<strong>General Desiccant Information</strong> <strong></strong>
This article is aimed at people who want to double or quadruple the number of years their seeds will stay viable. Most seeds will last 2-5 years or longer with no more care than keeping them consistently below 50% RH. However, most seeds' viable lifetime doubles with every 10% decrease in relative humidity from 50% RH down to 20-30% RH. Ideal storage conditions are consistently dry (15% to 30% RH), cool and dark.)
(A few types of seeds can only be sowed fresh, and/or must be stored moist. Don't try to store those dry!)
* * * * This blog entry is still being written. It needs photos, and then I'll offer it as an article * * * *
Any sealed container can leak over time, and even properly dry seeds do release some humidity over time, even for professionals. A desiccant inside the container solves that problem. The Kew Millennium Seed Bank Project suggests a humidity monitor inside each container so archivists know when to replace the desiccant. A 40 cent humidity card works great for that, if the container is see-through.
I use stapled paper coin envelopes to keep desiccants separate from seeds. Humidity flows freely through paper. I put a tablespoon or two or desiccant into a paper coin envelope and put that inside large plastic tubs with good screw-lids.
I keep seeds in plastic Ziplocs in the same tubs, trusting that the small amount of humidity released from dry seeds will diffuse through the Ziploc zipper and directly through the Ziploc plastic. The plastic Ziplocs also protect seeds from rapid humidity changes when I open the tubs. It also protects the seeds from excessive dryness if a large fresh batch of desiccant dries the air in a tub below 15% RH for a few days.
Probably storing seeds in paper, consistently dry but not too dry, is a little better than storing in Ziplocs. I like the resealable convenience of Ziplocs, being able to see the seeds, and the way they keep the seeds from going to 50% RH and back down to 20% every time I open a tub. Also, if I put the whole tub into a freezer to kill insects, there is no chance that condensation can reach the seeds.
You have to store silica gel and most other desiccants in a tightly-sealed jar, or they will absorb humidity from the atmosphere until they can't absorb any more. Bentonite clay seems to be able to keep its drying ability even if exposed to the atmosphere for months.
- <strong>Silica gel</strong> is my favorite desiccant. One tablespoon will last for months in a well-sealed jar if I don't open it too often. You can buy a few pounds of silica gel for $5-7 in a craft store's flower-drying aisle. In the United States, silica gel is the only desiccant approved by the FDA for use in food and pharmaceutical applications. It is literally safer than sand, since sand is crystalline and silica gel is amorphous. Even so, avoid breathing the dust.
You can regenerate silica gel by baking a shallow layer at 250 F for several hours, and sealing it back in glass before it sucks up much humidity. 250 F will melt most plastics!
Amazon charges three times what my craft store charges for silica gel.
https://www.amazon.com/dp/B0001EJIXU/
- <strong>Oil Dry</a></strong>- oil-absorbent Bentonite clay containing montmorillonite may not need to be regenerated, but it can be regenerated at 150 F. You can buy 40 pounds of Oil Dry in a Wal-Mart automotive section for $8. (Thanks, Dave, for pointing this out as an inexpensive desiccant.)
https://www.amazon.com/dp/B0001EJIXU/
- <strong>Drierite</strong> - anhydrous calcium sulfate - is a about the same price as silica gel, but more powerful. It's less dusty, but if you buy the indicating Drierite, the dust has a little Cobalt Chloride in it, so the dust is more irritating. This is the classic desiccant that you see in high school laboratory glass crocks.
<a href="https://secure.drierite.com/catalog3/page4a.cfm">https://secure.drierite.com/catalog3/page4a.cfm</a>
<a href="https://secure.drierite.com/catalog3/page4b.cfm">https://secure.drierite.com/catalog3/page4b.cfm</a>
If you use too much fresh Drierite it can desiccate seeds to death. That makes a humidity indicating card useful. You don't need seeds dried below 15% RH, so I don't let my jars go below 10% RH for long. (I store seeds in closed Ziplocs inside the dry jars, so the inside of the Ziplocs only dry further very slowly when the humidity difference is 10% vs 15% or 20%. Paper labels and the seeds themselves buffer that last few % of RH change.)
Drierite <strong> relative humidity cards </strong> cost 40 cents each, no minimum order:
<a href="https://secure.drierite.com/catalog3/page15b.cfm">https://secure.drierite.com/catalog3/page15b.cfm</a>
ULINE humidity cards, $18 / 100 cards
<a href="http://www.uline.com/BL_1002/Humidity-Indicator">http://www.uline.com/BL_1002/Humidity-Indicator</a>
<a href="http://www.uline.com/Product/Detail/S-8028/Damage-Indicators/10-60-Humidity-Indicators">http://www.uline.com/Product/Detail/S-8028/Damage-Indicators/10-60-Humidity-Indicators</a>
- <strong>Dry uncooked rice</strong> can be baked until it's almost-light-brown. When it's that dry, it is a useful desiccant. But if you over-bake it, it turns light brown, which greatly reduces its water-absorbing properties. You have to use a larger volume of rice to absorb as much humidity as a smaller amount of silica gel, but rice won't over-dry seeds.
I would love to know if dry, lightly-baked pasta absorbs humidity.
- <strong>Dry milk powder</strong><strong> </strong> is supposed to be able to take up some humidity. But I doubt whether it can be regenerated.
<strong>Moisture Barriers:</strong>
Professional seed banks and researchers use heat-sealed, tri-laminate foil bags.
Seal-A-Meal might come close, but both these trap humidity IN with the seeds.
Glass jars with a screw-lid and a rubbery gasket ring are good for many years. (Unless you open them - each time you open a container, you let in room air around 50% RH and the desiccant has to absorb that humidity. Eventually, any desiccant will be exhausted and stop protecting.)
Plastic jars with tight-fitting screw lids and non-paper seals are probably OK for a few years. That's what I use.
Double Ziplocs might leak humidity too fast for long term dry storage in humid air, but they protect seeds from condensation when coming out of a freezer or vegetable drawer. (By the way, the "crisper drawer" is the most humid spot in your refrigerator, not the driest.)
Big plastic tubs with snap-on lids leak so fast that desiccants will be exhausted rapidly. You might be able to slow the leaks down by clamping the lid over a thick plastic film like a gasket. And use lots of desiccant.
Kew Millennium Seed Bank Project, long-term seed storage containers:
<a href="http://www.kew.org/ucm/groups/public/documents/document/ppcont_014347.pdf">http://www.kew.org/ucm/groups/public/documents/document/ppcont_014347.pdf</a>
Sorbent Systems / IMPAK technical comparison of desiccants:
<a href="http://www.sorbentsystems.com/desiccants_types.html">http://www.sorbentsystems.com/desiccants_types.html</a>
Sorbent Systems / IMPAK CHART for comparing desiccant types:
<a href="http://www.sorbentsystems.com/desiccants_charts.html">http://www.sorbentsystems.com/desiccants_charts.html</a>
Kew: theory of desiccants and RH
<a href="http://www.kew.org/ucm/groups/public/documents/document/ppcont_014349.pdf">http://www.kew.org/ucm/groups/public/documents/document/ppcont_014349.pdf</a>
Many Kew Royal Botanic GardensTechnical Information Sheets about collecting and storing seeds, written by Millennium Seed Bank staff.
<a href="http://www.kew.org/science-research-data/kew-in-depth/msbp/publications-data-resources/technical-resources/technical-information-sheets/index.htm">http://www.kew.org/science-research-data/kew-in-depth/msbp/publications-data-resources/technical-resources/technical-information-sheets/index.htm</a>
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