The following is an excerpt from The Complete Guide to Saving Seeds by Robert Gough and Cheryl Moore-Gough (Storey, 2011). The excerpt is from Chapter 4: Seed-Storage Know-How.
Drying seeds to the desired moisture level for storage may take a few days to several weeks, depending upon the species, the atmospheric humidity, and the equipment you use. The faster you dry the seeds, the less likely they will be to succumb to pathogens. The lower the humidity of the air in which the seeds are placed, the faster the seeds will dry. Seeds dry quickly at first, then more slowly as their moisture content nears that of the air around them.
Packaging Seeds for Drying
Small coin envelopes work very well for holding seeds while they dry, better than standard paper letter envelopes, which often have small holes at their corners through which tiny seeds can spill out. Another option is to wrap small seeds tightly in pieces of paper towel and secure the seams with tape. Whether you use envelopes or paper towels, package only a few seeds together. If you have a lot of seeds, you’ll have to use a lot of envelopes. Be sure you carefully label each envelope or paper towel with the date and the crop or species name (and the variety name if there is one). Note that although plastic bags and glass jars work well for storing dried seeds, they are not good container choices for seeds during the drying process.
If the air in your home is damp, as is often the case in coastal states and in the South, set up your seeds in a small room with a dehumidifier. You can use an inexpensive hygrometer and a thermometer to estimate how dry seeds are by tracking the relative humidity and maximum and minimum temperatures in the drying room. Measure the relative humidity and temperature daily.
Leave the seeds in the drying area for at least a week to be certain their moisture has come into equilibrium with that in the air. Ideally, follow this rule of thumb: The sum of the relative humidity and the storage temperature in degrees Fahrenheit must not exceed 100, as long as the temperature is less than 50°F (10°C). For example, if the relative humidity in your drying room is 40 percent and the temperature is 40°F (4°C), then the sum is 80 (40 + 40), which is less than 100, so your seeds should dry just fine.
Most orthodox seeds will dry to an ideal 6 to 8 percent moisture if they’re held at 38 to 40°F (3.5 to 4.5°C) and 30 to 35 percent relative humidity. Seeds of recalcitrant species like those of the oaks will dry to their appropriate moisture content if held at 38 to 40°F (3.5 to 4°C) and about 40 percent relative humidity. As a homeowner, you don’t have the wherewithal to purchase expensive moisture meters to be absolutely sure of having perfectly dried seeds, but these schemes will get you into the ballpark. The graph on page 52 illustrates the relationship between the relative humidity of the drying room and the approximate moisture content of the seeds. You can easily see that keeping the relative humidity below about 40 percent will reduce the seed moisture to below about 8 percent, although more exact percentages depend upon species and seed lot. The table below gives some specific examples of the expected moisture content of vegetable-crop seeds when dried at 40°F (4.5°C) at 45 percent relative humidity.
If you live in a fairly humid area and have no dehumidifier, you can try drying seeds in your oven instead. This is easy, works well, and is safer for the seeds than is drying them in direct sunlight. Note that this method is primarily for larger seeds such as those of squash and beans. Tiny seeds such as those of cabbage and carrot seeds should dry sufficiently without heating in an oven.
To prepare seeds for oven drying, do not put the seeds into envelopes or folded paper towels. Instead, spread the seeds in a thin layer on a cookie sheet and place the cookie sheet in the oven for 24 hours at 100°F (38°C). Stir the seeds once or twice during this time to be sure all sides are exposed to drying air, and that should do it. Once the seeds are dry, you can package them in containers for storage. Some pinecones may require a higher temperature, 130°F (55°C) or more; we note exceptions like this in the plant entries in part 2.
Using Silica Gel
Following a dry-air treatment with a silica-gel treatment provides extra insurance that your seeds will be dried well enough. This is advisable if you live in the mid-Atlantic or southern states, or in other areas with very high relative humidity, or in general if you want to be absolutely sure your seeds are ready for storage. Silica gel is widely available at photosupply stores or by mail order from some garden seed companies and is very easy to use. The best bead size for drying seeds is ¹⁄₁₆- to ⅛-inch diameter.
To set up seeds to dry with silica gel, first measure out an amount of the deep blue silica gel equal to the weight of the seeds. Place either the silica gel or the seeds in a porous bag, then place both in an enclosed space such as a glass jar with an airtight seal. Make sure the silica gel is not touching the seeds. Place the containers in a room held at 59°F (15°C) for drying. You’ll need to replace the used silica gel with fresh, either daily or when the color turns from deep blue to pale blue or pink (which indicates that the gel has absorbed its fill of moisture).
Allow the seeds to sit, changing the silica gel as needed, for a few weeks to be sure they have dried properly. Larger seed lots and bigger seeds require longer drying times.
Some folks use powdered milk as an alternative drying agent to silica gel, but it is perhaps one-tenth as effective. Use it as you would the gel. Powdered milk can’t be redried very well, so dispose of it.
Reusing Silica Gel
You can reuse silica gel over and over again as long as you dry it out between each use. To do so, heat used silica gel in a warm oven to drive off the moisture. Set the oven at a temperature above 212°F (100°C) but below 275°F (135°C). Place the gel in a thick-walled Pyrex dish in a layer no more than an inch deep. Stir the gel a few times during the drying process; 1 quart (1.9 pounds) of gel should take about 2 hours to dry. You’ll know it’s dry when it has turned deep blue again. Alternatively, heat the gel in a microwave oven at a medium or medium-high setting for 3 to 5 minutes. If it is still not dry at the end of the cycle, stir it and heat it again for the same period. It may take about 10 minutes to dry a pound. Store the dried gel in an airtight container for future use.
Stay Out of the Sun
Avoid placing seeds in direct sunlight while they are drying. The strong ultraviolet light may damage seed embryos, affecting long-term seed viability.
To Dry or Not to Dry?
Should you dry the seeds that you’ve collected from your garden? In most cases, yes. If you plan to store the seeds for a while before planting, begin drying them as soon as you can after you clean them. However, if you intend to plant the seeds soon after cleaning, take care not to dry them, because this may induce dormancy that does not exist in some fresh seeds. (We talk more about seed dormancy in chapter 5.)
Excerpted from The Complete Guide to Saving Seeds by Robert Gough and Cheryl Moore-Gough Copyright © 2011 by Bob Gough and Cheryl Moore-Gough. Used with permission from Storey Publishing.