Freezing your seeds will actually prolong the life of the seeds. But nearly all of the moisture in the seeds must be removed first, otherwise freezing could end up killing your seeds.
If there is still too much moisture in the seed, freezing could cause ice crystals to form in the seed and damage the cells, killing the seed. With low moisture content, however, freezing simply cools down the seed and slows the metabolic processes, prolonging the life of the seed without damaging it in any way.
Drying your seeds is easier than you think. It can be a pretty fun science project for kids, too!
There are a variety of desiccants you can use, including rice & powdered milk, which you probably already have in your kitchen. But in order to dry seeds enough to freeze them, we prefer to use silica gel beads.
There are color-indicating silica gel beads that will tell you when they’ve absorbed moisture from your seeds, which is very useful. Cobalt chloride used to be used, and the beads would turn from blue to pink when they’d absorbed moisture. But because of worries about carcinogenicity, the beads you’ll find now are orange, and turn green when they’ve absorbed moisture.
Silica gel beads aren’t just useful for seed saving. You can use them to keep moisture from damaging instruments, guns, tools, etc.
The best thing about the silica gel beads is that you can reuse them over and over. You simply reactivate them by placing them in an oven, between 212℉ and 275℉, stirring every 10 or 15 minutes, until they’re dry (about 2 hours).
You can do the same thing with rice as well, though they’re only about half as effective as silica gel beads. (Powdered milk is only about a fifth as effective.)
Step by Step Instructions
1. Harvest the seeds
The first step is to simply harvest and clean your seeds. Depending on the type of plant/seed, you’ll need to use either the wet or dry method. We have a whole article devoted to that, so check it out here: Wet vs. Dry Seed Harvest.
2. Package & Label
Once your seeds are harvested, air dried, and cleaned, then the fun really gets going. (This is a good time to involve your kids.)
Package your seeds in an envelope of some kind. You can purchase the self-sealing envelopes that are designed specially for seed saving (and they’re really economical), use an envelope you already have, or make one yourself out of a piece of paper. Or you can use more expensive, but more durable foil pouches. (Don’t use plastic bags.)
Be sure to note the date (at least the year) that the seeds were harvested, the specific variety, and any other notes about the seeds that you might need to know in subsequent sowings.
3. Put the Seeds and Beads in an Airtight Jar
Once your seed packets are labeled, filled, and sealed, weigh them all together.
I usually use my digital kitchen scale (I would highly recommend this one if you don’t have one already… it’s inexpensive and I absolutely love mine!), but I wanted to get the kids involved in the fun.
We used their Educational Insights Number Balance beam. First we placed all the seeds in a jelly bag.
Then we added the bag to the scale.
We added the empty bag to the other side.
The kids took turns adding scoops of silica gel beads to the bag until it balanced.
Just like we did here, you’ll need the same amount (by weight) of silica gel beads to match the weight of the seeds (packets included).
Once you have everything weighed out, place the seed packets in a jar with an airtight seal. (If your seeds are in foil pouches, be sure to leave them unsealed while in the jar to allow the moisture to escape from the seeds.)
Pour the silica gel beads around the seed packets in the jar.
Seal the jar.
4. Let It Sit
Then you wait. It takes 6 to 8 days or so for the seeds to reach an ideal moisture content. Larger seeds, such as beans and corn, will end up at about 7-8% moisture, and smaller seeds will end up with around 4-5% moisture.
Drying your seeds too much can actually damage the seeds. After 7-8 days, pull a couple of seed packets out of the jar and test the seeds to see if they’re dry enough.
For large seeds, smash them with a hammer. If they shatter, they’re dry enough. If they form a pancake when they’re smashed, they need to be dried some more.
For flat seeds, you can bend them. If they snap, they’re dry enough. If they bend, they’re not dry enough yet.
5. Remove the seed packets & place them in another airtight jar, and place in the freezer
Once you’re sure the seeds are dry enough, transfer them from the jar with the silica beads into another air-tight jar, seal it up, and put it as far in the back and bottom of your freezer that you can. That way they’re exposed to as little temperature change as possible when the freezer door is opened and closed. (If you’ve used foil pouches instead of paper ones, be sure to seal them before placing them in the second jar.)
Seeds stored this way can keep for many times longer (years and years!).
When you remove a jar of seeds from the freezer, leave the jar sealed until it has had time to reach room temperature, otherwise moisture would gather on the cold seeds and re-hydrate them too quickly. You don’t want to shock your seeds after you’ve removed them from the freezer.
For Larger Quantities of Seed
If you have a lot of seed of one kind, you could put the seeds in a jar (preferably a dark-colored jar), and put a bunch of silica beads in a tea bag inside the jar with the seeds (bury it in the seeds a bit, just to help even out the moisture absorption). After 7-8 days, test the seeds as usual. If they’re ready for freezing, remove the silica beads, seal the jar, and place in the freezer.