As I write this column, three boxes of seeds sit stacked on the corner of my desk, two containing seeds received from various seed houses over the past month and one box marked “old seeds”, unsealed packets of seeds left over from previous years. As I gear up to sow seeds indoors for the coming season’s garden transplants, do I dare take a chance on these old seeds or should I toss them out?
How many years do seeds remain viable? The answer depends on the types of seeds in these old packets and how they were stored since the packets were opened.
For the seeds of most garden vegetables, annual germination tests will show steady decline in percent germination even under the best of storage conditions. Up to a point, the gardener can compensate for this by sowing older seeds more densely based on the decline in percent germination. The following lists show the approximate number of years that seeds of specific crops retain sufficient viability to be used, assuming that they are stored properly.
corn, onions/leeks, parsley, peppers
beans/peas, beets, broccoli, Brussels sprouts, cabbage, cauliflower, carrots,
celery/celeriac, eggplant, kale, kohlrabi, rutabaga, squash/pumpkins, Swiss chard
tomatoes, turnip, watermelon
cucumbers, endive, lettuce, melons, radish, spinach
The first step is to toss out any seeds that have exceeded the above limits, assuming that you can still find the “packaged for” or “sell by” year stamped on each packet when it was sold. Sometimes this information gets torn off when you open the seed packet at the wrong end. And sometimes leftover seeds get transferred to a small jar without this information. In these cases, you are left wondering how old the seeds might be.
The viability of leftover seeds can be quantified with a simple home germination test. Start with a minimum of 10 seeds from the packet to be tested, although a sample of 20 seeds will produce more accurate results. Moisten a stack of four paper towels, gently pressing out any free water, then spread the seeds over the surface of the stack, taking care to keep the seeds separated. Roll the seeds up in the towels and place the roll in a plastic bag. Remember to label the bag with the variety of the seed and the date that you began the test.
Store the bag at 70º to 80º F. Check the seeds in two days and every day thereafter for up to two weeks. When a root emerges from the seed, the seed has germinated and it should be removed from the towel and discarded. Keep a running total of the number of seeds that germinate each day. When a week passes without any additional germination, you can discard any remaining seeds and calculate the percent germination by dividing the total number of germinated seeds by the number of seeds tested, multiplying the result by 100 to get percent germination.
There are two factors worth considering when evaluating the germination test results: percent germination and the uniformity of germination. The ideal garden scenario is that all seeds germinate and they all germinate on the same day so that growth is uniform throughout the row or bed. Both factors, percent germination and uniformity of germination, are important. If germination is high, say 80%, but drags out over a two week period, it would be better to purchase fresh seeds that should all germinate within a 48 hour period.
If percent germination is somewhat low, you can still use the seeds, sowing them at a greater density (less space between seeds). For example, if germination has been reduced during storage to 50%, you would sow twice as many seeds in the same space than you would if germination were 100%.
Late winter is the ideal time to run germination tests on all of your old seed. By mid-March you will have your results with plenty of time to order new seed as needed.
How should you store leftover seeds from one year to the next? Certainly not by throwing opened packets in a box on your desk!
The most important factors are temperature and humidity: seeds are best stored from one season to the next at 40º F and 50% relative humidity. These conditions can be met by storing the seeds in a sealed jar with a layer of desiccant in the bottom and keeping the jar in the refrigerator or coolest part of the basement. Dry uncooked rice kernels work well as a desiccant and can be recharged by drying in the oven at low temperatures.
To maintain their identity, seeds are best kept in their original packets when placed in the storage jars. If you do have to store loose seeds, be sure to include an identifying label and use a layer of paper toweling to keep the seeds separate from the desiccant.
I’ve decided to run germination tests on some of my old seeds, mostly sweet peas (which have notoriously low germination even when fresh), a few packets of lettuce, spinach, cilantro, beans, and basil. They were tossed in a box, the open end of each packet rolled down to prevent the seeds from spilling out (which some did anyway), not stored in sealed jars over a layer of desiccant, not stored at the recommended temperatures.
Don’t say it, I know that I should practice what I preach.