The rule of thumb you are aiming for is 100. The two numbers that need to add up to 100 are humidity and temperature. An extreme of this is if you are storing your seeds in a 95% humidity situation, the storage temperature needs to be 5ºF. After speaking with many local seed banks, the numbers most are looking for is 50% and 50ºF.
So how can you tell the humidity of seeds? Put them in a sealed glass jar with a hygrometer. These can range in price from a few dollars to a whole lot of dollars.
Above is the hygrometer we use even though we need to tweak them a bit to correct their humidity.
How can you change the humidity of seeds? Dry them. The general rule, but not absolute rule, is to not allow the temperature to exceed 100º. A food drier that has temperature setting or a bread proofing setting on your oven would do. Also see posting below for the bead seed dryers.
Now you have your seeds dried to 50ºF humidity, now what? Keep them in a glass jar and add some desiccant (rice, or reusable flower drying beads). We put the desiccant in a bit of fabric so it doesn't touch the seeds for two reasons: it's hard to get the desiccant out of the seeds and the flower desiccant should not directly touch the seeds.
All sealed up and ready for storing. But where? We have a used wine refrigerator that is only used for seed saving. Use the white wine setting which is (50ºF). Can't do that? Find the warmest part of your refrigerator and keep them there. We used to use gizmos, like the refrigerator thermometer above, to find the warmest part of our frig.
How long will they live? That depends on how you treat your seeds. In perfect conditions, at least three years for most seeds. But what if you keep taking the jars out to the garden to plant seeds without readjusting the humidity? Perhaps they will see their first or second birthday. Leave them in your car in the middle of August while you hop an airplane to a cooler place? Start planning the funeral.
