Seed viability is all about how many of the seeds are alive and will grow into plants. It varies based on the type of seed and how it was stored. Most seeds are intended to be planted the year they're packaged and you'll see that date printed on the seed packet. You can expect close to 100 percent viability in that first year. This year, initial results show that just about every tomato seed I planted has sprouted.
In a laboratory setting you might start with 100 or 200 seeds for your test, but for a typical home test I suggest using 10 seeds. Randomly select 10 seeds from the packet of seeds you're wondering about. Put a paper towel or coffee filter on a plate and place the seeds spaced apart on top. I use a paper towel.
Place the seeds in a warm spot out of direct sunlight. If the seed packet tells you how many days until germination, you can mark that on the label and come back to check within that time. If you're not sure how long it will take, you'll need to check the seeds after five or six days and again every few days after that. What you're looking for are signs that the seeds are sprouting.
When you're ready to determine the viability, after indication that germination has taken place, open the paper towel and count the seeds that have little white growths popping out. The math should be easy if you use 10 seeds for your test; the number of germinated seeds tells you the percentage. If eight seeds germinated then you have 80 percent viability, if you had five seeds sprout then you have 50 percent viability. In my test of 10 old squash seeds, three seeds had fully germinated with little roots and two more were showing signs of new white growth, for a 50 percent viability.
Old seeds may also be dormant or dead. When you do your viability test the dormant seeds should swell or remain hard once they get the boost of water. Dead seeds will flatten, soften, and may begin rotting. The dormant seeds can still germinate when all of the conditions are good, they may just need extra care. The five seeds in my test that didn't germinate were plump and showed no signs of decay. Given warmer conditions and more time they may still germinate.
You will probably have leftover seeds each year after planting. Rather than throw them in a shoebox, save them to reuse. By doing it right you can increase viability. You want to store seeds in cool, dark, dry locations. Left in a shoebox on a shelf, vegetable seeds may only remain viable for one or two years; temperature and humidity fluctuations can spell doom for them. If you want to keep seeds for more than a year, put them in a sealed jar with a package of desiccant, those little packets that come with shoes, electronics, and many other products. Place the jar in the refrigerator or freezer.
When frozen, with humidity below eight percent, many seeds will remain viable for decades. The National Seed Storage Laboratory in Fort Collins, Colorado, was built in 1958 to store seeds and test for seed viability. The name was changed in 2002 to the National Center for Genetic Resources Preservation. Their collection includes more than 500,000 seed samples of nearly 7,000 species. The seeds are stored at a temperature of minus 18C degrees. They regularly test the seed viability and replace samples that fall below 60 percent.
Home gardeners certainly can't match their expertise, but when properly stored you can extend the life of seeds by many years. When it comes time to plant, you can guess at their viability or conduct your own home test. It's a great way to save money by not buying new seeds every spring.