Seed viability chart: how long vegetable seeds last

On this page: Seed viability chart · Where the tables disagree · Storing seed · Paper-towel germination test · The germination math · Checklist

Seed viability chart: years by vegetable

Each column is one extension table. The first five give years under good storage: "favorable conditions" in Iowa State's 1999 table (taken from Knott's Handbook for Vegetable Growers) and in Purdue's, "when stored properly" for South Dakota State, "ideal storage" in Iowa State's 2023 guide, and relative longevity for Colorado State. UW–Madison Extension in Dane County gives the same figures as Iowa State's 2023 guide for every crop both list. The last column is different: Illinois Extension's years under average storage, meaning a cool, dry place away from moisture. A dash means the table does not list the crop.

*Iowa State 2023 only; UW–Madison's list does not include cauliflower. †Illinois lists "melons." ‡Illinois gives one year for "most herbs." The range column covers the five good-storage tables, not Illinois.

Treat the figures as a planning guide, not an expiry date. Oregon State Extension says seed kept in a cool, dry place is generally viable for at least a year past the season it was packed for and may be good for up to five years; the packed-for year is usually printed on the top or bottom flap of the packet. Older seed also loses vigor, not just germination (Minnesota, South Dakota State).

Where the tables disagree

For most crops the good-storage tables agree within a year. Beans, peas, peppers, cucumbers, melons, squash, pumpkins and tomatoes get the same figure in every table that lists them. The exceptions:

• Lettuce: 1 to 6 years. Colorado State gives 1 year, South Dakota State 2, Iowa State's 2023 guide and UW–Madison 5, the Knott's-based Iowa State table and Purdue 6. Clemson also names lettuce, with corn and onion, among seed that can lose viability after one or two years. Test lettuce seed older than a year before relying on it.
• Spinach and broccoli: 3 or 5 years. Three in the Knott's table, Colorado State and South Dakota State (and Purdue, for spinach); 5 in Iowa State's 2023 guide and UW–Madison.
• Onion, parsnip and parsley: 1 or 2 years. One year in most tables, 2 in South Dakota State's. Cornell Small Farms writes that alliums and parsnip "often lose their germination by 50% each year, regardless of conditions," and that celery and fennel are sometimes best not stored beyond one season.
• Carrot, sweet corn and radish: one year apart. Purdue gives carrots 4 and sweet corn 3 where the others give 3 and 2; Colorado State gives radish 4 where the others give 5.
• Average storage cuts the figures in half or more. Illinois gives 1 year for beans, broccoli, carrots, corn, leeks, lettuce, okra, peas, peppers and spinach, and 2 for beets, cabbage, cauliflower, kale, cucumbers, melons, squash and tomatoes. By the second year there is "most likely a loss in viability," and by year three "all bets may be off."

Much of the spread comes down to storage. Purdue notes that storage conditions can affect germination as much as or more than the species does, and Colorado State shows the size of the effect: at 15% relative humidity seed dries to about 6% moisture and stores safely for several years, while at 90% it holds about 19% moisture and germinates poorly after one year.

Storing seed: cool, dry, sealed

The sources agree on cool and dry, and most add a sealed container. The numbers behind them:

Cornell Small Farms rolls cool and dry into one check, the "100 rule": if the temperature in °F plus the relative humidity in % comes to less than 100, storage conditions are good. Its own example, 40°F and 30% humidity, totals 70. A 75°F room at 50% humidity totals 125 and fails. South Dakota State advises against sheds and garages, which can get humid in summer.

Refrigerator or freezer? The sources split. Colorado State keeps sealed seed in a refrigerator or deep freezer; Purdue says the refrigerator, not the freezer; Cornell Small Farms says seed must be completely dry before it is frozen. Utah State warns that a refrigerator holds about 40°F but high humidity, so the container's seal matters, and South Dakota State lets a cold container reach room temperature before opening it so that moisture does not condense on the seed. Keep seed in its original packet, or label it with at least the crop and the year it was bought (Purdue).

Treated seed keeps less well: Utah State uses primed seed in the same planting year and tests pelleted seed held more than 2 years, which may germinate less. Colorado State also dries beans and peas less than other seed, because overdried seed of these crops can turn hard.

The paper-towel germination test

1. Count the seeds. Iowa State uses at least 20 randomly picked seeds, calling 50 better and 100 best; UW–Madison and Clemson use 10, which makes the math simple.
2. Wet the towel. Very damp but not dripping (UW–Madison). Lay the seeds along one edge, or between two moist layers, so they stay apart (UW–Madison, Iowa State, Clemson).
3. Roll, bag and label. Roll the towel loosely, slip it into a plastic bag and write the crop, variety and date on it (Iowa State, UW–Madison). UW–Madison seals the bag about 70% so a little air can circulate.
4. Keep it warm. 70–80°F (Iowa State), in a warm, dark place (Clemson). Seed that needs light to germinate goes on top of a moist towel in a bright spot; the packet says which kinds need light (Clemson).
5. Check it. After 2–3 days, then daily for about a week (Iowa State); every 2–3 days, making sure the towel neither dries out nor molds (UW–Madison). Most vegetable seeds germinate within 7–10 days (Clemson), and the packet gives the expected days (UW–Madison).
6. Count the sprouts. A seed has germinated once a root or seed leaf emerges; Iowa State makes the final count when no new sprouts have appeared for about a week.

The germination math

Germination % = seeds that sprouted ÷ seeds tested × 100 (Iowa State). With 10 seeds each sprout is worth 10%, so 8 sprouts is 80% (UW–Madison). With 20 seeds, 15 sprouts is 15 ÷ 20 × 100 = 75%. Iowa State also suggests comparing the result with the germination rate printed on the packet: if it is still close, the seed is worth sowing.

What a result means depends on whose cut-off you use:

None of the sources gives a figure for "sow more." The arithmetic is seeds to sow = seedlings wanted ÷ germination rate. At 80%, 12 seedlings take 12 ÷ 0.80 = 15 seeds; at 50%, they take 24. For seed started in cells, Minnesota Extension sows two or more older seeds per cell and cuts all but the strongest at soil level (see seed starting indoors).

The same math shows why short-lived seed is worth testing every year. If a packet of onion seed tests 80% this spring and loses half its germination in a year, as Cornell Small Farms says alliums often do, it would test about 40% next spring: below Iowa State's 50% line and at UW–Madison's buy-fresh threshold.

Checklist

• Check the packed-for year on the packet flap; write the year on any seed you save or repack.
• Store in a sealed glass jar with silica gel or powdered milk, in the refrigerator where possible; aim for °F plus % humidity under 100.
• Let a cold jar reach room temperature before opening it.
• Plan on fresh onion, leek, parsnip and parsley seed every year or two.
• Test lettuce, spinach and broccoli, and any seed older than 2 years, with 10–20 seeds before sowing.
• Replace below 50% germination (Iowa State), 40% (UW–Madison) or 70% (Clemson); in between, sow more.

Sowing dates for seed that passes the test are on the planting calendar; for crops started indoors, the seed starting calculator counts back from the transplant date.