How To Test And Extend The Shelf Life of Seeds

Cravings for snappy fresh beans, luscious tomatoes and brightly colored flowers hit hard as the days lengthen and winter starts to lose its grip. Perhaps it leaves you rifling through leftover seeds, wondering whether they’re still good. Seeds expire, but expiration dates are rough guidelines; experts say it depends on the kind of seeds and how they were stored.

For advice we turned to two people: Randel Agrella, a senior horticulturalist at Missouri-based Baker Seeds, which sells more than 1,200 varieties; and Phil Kauth, director of preservation at Iowa-based Seed Savers Exchange, which sells about 600 varieties to the public. Seed Savers also has a gardener-to-gardener seed exchange with up to 5,000 rarer varieties.

Can I Plant Last Year’s Seeds?

Maybe. Most vegetable seeds will last through their expiration date if kept cool, dry and away from sunlight. Baker Seed guarantees seeds for at least two years after purchase. Most seeds last three to five years after purchase, but those dates can vary depending on the variety.

Lettuce, peppers, parsnips and onions have a short lifespan and should be planted within a year or two. Corn and beans are among the best for long-term storage. “I’ve had them last for 10 years,” Agrella said.

How Do I Test Seed Viability?

To test whether seeds will germinate, grab a paper coffee filter or a wet paper towel. Squeeze it thoroughly so it’s damp but not soggy. Place five test seeds on the paper towel and slide it into a plastic storage bag or sealed container to keep it from drying out.

Viable seeds should germinate in roughly six to 10 days, but you should check the time frame listed on each seed packet. You can also find rough guidelines through The Old Farmer’s Almanac.

Cold-tolerant plants such as peas, greens and cabbage family crops can germinate in lower temperatures, such as 55 to 65 degrees F. But tomato, cucumber, squash and other heat-loving crops need temperatures closer to 70 F to germinate. Try to replicate those temperatures indoors for the seed germination test.

Seeds needing a little more warmth can be set near a lamp or in the kitchen. You can find heating mats that can go beneath seed-starting trays when you have viable seeds ready to plant.

A quick alternative test if you’re eager to start a tray of seedlings is to drop a few seeds into a glass of water. If they sink, they’re good. If they float, toss them. If you have a mix of failed and germinating seeds, you can give them a try, but plant extras to make up for diminished returns.

How Do I Keep Seeds Longer?

Most seeds will last a couple of seasons if kept somewhere cool, dry and away from sunlight. “For longer-term storage, I steer [gardeners] to freezer storage,” Agrella says.

A freezer, especially a less frequently opened chest freezer, stays dryer than a refrigerator with fresh produce. Kauth advised using a glass jar with a tight screw top or lid with a rubber gasket. Plastic freezer bags can let in condensation that compromises seeds.

What About Grass Seed?

Like other seeds, how long grass seed stays viable depends on the variety of seed and how well it’s stored, Kauth says. It may stay viable for three to five years if kept cool, dry and safe from rodents or insects that spoil it. Try a sturdy, tightly sealed storage container or bin.

How About Flower Seeds?

Treat flower seeds the same way as vegetable seeds. Keep them in a freezer for long-term storage and test older seeds using the damp paper towel method.

Be sure to keep them in their original packets so you know the temperature and amount of time needed to germinate, along with any specific instructions. Some with hard outer shells, such as nasturtiums, morning glory and moonflower seeds, may require a 24-hour pre-soaking.

Look for Seed Swaps

If you want to try several varieties of vegetables or flowers in a small garden without a lot of leftover seeds, consider sharing seed packets or trading with neighbors or friends. Fellow gardeners in local garden clubs can be another good source for seeds, especially for unusual or heirloom varieties or any seeds in high demand or back-ordered.

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