I have leftover seeds from last year. How long do seeds last, or remain viable?
There is no hard-and-fast answer on how long leftover seeds last; every expert has a slightly different take, making memorization of any absolute rules impossible. Most seed will last a couple to several years—but there are disclaimers to even that general a statement.
As living things, seeds are perishable, particularly if not kept cool and dry (such as in a tightly sealed jar in the refrigerator or freezer). Humidity, in particular, is death to seeds.
Treated and pelletized seed will also have a different shelf life from seed in its natural state. The condition of the original crop the seed was harvested from will also, of course, affect how perishable it is.
Onion seed lasts just a year, everyone seems to say, with parsley and parsnips also very short-lived in storage. Muskmelons last about five years; watermelons slightly less. Corn about two, maybe three, and likewise for peas and beans.
The best way to be sure: Do a germination test. (Details also below.) But this is tricky when you bought some great heirloom pumpkin and there were just 20 seeds in the packet and now only half those remain.
Next best is an educated guess based on what’s the date on that packet, and how carefully you cared for it the last year. Safest of all: the educated guess and a backup supply of anything where you might be caught short if the calculation proves wrong.
If it turns out you don’t need the new backup seed, you can always use it next year.