Sometimes, we have seeds that have been around for a while and we either aren’t sure of their age or are unsure of how viable they still are. Seeds from professional seed houses and gardeners will usually have a 90% germination rate within a 2-3 year span of their packaging.
If you have old seed packets or want to test your own harvested seeds for viability, then there are two common methods for doing so.
Each of these methods can be used with a random sampling of your seeds (10-20 seeds or 10-15% of your total seed in that variety) or with all of your seeds you intend to plant into the garden beds or starter pots right away. The random sample will tell you the percentage of your germination. So if you try 10 seeds and 8 of them germinate, you have an 80% germination rate.
Be aware that once the seeds are tested, you will need to either plant them immediately or be prepared to throw them out. So if you aren’t near gardening season yet, use a random sampling rather than all of your seeds.
Soak Test for Old Seeds
This is the easiest of the methods. Simple put the seeds into a cup, jar, or bucket of water and allow them to soak. Bad seeds will continue to float indefinitely while good seeds will sink within a few hours.
Give the soak about 12 hours to work, then skim the bad seeds off the top and throw them onto the compost heap. Drain the water through thick cloth, a very thin sieve (depending on seed size), or similar and retrieve the good seeds. They will need to be kept moist and planted within a few hours.
Damp Towel or Cloth Test for Old Seeds
This germination test requires a bit more work and time, but is more reliable than the soak test method. Using a damp cloth or paper towel, place seeds in a single layer across half of it. Then fold the other half over them to cover. Place in an area that is room temperature (closet, cabinet, etc.) and keep moist. Many put their seeds into an unsealed plastic zipper or freezer bag.
Check daily to keep it moist and after 3-4 days, begin lifting the towel gently to look at the seeds. In most cases, all of the seeds which will germinate will do so within 10 days.
Using Less Reliable Seeds
If you used a random sampling and found your reliability to be 50% or better, the seeds are still likely good enough for gardening. Simply increase the rate of seeds per foot/row/hole to make up the difference. So if your germination rate was 50%, double the number of seeds used.
If your germination rate was below 50%, you can either use the above heavier seeding method or throw the seeds out. It’s advisable to throw them out and replace them with others if possible as the extra seeding and resulting culling will require a lot of extra time and effort.
Want to learn more about testing old seeds?
Check out these Resources:
Growing Vegetables From Seed from University of Tennessee Agricultural Extension Service
Seed Germination from Washington State University
I have always used the method with the wet paper towel. I think I might try soaking some old seeds for awhile first next time to see if that helps to increase the germination rate.