable collection of seeds of rare and endangered plants of the region.
In 1985 the Garden was invited to become a Charter Participating Institution of the Center for Plant Conservation. This non-profit organization, now based at the Missouri Botanical Garden, was set up to use United States botanic garden resources to help conserve native endangered plants.
Over the years many changes have taken place. In 1989 Edward Guerrant took over as Curator of the seed bank and his timely visit in 1991 to the seed bank of the Royal Botanic Gardens, Kew at Wakehurst Place, Sussex, U.K., inspired us to construct a temperature and humidity-controlled room to process seed as part of a major renovation at the Garden. We added a concrete shell for fire protection and now store seed in a freezer in what we call the "Seed Vault".
Berry is a small, non-profit institute, with limited funds, so we have had to be frugal in designing the seed bank. If a less expensive solution is scientifically defensible it is the one we will implement. For example, rather then investing in an expensive back-up generator that automatically switches on in a power failure, we have simply half-filled the freezer with plastic gallon milk-jugs of water. In the event of a long powercut, the large thermal mass of the ice will slow any warming and give us time to use a small, portable generator. Many of the procedures we have developed are readily transferable to other circumstances where funds are limited. Always, good science has been the goal, even when the solutions seemed low-tech.
Seed Bank Operation
The procedures described here for long-term storage of orthodox seeds are based on those developed by Kew's seed bank at Wakehurst Place. Once seed has been collected, the basic sequence of events is to dry the seeds, clean and count them, and then package them for long-term frozen storage. Developing germination procedures for each species is important. Viability of the seed should be tested at various stages, and not assumed. Details of seed banking procedures, with considerable supporting material, can be found in the Center for Plant Conservation's Guidelines for the Management of Orthodox Seeds (Wieland, 1995).
Most seeds fall into two discrete categories. Orthodox seeds can survive drying to such a low moisture content that freezing will not kill them because no ice crystals are formed. Recalcitrant seeds such as acorns, and fruits of many tropical trees, are much more demanding and cannot survive drying, and cannot be stored frozen. Low-tech seed banking is thus only for orthodox seeds.
The primary determinants of how long stored seed remains viable are seed moisture level and storage temperature. The optimal moisture content for long-term storage is a complex mix of many things, including the chemical composition of the seeds. Fortunately, drying seeds to equilibrium at the correct relative humidity (RH) and temperature will prepare them well for long-term storage: 15% RH and 15ºC. have long been the international standard. However, recent research at the US Department of Agriculture's National Seed Storage Laboratory at Fort Collins, Colorado, suggests this may dry out the seed too much. Until the matter is finally settled, which may take many years, advocates for both positions appear to agree that 20% RH and 15ºC. is an acceptable compromise.
In the time between collection and when seeds enter long-term cold storage, high temperatures may be the greatest threat to seed viability. Of particular concern is the journey from collection to storage site. Simply leaving seeds unprotected in the back of an automobile may expose them to lethally high temperatures. It is also important to collect seeds in containers that 'breathe', such as paper or cloth bags. The combined temperature and humidity in plastic bags can cook the seeds on the trip home.
Drying seeds properly is a crucial step in seed banking. The important thing is to remove moisture rapidly and then to store seeds in such a way that they do not pick up new moisture until they are needed for use. Drying can take place in any space that can be made airtight. Plastic boxes (desiccators) made for this purpose are available commercially. Suitable alternatives include large glass jars with good lids and a gasket, or even fish tanks sealed with a sheet of glass and petroleum jelly.
Some sort of desiccant is necessary to draw moisture out of the seeds. Silica gel is most commonly used. Resembling coarse sand or fine gravel, it is generally white but is also available with an indicator blue dye that changes to pink when saturated with moisture. To save money, we mix some blue with a larger quantity of white gel. Once the gel is saturated, it can be baked at 350ºF. until the colour changes back to blue, and be used again and again. We have heard that beans and rice, if baked dry, will also work as dessicants.
Relative humidity can be measured with a hygrometer. We have found, that dial hygrometers occasionally require calibration, so it may be safer to have two. Electronic versions are available. When choosing any hygrometer, it is important to know how accurate it is, especially at low humidities where many perform poorly.
Ideally, for long-term storage, seeds should be placed in moisture-proof, air-tight containers. In practice, complete impermeability is difficult if not impossible to achieve. We use heat-sealed, metal-plastic laminated pouches. Although expensive sealers that deliver the correct temperature either constantly or in short bursts are available, a humble home clothes-iron will work at the correct temperature setting.
After seeds have been cleaned, counted, dried and packaged, they are ready for long-term cold storage. Like the controversy over how dry seeds should optimally be, there are two schools of thought about storage temperature. Some advocate using liquid nitrogen, which approaches -200ºF., while others maintain that 0ºo (-18ºC) is adequate for most purposes. The advantage of liquid nitrogen is that all metabolism and other chemical reactions effectively cease at such low temperatures.
Disadvantages include possible mechanical damage to the seeds, and the relatively high cost of the equipment. We, like the Kew seed bank, store most of our seed at -18ºC., which can easily be achieved with a standard home freezer. Chest freezers are preferable to cabinet types, because the cold air does not flow out every time the door is opened. Do not use 'frost free' machines, because they have periodic warming cycles to remove ice build-up!
One final note, the importance of good record keeping cannot be overstated. You may have the last viable seed of an otherwise extinct species in your freezer, but unless you know what it is, it is of no value. Seed banking for conservation can use very sophisticated techniques and machinery, but much can also be accomplished using humble techniques.