According to the National Seed Storage Lab at Ft. Collins, CO, seeds have three enemies: fresh air, moisture and heat.
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The NSSL advises the moisture content most optimal for long term seed storage is is 4-8%. These moisture levels can only be obtained by slow drying seeds, over the course of 10 to 14 days (in a controlled environment).
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Air drying, at room temp ( 72° F ), will only get you down to 12 - 20 % at best. At these moisture levels, freezing can harm seeds, because ice crystals can form inside them. If your seeds are only air dried at room temp, don't freeze them..
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If stored at room temperatures, seeds will last perhaps 2-3 years. For each 10° F the temperature is lowered, the seed life can double (but only if the moisture content of the seed is 8 % or slightly less).
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Here are the general timelines if the moisture content is proper, and they are stored in airtight containers (having a small desicant pack in with them is a smart move).

32° 50+ years

42° 25 years

52° 12 1/2 years

62° 6 years

72° 3 years

82° 1 year

90° and above-not recommended

(If your seeds are only air dried, at room temp, divide the times listed above by 2 to 2.5, to correct the average survivial / with very good germination rates (but do not freeze them).

When chosing a storage container, know that it is just as important as the moisture content & storage temp. The reason the storage container must be airtight is because fresh air carries moisture (which can lessen life, or cause false starts). It also carries oxygen, which will oxidize your seeds over time.
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Your airtight storage container needs to be made of a closed cell material, such as glass, tin, or mylar for best results. Most plastics don’t work, since they are an open celled material, which air can and will penetrate over time. These include tuperware, 35mm film canisters, disposable food containers, and zip lock bags.
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If you google airtight storage containers, you can find plastic ones made specifically for seed storage.
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So, if you're planning on storing some special seeds for later down the road, know what the enemies are and how to eliminate them.

The requirements are basically clean, dry and not above room teimp.
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To acheive this you can :
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Place your seeds in a clean / new paper lunch sack, label it, and set it in a cool dry place (out of direct sun light) for several days.
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Next place it in a clean / sterilized ice chest (hot water, soap, vinegar or hydrogen peroxide (not bleach), with a small Eva Dry dehumidifier (they don't require electricity to work).
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Place the ice chest in an area out of direct sunlight, with a temp of 72 F or under (but above freezing), for eight days. During that eight days, (every two days) open, remove sack, shake lightly, and return promptly to the the ice chest. This will loosen any plant material still on the seeds, that you might have missed.
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The next step is to place them in the final stage dryer.
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You can make your own seed dryer from any of the items listed below, by cleaning / sterilizing them first (hot water, soap, vinegar or hydrogen peroxide (not bleach), then adding a small dehumidifier to it.
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Refrigerator - ** non functioning, but door gasket must be in good condition
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Old freezer - ** <- this is what I use.
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Large dishwasher **
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^^ the nice thing about all of these is, they are gasketed, have shelves, and can be had for free (curb score or recycling centers).
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Place your seeds in the bottom of a clean cardboard box, or a large paper sack (wear gloves to avoid moisture & oils from your hands getting on the seeds). Place your box or sack in your clean / sterilized, gasketed, drying box.
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Set the dehumidifier (*) to 70 % for two days, then drop the setting to 60 % for two days, and keep dropping it every two days, until you hit 20 %. Very few home models will dry the air to below 20 % humidity.
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(*) If your dehumidifier doesn't have a digital readout, no biggie, you can buy a cheap digital temp / humidity gauge to help you slowly drop down the humidity.
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After eight days, remove your seeds (wearing gloves), weight them on a scale, place them in small lots in your airtight containers, add silica desicant packs (twice the weight of the seeds you are storing), and seal your containers. Small silica desicant packs can be purchased at hobby shops, or from online packing / shipping businesses or coin dealers.
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Place the small airtight containers (either numbered or with abbrevated strain names written on them in permanent marker) into a tupperware container (just to keep them together & this gives you a place to put a sheet of paper listing what is in each numbered or abbrevated containers).
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I say small lots, because it makes more sense to have 20 seeds in 10 airtight containers than it does to have 200 seeds in just one.
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Most people only need to pull 10 to 20 seeds at a time, and you don't want to expose all of them to air / moisture, so small lots are the way to go.
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Last step, into the refrigerator........


I found this at How to Store Your Home-Grown Heirloom Seeds for Best Keeping hXXp://howtosaveseeds.com/store.php

Using Silica Gels For Final Drying

Seeds air-dried during humid weather require additional drying with desiccants such as silica gel before final storage (but don't use heat!). Most seeds benefit from drying with silica gel if they are to be stored for very long. The longest storage life for desiccation-tolerant seeds is achieved by drying them to between 5% and 7% moisture content (by weight) and then storing them at several degrees below freezing. As storage temperature rises above freezing or moisture content rises above 5 to 7%, longevity in storage goes down and the incidence of mutation rises. Seeds dried to a low moisture content with silica gel and then stored in a freezer can usually retain viability for many years.
To use silica gels for drying seeds, place equal weights of dry silica gel and seeds to be dried in a well-sealed jar for 7 to 8 days. Then transfer the dried seeds quickly into airtight storage jars and place in a freezer, refrigerator or other cool, dark place.
Carefully follow the instructions which come with your silica gels for drying them after use (to avoid burning them and rendering them useless). Silica gels suitable for drying seeds can be purchased from Southern Exposure Seed Exchange, PO Box 158, North Garden, VA 22959.