Day: October 17, 2015

Saturday, 17 October 2015

09:01 – The last of the open-pollinated seeds arrived yesterday. Today, we’ll start freeze-testing on all of them.

Like a lot of things that appear simple on first glance, seed viability is actually extremely complicated. Initial germination percentage varies dramatically from species to species and even from lot to lot, as does the curve of decreasing viability over time. Some species have initial germination percentages of 99%+, while others are down around 10% or even less. Fortunately, the latter are very unusual. Typical initial germination rates vary from 70%+ to nearly 100%, depending on the species and how it was initially dried and otherwise treated. All seeds show decreasing germination percentages over time, but the rate of decrease varies hugely and is not related to the initial germination percentage. For example, one lot may show an initial germination percentage of, say, 96%, with germination percentage down to 68% after one year stored at room temperature, 52% after two years, 40% after three years, and so on. Another may show a 73% germination percentage initially, but drop only to 68% after one year, 63% after two years, 60% after three years, and so on. The first seed has a very high initial germination percentage, but loses viability quickly over time. The second seed has a lower initial germination percentage, but loses viability much more slowly over time.

Then there’s the effect of moisture. Seed must be dry to store well. If the percentage of loosely link moisture is above 25% or so, the seed will rot, grow fungus, etc., so commercial seed is normally dried to an average moisture content of below 20%. That’s sufficient if it’s to be stored short-term (year-to-year) at above freezing. Getting it somewhat drier extends viability, but you have to be careful not to dry it too much. If it gets below 8% or so average moisture, seeds tend to “harden”, which means they’re too dry to absorb the water they need to germinate. Drying seeds with warm air exacerbates the problem because seeds dry unevenly, partially or completely hardening some of them while leaving others too moist.

That’s why those big seed banks located up above the Arctic Circle have huge drying rooms that constantly circulate cool, very dry air around racks of screens that hold seeds. These facilities keep seeds on the drying racks for weeks or even months on end, drying them very slowly but evenly. After drying, the seeds are stored at well below freezing. Under those conditions, they maintain high viability and are essentially immortal. Unfortunately, doing that on a small scale is not practical.

There’s an interesting correlation between moisture percentage and storage temperature. Seeds that have a moisture content well above the ideal 8% store well at temperatures just above freezing, the temperature in a standard refrigerator. They maintain pretty high germination percentages for about four times as long as they would if they were stored at room temperature, or perhaps five to ten+ years. But if you put those same seeds in a freezer, you immediately reduce the germination percentage significantly while at the same time increasing their shelf life significantly. That seems self-contradictory, but it’s true.

The issue with freezing is that it causes microscopic water ice crystals to form, which may (or may not) kill the seed. Those seeds that are fatally damaged by ice crystal formation are deader than King Tut and will never germinate. But those seeds that are not killed by freezing become essentially immortal, and will be viable 10, 20, 50, or 100 years after being frozen, assuming they’re kept frozen the entire time. But always remember that each thaw/freeze transition will kill more of the seeds, so freezing/thawing/refreezing such seeds is a very bad idea.

That’s why we’re doing a freeze/thaw transition followed by a germination test for each of the species we’re including in the open-pollinated seed kits. Some kit buyers will choose to keep the seeds refrigerated, which is what we’ll do and recommend. Stored that way, the seeds will very gradually lose viability, but germination rates should remain reasonably high for at least 10 years out, and probably much longer. Buyers who choose to freeze their kits will do so with the knowledge that they may be killing some percentage of those stored seeds in exchange for keeping some percentage of them good essentially forever.

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