How long seeds last, how to store them and how to test germination at home

Seeds & saving · Published 2026-10-23

Ears of multicoloured flint corn in orange, red and gold hanging in a row to dry — Corn hung up to dry. Seed keeps best when it is thoroughly dry and cool. Photo: Roderico Y. Díaz (Unsplash), via Wikimedia Commons, CC0.

Almost every gardener has a box of half-used seed packets. Are they still good? Often, yes: many seeds stay viable for years if they are kept cool and dry. A simple test will tell you for sure before you sow, and before you spend money reordering seed you already have.

Why seeds age

Seeds are alive, just dormant. They slowly use up stored energy, and the processes that age them speed up with heat and moisture. Colorado State University Extension describes good storage as keeping seed dry, below about 8 percent moisture, and cool, below about 40°F; it notes that seed kept at 15 percent relative humidity dries down to about 6 percent moisture, while seed at 90 percent humidity stays at about 19 percent. In short: humidity and warmth shorten seed life, dryness and cold extend it.

How long common seeds last

Viability varies by crop. Iowa State University Extension gives these approximate figures for seed kept in good conditions:

Years	Crops
1	Onions
2	Corn, peppers
3	Asparagus, beans, carrots, peas
4	Beets, pumpkins, squash, tomatoes, watermelon
5	Broccoli, cabbage, cauliflower, cucumber, lettuce, muskmelon, radishes, spinach

Other extension tables differ for some crops; Colorado State, for example, lists lettuce at about one year. Treat any chart as a guide only. Germination declines gradually within these periods, and storage conditions matter more than the calendar. Onion seed is the usual exception to the "it will keep" rule: buy it fresh every year or two.

Storing seed well

Keep packets sealed and dry. Put them in an airtight glass jar or tin; Colorado State recommends sealed cans or jars rather than plastic bags.
Keep them cool. A cool closet, a root cellar or a refrigerator works. Avoid sheds, garages and greenhouses, which heat up.
Use a desiccant for home-saved seed. Iowa State suggests sealing dried seed in a jar with an equal amount of silica gel in a separate envelope for one to two weeks, then removing the gel.
Avoid heat when drying. Iowa State warns that food dehydrators run too hot and can damage seed.
Let cold jars warm before opening, so moisture does not condense on the seed.
Label everything with variety and year.

The paper-towel germination test

A germination test tells you what percentage of a batch of seed will sprout. It takes a few minutes to set up and about a week to read. Following Iowa State University Extension's method:

Count out seeds at random: at least 20, or 50 to 100 for a more accurate result. Do not pick out the best-looking seeds.
Lay them on several layers of moist paper towel, spaced apart, and roll the towel up loosely.
Put the roll in a labelled plastic bag to keep it moist, leaving it partly open for air.
Keep it warm, at about 70–80°F, for example on top of a refrigerator.
Check after two or three days, then daily. A seed counts as germinated when a root or seed leaf breaks through the coat.
Count after about a week with no new sprouts, or after the time given on the packet for slow crops such as peppers or parsley.
Calculate: divide the number that germinated by the number tested.

Reading the result

Iowa State considers 90% or higher very good for most species, 70–80% acceptable for small or abundant seed, and 50% or lower generally not worth using. Between those, sow more thickly to compensate, and use the seed up this season.

What the packet tells you

Under federal seed regulations, vegetable seed in packets of a pound or less that meets the germination standard does not need to show its germination percentage, but seed below standard must be labelled "Below Standard" with its germination rate. Many seed companies print the year the seed was packed for, which tells you its age. See how to read a seed catalog.

Before you reorder

Sort your seed box in late fall or early winter, before catalogs arrive.
Discard seed that is clearly past its time, such as onion seed several years old.
Test anything you are unsure of.
Make your catalog order for what is missing or weak, and buy quantities you will use within a few years.

Old seed: sow thicker, not never

Seed that tests at 60 or 70 percent is still useful: sow it more thickly and thin later, or start it in cells where you can see what comes up. Seed that germinates slowly or produces weak seedlings in the test is losing vigour even if it still sprouts, so use it soon. For valuable or rare seed, test a small sample rather than sowing the whole packet, and keep the rest in the best conditions you can.

Buying the right amount

Catalogs often offer several packet sizes. For short-lived seed such as onions, buy only what you will sow this year. For long-lived seed such as cucumbers or tomatoes, a larger packet can last several seasons if stored well, which makes it better value. Sharing packets with friends or a garden club is an easy way to keep seed fresh without waste.

Cool, dry and sealed: and when in doubt, test.

Sources

Iowa State University Extension and Outreach, How to store seeds and test germination rates.
Colorado State University Extension, Storing vegetable and flower seeds.
University of Alaska Fairbanks Cooperative Extension, Procedures for the wet paper towel germination test.
Federal Seed Act regulations, 7 CFR § 201.29.