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The short answer: grass seed does not rot or spoil, but it steadily loses the ability to germinate — roughly 10 to 20 percent per year — so a bag that was 90 percent viable when packaged may only sprout half its seeds after 2 to 3 years. Stored cool, dry and sealed, most grass seed is reliably good for about two years; kept in a hot, damp shed it can fade in a single summer. Old seed that still sprouts grows perfectly healthy grass, so the only real cost of age is a thinner stand unless you sow more heavily.
Before you toss last year's leftover bag, it is worth a two-minute test. We do not test seed on plots; our seed rankings come from aggregated owner reviews and germination reports, described in how we rank. The seasonal picture is at the lawn care hub.
How long does grass seed stay good?
Germination falls off gradually with age and much faster in poor storage. The bands below assume cool, dry, sealed storage; halve the timeline for seed left in heat and humidity.
| Age (well stored) | Typical germination | Verdict |
|---|---|---|
| Fresh, within test date | 85 to 95% | Sow at normal rate |
| 1 year old | 70 to 85% | Good; slight rate bump |
| 2 years old | 50 to 70% | Usable; sow heavier |
| 3+ years old | Under 50% | Test first or replace |
Every bag of grass seed carries a seed label with a test date and a germination percentage measured near that date, which is more useful than any generic expiration stamp. A bag tested at 90 percent last spring is a very different thing from one tested three years ago, even if both look identical.
What ruins grass seed fastest?
Three things degrade seed: heat, moisture and pests. Warmth speeds the seed's metabolism so it burns through its stored energy; humidity can trigger premature swelling, mold, or partial sprouting that then dies; and mice and insects simply eat it. The classic mistake is leaving an opened bag in a garage or shed that bakes over summer.
- Cool: below about 60°F is ideal; a basement or unheated interior closet beats a shed.
- Dry: seal the bag inside a lidded plastic or metal container to keep humidity out.
- Dark and pest-proof: a closed container also keeps rodents and insects from the seed.
- Discard if: you see mold, smell mustiness, or find insect webbing or droppings.
How do you test old grass seed?
A paper-towel germination test tells you exactly what you have. Count out 10 seeds and space them on a damp paper towel, roll it up, seal it in a zip bag, and keep it somewhere warm around 70°F. Check daily and re-moisten as needed for one to two weeks, since some species are slow to sprout. Count how many germinate: 8 of 10 means about 80 percent viability and normal seeding rates; 5 of 10 means sow roughly twice as heavy; below 4 of 10 it is usually cheaper to buy fresh. This mirrors how germination timing works for new seed, covered in how long grass seed takes to germinate.
Should you just buy fresh seed?
If your test comes back strong, use the old seed and save the money — aged seed that sprouts grows a lawn indistinguishable from fresh. If viability has dropped below about half, fresh seed is the better bet, because sowing very heavily to compensate wastes seed and crowds the seedlings. Matching the species to your climate matters far more than seed age, and our current picks by region and use are in best grass seed.
Store what you have properly and it will carry over for a season or two with little loss. Related questions: how long grass seed takes to germinate, how long to wait to mow after seeding, and fescue vs Bermuda grass.