If we are going to go to the expense and trouble of assembling a stash of seeds for use in the future then it is important to store them in a manner that is likely to provide viable seeds when we are ready to plant.
A great seed storage strategy should take into account all of the ways that seeds are commonly lost. In my personal experience, seeds are most commonly lost or damaged in the following ways. My list is sorted from most common to least common for my personal seed losses. I have never experienced a disaster, but included it on the list for the sake of completeness: human foibles, animals, bugs, moisture, heat, decay and disasters.
From my observations, the most common way that seeds are lost is due to human foibles. Grandpa dies and the people cleaning the house don't realize that they are throwing away a box of seeds containing precious family heirloom landraces. People get divorced and the non-gardening spouse takes the seed stash. Seeds get misplaced. They get left in the back of the truck during a rain storm. Thieves steal. Things get dropped or broken. The rent doesn't get paid on the storage unit.
I think that one of the best ways to avoid losing seeds to human foibles is to live a life of peaceful cooperation with others. Many times in my life I have lost a treasured variety due to absent-mindedness, or crop failure, or mice, or whatever. When my collaborators find out they say things like “You gave that variety to me 5 years ago and I love it! I've sent you a packet of seed.” I keep archive copies of my garden seeds at the homes of friends and relatives. If something bad happened to my main seed supply I'd still have backup seeds. I also send archive copies of my garden seeds to collaborators. They can stash the seeds, or plant them, or donate them away. More than a few times I've had seeds come back to me from the collaborator's stashes.
Twice in my life mice have gotten into my seed stash and eaten nearly every seed. Both occasions occurred after I moved and a box of seeds got left in the garage. The mice chewed into plastic totes, and cardboard boxes and ate the entire seed stash except for one bottle of seeds stored in a glass mason jar. I still have that jar of seeds today, 13 years after the first attack.
The two methods that I have found which reliably deter mice is to use either glass jars or steel cans. Sometimes I seal seeds into #10 steel cans. Around here the cans and sealer are available a few times per year at the LDS church meetinghouse. In other areas the canners are available by appointment at the Bishop's Storehouse.
My preferred storage method for seeds is glass jars with steel lids, because they are re-usable but steel cans are not. I use sizes from 4 ounces to one gallon. I could use larger sizes if they were readily available.
Once in a while I drop a bottle of seeds in the field and it breaks. These days I tend to transfer the amount of seeds I want to plant into a plastic bag and return the excess to the glass jar when I get home. I'll typically stuff lots of small packets of seed into gallon jars with wide mouths.
Bugs are the next most common way that I lose seeds. They likewise chew through plastic, paper, and cardboard. They sneak through tiny little cracks. I often can't tell by looking at a packet of seeds if it contains bugs or baby bugs. There are lots of different species of bugs that attack seeds. Some get into my seed stash as eggs that are harvested with the seed. Others arrive during processing or storage.
Regardless of how bugs get into the seed, I have one strategy that has always worked for me: Freezing the seed kills the bugs. The seed should be dry and ready for storage before freezing, because freezing damp seed could damage the embryo. A couple of days in a typical home freezer is usually sufficient. Once in a while I'll run seeds through the freezer twice separated by about a week. This may catch the bugs at different stages of their life cycle when they are more susceptible to freezing. The seeds should be in a hermetically sealed waterproof container (for example plastic bag or glass jar) so that they do not absorb moisture after being removed from the freezer.
I have run germination tests on dry seeds both before and after freezing. I haven't observed detrimental effects on the varieties I've tested.
Vigorously shaking a bottle of seeds will mechanically crush many bugs and eggs. I shake seeds both before and after freezing.
Once the bugs on the seed are dead, then the seed needs to be protected from reinfection. Glass mason jars are my preferred method. They are inexpensive, readily available, and impervious to bugs. I have often wondered if steel ammo cans would be bug proof.
Many different kinds of seed eating bugs arrive in my home from the grocery store. I do not allow infestations to continue. A thorough cleaning of the pantry is done whenever I notice bugs. If I keep the population of bugs low then they are less likely to eat my seeds. When I have available freezer space I freeze incoming grain products to reduce the number of bugs arriving from the grocery store. Incoming seeds are always frozen before being put in the seed stash.
Excess moisture during storage is detrimental to seeds because it reduces their life-expectancy or encourages growth of microorganisms. I use a couple of seat-of-the-pants methods to estimate how dry seeds are. I'll do a bite test. If the seed is still soft enough to bite then it is too moist to store. Another test I use is to put a glass jar or plastic bag of seeds outside in the sunlight. If moisture beads up on the inside of the container then they are too moist. Carol Deppe in `The Resilient Gardener` recommends hitting bean seeds with a hammer. She says that if they shatter they are dry enough. If they smash they are too damp.
In my super-arid climate seeds readily dry to low moisture. People that live in damper climates may need to take active steps to dry seeds. I like using a dehydrator set at 95°F. I also dry seeds by spreading them out on a tarp or cookie sheet exposed to the sun.
The moisture in seeds can be reduced by the use of desiccants. I like using white rice because it is readily available: Dry the rice in the oven at 225°F for about 4 hours. Cool. Place in airtight container such as a gallon sized glass jar. Add seeds in paper or fabric envelopes. Allow to dry for about a week. For large sized batches, equal weights of rice and seeds will reduce the moisture in the seeds by 50%.
Commercial seeds that are sold in paper envelopes typically have too much moisture in them for optimal storage, therefore I recommend drying them before storage. They can be dried paper envelope and all.
Once dry, the seeds need to be protected from atmospheric moisture. I prefer glass jars with steel lids.
Most species of dry seeds store very well for me at room temperature. The physical chemistry of biological systems roughly operates on the principle that for every 18°F increase in temperature the rate of reaction doubles. So a variety of seed that is expected to last 8 years at 70°F would only be expected to survive 4 years at 88°F, 2 years at 106°F, and 1 year at 124°F. So if you have a choice between storing your seeds in a warm place or a cool place, choose the cooler location.
In like manner, for every decrease in temperature of 18°F the rate of reaction is cut in half. So seeds that are expected to be viable for 8 years at room temperature can be expected to survive for 32 years in the refrigerator or 128 years in the freezer. Freezing dry seeds basically puts their life-expectancy on hold. If the power ever goes off, then their biological decay starts up again.
I have never lost seeds to a disaster, but it is something that I plan for. I keep seed stashes in three different counties. One stash is quite susceptible to flood, wildfire, and theft. Two stashes are immune from flood but susceptible to earthquake. All stashes are susceptible to house fires. I figure that by spreading the seeds out that I am mitigating against all of them being destroyed at the same time. The shelves of my primary seed bank are bolted to the wall, and have a lip around them so that the seeds are less likely to be damaged by an earthquake. They are in glass jars. If I wanted an extra measure of security, I could put plastic bags inside the jars so that even if the jars break the seeds would still be contained. People in other areas should include plans for getting their seeds through their most likely disasters: For example burying seeds in Tornado Alley.
With careful forethought, it is possible to store seeds in a manner that will allow them to avoid or survive common seed destroying events. If genetically locally-adapted genetically-diverse seeds of many different species are stored then rebuilding of a local food system would be straight forward. This is part of the reason why I believe that landrace gardening is a path towards food security through common sense and traditional methods.