I suppose most of you have heard of the Doomsday Vault in the far north, where millions of seeds from around the world are being stored in case of global catastrophe. That’s all good and well for the future of our species (and, of course, the thousands of species being stored there,) but what would you do if such a catastrophe happened? Hike and boat your way to the Arctic Circle and fight off the polar bears, only to find that you can’t open the vault, or that others have beat you to it and eaten all the seeds? Even a relatively small and localized catastrophe, lasting longer than a few days, can mean food supplies will be in short supply. In the worst case scenario, if you were on your own for months or years, a personal seed stock would be vital.
There’s another reason to save your own seeds. Many old-fashioned varieties have already become extinct due to the promotion of hybrid varieties over the last century. Some seed companies specialize in open-pollinated and heirloom varieties, which come true from seed, but what’s available one year may not be the next. For the sake of preserving your favorite varieties of vegetables and grains (some tropical seeds and tree seeds don’t store under the same conditions) that are in danger of extinction, you should consider storing rare seeds long-term in your own personal seed bank.
There are two variables that are most important for seed longevity: temperature and moisture. Harrington’s Rule states that for every 1% decrease in seed moisture content, or every 10 degrees F decrease in storage temperature, seed life doubles. So by simply keeping the seeds a little dryer and cooler than the conditions inside your house, the lifespan of the seeds increases dramatically. Frozen seeds can last for generations. At room temperature, without proper drying, a few years is all you can expect.
The moisture content of dried seeds stored in your house is probably roughly 10-12% or more (depending on where you live.) It’s best to keep your seeds around 7-8%, or even as low as 5-6% if you want to freeze them (less than that the seeds will likely die.) If a seed breaks when you bend it, it is dry enough. You can lower the moisture in your house with air conditioning, which is convenient when you harvest seeds in the summer. There are many food-grade desiccants available to help keep moisture content low in seeds stored at room temperature. You can re-dry the desiccant in the oven if necessary.
When storing at room temperature, there are insects and microorganisms to consider, and the metabolism of the seeds. Problems relating to these factors can be greatly reduced by removing a single element: oxygen. Cultures around the globe have various traditions to deal with this. Often they fill the airspace around the seeds with something inert, like sand or dust. Small seeds would be hard to find in sand, whereas beans and some grains would not be difficult to sift out. Some of these materials, such as lime or wood ashes, also have the added benefit of acting against insects. One might use Diatomaceous Earth instead. Any of these dusty materials can first be oven dried to also act as desiccants.
A traditional method for removing oxygen from sealed containers is to place a small oil lamp or candle inside and light it just before sealing. This burns out oxygen in a short time. This method is used in cultures where the seed is normally sown within a year or two, so I would wonder how much heat is released in the process, and if it is enough to shorten the seeds’ lifespan?
A more modern method to reduce or remove oxygen in a sealed container is to simply replace it with another gas. Some have used carbon dioxide, which can be purchased in bottles or in the form of dry ice. If you use dry ice, remember not to seal the container until after it has completely evaporated, or the container may burst from pressure. Another gas used to replace oxygen in seed storage is nitrogen. Liquid nitrogen is used in cryogenic seed storage systems. Those are very expensive systems…
In my experience as a carpenter, I encountered a product called BLOxygen, which is used to replace oxygen in oil-based paint and stain containers to make them last longer. As it happens, this was inspired by something similar used in the winemaking industry. As it further happens, Bloxygen is a medical grade equivalent mixture of nitrogen, argon, and carbon dioxide, which is perfect for use in extending seed longevity! A can is about ten bucks, and has over a minutes’ worth of “air” in it. It doesn’t take much to replace the oxygen. I don’t have any hard data for you, but to my thinking a half-second to a one second blast will do the job for small to medium sized seed containers, so you could get over a hundred applications from one can. Not too expensive, but I think it would be most economical to only use it for seeds one intends to keep for more than 3 years or so. http://www.bloxygen.com/faq.html
Oxygen content can also be reduced by vacuum. The easiest way to do this is with a vacuum sealer designed for kitchen use. If you don’t have one you can use a piece if small tubing to suck out as much air as possible. I found an innovative system that uses a modified bicycle pump. You can check out the plans here: http://www.saveseeds.org/tools/tool_vacuum_pack.html
As for temperature, obviously you can only go so far without refrigeration. Without it, you can keep your seed bank near the floor rather than the ceiling and hope for the best. Underground rooms usually stay cooler than those above ground.
But most of us at least have a refrigerator or freezer. Depending on the size of the collection, this is often the best option. A chest freezer is better because the cold air stays put when opened, whereas in an upright freezer it falls out. This is especially important during power outages.
What happens if there is an extended power outage, or if you happen to live where there is no power? If you are really serious about long-term storage, you might consider a solar-powered chest freezer. They are available in a variety of sizes. NASA designed the best one I found. You can read about it here: http://www.alphasolar.com/alpha_solar_118.htm It comes in 5.8 and 8 cubic feet sizes, so this one would be practical for a fairly large amount of seed. It is heavily insulated, so it can keep cold for a week with no sun. It is tremendously cheaper than liquid nitrogen storage, and could operate for years without grid power.
Always allow frozen sealed seed containers to warm to room temperature before opening, to avoid condensation.
Between keeping your seeds dry and cold, versus storing them at room temperature and ambient moisture, you can expect them to last decades or longer instead of just a few years.
Another important factor in seed longevity is choice of containers. They should be airtight and not promote condensation. Glass jars with rubber-gasket lids are probably best. For individual packets, if you aren’t using vacuum packaging, foil packs are best. Zip-lock packets are okay; at least squeeze the air out of them before sealing. If they are going to be sealed in an airtight jar anyway, especially when you have taken precaution to replace oxygen, coin envelopes would be sufficient.