As spring approaches, the days get longer and gardeners’ mailboxes get heavier. They fill and refill with all manner of catalogs offering flowering bulbs, live plants and so many seeds.
While it’s tempting to order a few packets of, well, everything, that kind of spending really adds up. Besides, you might already have scores of different seed varieties leftover from previous years. Or maybe you collected seed from last season’s heirloom veggies and flowers.
Depending on the kinds of seeds you have and how carefully you’ve stored them, many are still perfectly viable. For instance, lettuce seed can last up to six years. Cucumbers, radishes, endive and collard greens usually will last about five years. Calendula, nasturtium and zinnia seeds can also survive for about five years.
Seeds of pinks, poppies and celosia may be good for four years. That goes for tomato, squash, cabbage, beet and eggplant seeds, too.
Provided you stored your old seed in a cool, dark dry environment—and away from nibbling insects, rodents and other pests—it may well have some life left in it. You can perform a simple germination test to find out.
Large seed-packing companies routinely test the germination of their stock and will include the results, expressed as a percentage, on the seed packet. To calculate your own germination percentage, you’ll take the number of seeds that sprouted during your test and divide this by the total number of seeds you tested. Now, take that number and multiply it by 100. Higher percentages indicate better germination rates.
Seed companies typically use very large sample sizes and may complete multiple rounds of tests for a single seed stock. Fortunately, you don’t have to go to those lengths in order to know whether it’s time to splurge on new seeds or make do with your old stash.
First, you’ll need 10 to 20 seeds of each variety you wish to test, as well as some paper towels or coffee filters.
Cut one long strip of paper towel, space your seeds evenly along half of it, and then fold the remaining half over the top. Next, carefully wet the towel with your seeds sandwiched in between, then gently press the wet towel between your hands to squeeze out excess water.
(Your seed test strip should be about as moist as a wrung-out sponge. If it’s too wet, your test seeds could rot before they have the chance to sprout.)
Now, place the moistened test strip in a clear, plastic bag or container. Use a permanent marker or temporary label to identify the seeds being tested and the test start date. Place your germination tests on top of a seedling heat mat if you have one. Otherwise, keep them in a warm spot and make sure they remain moist.
Check them every day or so for signs of germination. Viable seed should swell with the addition of water. Seed coats should soften and eventually split to reveal roots and first leaves. Make note of the number of seeds that germinate along with the date. Remove any sprouted seedlings from your test strips to give any remaining seeds a little more time and space. Different varieties require different amounts of time to sprout. As a result, the duration of each of your germination tests will vary accordingly. Zinnias, for example, should sprout in about 10 days. That means, if you’re testing zinnia seeds, you should have a good idea about their viability by 10 to 15 days.
If your seeds fail to sprout, that doesn’t necessarily mean they’re duds. Some seeds require special treatment such as exposure to cold temperatures in order to mimic a period of winter dormancy or presoaking to soften up extra tough shells. And some are more likely to sprout when planted in a sterile potting mix, rather than on a moist paper towel.
But what if you provide the variety you’re trying to sow with the extra attention or conditions required and you still fail to see sprouting? Then your seed is probably not viable.
As for seeds with very poor germination rates, you still can choose to heavily sow them in hopes of growing some new plants. Just remember that any plants that do grow from questionable seed may not be very robust. So, unless a variety is very rare or has sentimental value, you might be better off with those new seed catalogs after all.