Seed Viability

As you prepare to order seeds, or start planning your seedling sowing schedule, you may be wondering if you can still use the seeds leftover from last year. To my surprise, I have not addressed this topic for 12 years! Naturally, seeds have not changed much in that time! But we have more information about the factors influencing seed viability, and things you can do with old seeds that don’t pass your germination test or have dates too long ago to be reliable.

Factors influencing seed viability

It’s a waste of money to throw out all seeds leftover at the end of the planting season and buy all new for next year. Yet it’s a bigger waste to keep everything and risk poor germination of a vital crop. Here are nine steps to minimize the chances of throwing out good seed, keeping bad seed, buying too little, or buying too much. The key components are:

1. Store your seeds well, so you know you can rely on them to grow next year
2. Decide which old seed to keep and which are unreliable, See Johnny’s Seed Viability and Seed Storage Guidelines
3. Know how much seed is usually sown per 100′ (30 m). See Johnny’s website
4. Calculate how much your farm uses per 100′ (30 m) (optional fine-tuning)
5. Quantify how much seed you have left
6. Plan your planting quantity of each crop for next year
7. Calculate how much seed to buy
8. Decide how much extra to buy in case a sowing fails
9. Place orders early, to increase the chances of getting exactly what you want.

Opinions vary about how many years seeds of different vegetables are good for. The fuller story is that storage conditions make a big difference. You can make your own decisions, weighing up the information supplied, your knowledge of how carefully you stored the seeds, the information on each packet about percentage germination when you bought it, and the economic importance to you of that particular crop. If you always transplant lettuce, as we do, and you sow several varieties each time, you can risk one of your four varieties in that sowing coming up poorly and just plant out more of the other three if it fails.

Many seed catalogs include information about seed longevity, as does Nancy Bubel in The New Seed Starter’s Handbook. My updated book Sustainable Market Farming, Second Edition is the source of most of what I have here.

A simplified version is as follows:

• Year of purchase only: parsnips, parsley, salsify, scorzonera, and sea kale, as well as any pelleted seed. Pelleted seeds are primed (nudged to start germinating), then coated in clay. The priming is necessary to support the seed in getting through the clay coating.
• 2 years: corn, peas, and beans of all kinds, beets (2-5 years), onions, leeks, chives, peppers (2-5 years), okra, dandelion, martynia. Lettuce is very variable at 1-6 years.
• 3 years: carrots (3-4 years), asparagus, celery and celeriac (3-5 years), Asian greens, turnips, and rutabagas.
• 4 years: spinach (1-5 years), chard (2-5 years), pumpkins, squash, zucchini, watermelons, basil, fennel. Artichoke and cardoon seeds (1-4 years).
• 5 years: most brassicas, beets (2-5 years), tomatoes (maybe up to 7 years), eggplant, cucumbers (3-6 years), muskmelons (3-6 years), lettuce (1-6 years), endive, and chicory.

Rather than deteriorating with age, some very fresh seed has a dormancy that needs to be overcome by chilling (lettuce). Other seed contains compounds that inhibit germination; these can be flushed out by soaking in water for about 1 hour (beets).

A challenge with seed ordering is converting between grams, ounces and seed counts. Look in the description (perhaps under “Additional Information”) for Seed Specs info such as “Avg. 52,000 seeds/lb” then use your online calculator, slide rule or abacus to easily convert all to the units you are most comfortable with.

Seed germination testing

If you hate to throw seeds away, consider running some germination tests on critical packets of seed. Take a sturdy paper towel, folded in half, or a coffee filter, and dampen it by spraying with water. Count out 10–100 seeds and set them in a line in the fold or seam of the paper. The more seeds you can test, the more representative your result will be. Roll the paper loosely and put the roll in a plastic bag in a warm place, at a similar temperature to seed starting for that crop. Once I made the mistake of trying to germinate watermelon seeds in winter on the bathroom counter — it was too cold!

Unroll and check the seeds twice a day. Initially your inspections just provide an air change for the seeds, but soon you should see little sprouts emerging from the seeds. Spritz the seeds with water as needed. Count how many germinate and notice if they are quick (vigorous) or slow (low-vigor seed, not for use in challenging conditions). Old seed may still germinate but only produce weak seedlings.

When no more germinate, convert your final figure to “germinated seeds per 100” (percentage). Compare your germination rate with that of fresh seed you would buy to replace it. This will help you decide whether the seed is worth relying on next year and whether you need to sow it thickly.

Ideas for using old seeds

We sometimes use a mixture of old seeds for a short-term summer cover crop. We consider the preceding and following crops and customize a mix that avoids those crop families, for a good rotation. Naturally, some small, slow-growing crops like parsley never make good cover crops, but peas, beans, corn, and brassicas can all make quick growth. Be careful not to leave your free cover crops growing so long that they become problematic. One year we had a bed of southern peas, corn, and random old lettuce seed sown beside our sweet corn. Our plans for that bed changed, and before we knew it, we had cover crop popcorn tasselling out next to our sweet corn. Luckily, we noticed in time and cut them down before hey crosspollinated our sweet corn and ruined the flavor.

Consign unwanted old seeds to a special Old Seeds Bucket, which may provide you with a backup plan if you somehow run out of seed during the growing season. You can reorder from a quick-to-respond seed company and meanwhile make a sowing with your old seed. If it germinates, you will be ahead of where you would be if you had to wait for the new seed to arrive, and you can keep the new seed for next year. If the old seed doesn’t come up well enough, you can use the new packet and be no worse off than if you had composted the old seed back in the winter.

A third use for old seeds is microgreens or salad mix. We have used leftover brassica, endive, beet, and chard seeds to get a quick crop of salad mix from an unexpectedly empty spot in our hoophouse. But beware of turnips and radishes with bristly leaves! Having these three worthwhile uses for old seeds has helped us be more ruthless about buying new seeds at the beginning of each year.