How Long Does Grass Seed Last? | Opened & Unopened | UK

If you have an old bag of grass seed, and are looking to plant it or overseed your existing lawn, you might be wondering whether or not it will grow.

Grass seeds do have a limited shelf life. If you plant them when they’re expired, they probably won’t germinate. You will either end up with very patchy growth, or no growth at all.

In this guide, we’ve explained how long grass seed lasts for before they go bad, whether you store them in a sealed or unsealed bag.

How long does grass seed last in a sealed bag?

Person scattering grass seeds by hand.

Grass seed in a sealed plastic bag from the manufacturer should last 4-5 years of storage in a cool, dry location, depending on the variety.

When you keep your grass seed in an unopened container, they’re much more likely to retain their original moisture level, and avoid being affected by humidity in the air. Grass seeds are sensitive to dampness, as we’ll discuss below, which is why keeping them in a sealed bag helps them to last for longer.

How long does grass seed last in an opened bag?

Person scattering grass seeds with a spreader.

Grass seed stored in an opened bag will typically only last 1-2 years, depending on where it was stored.

When left in an open container, grass seeds are left much more exposed to their surrounding environment, meaning that these surrounding conditions become much more important.

If you’ve left the grass seed in an open bag in a humid shed exposed to direct sunlight, then it might last a year from the day that you bought them, or even less. However, if your seed storage is a cool, dry spot in your garage, then it might last up to 2 years before it will go bad.

Other factors affecting grass seed longevity

Apart from whether or not you store your grass seed in a sealed container, there are other factors that affect how long the seed may last for.

  • Temperature. You want to keep grass seed relatively cool, at a temperature of under 16°C but less than 2°C. This is why it can actually be a good idea to refrigerate your seed bag in storage. At the very least, if you can’t keep your seeds in the fridge, you want to keep them in a dark location, out of direct sunlight.
  • Moisture. Grass seeds prefer to have an internal moisture content of about 15%. If you store the seed in very dry conditions, or in a very humid location, the seed moisture content can go out of balance, preventing the grass seed from growing when planted. It’s important to keep your seed in a sealed container to prevent condensation buildup.

How do you know if grass seed is still good?

Grass seeds germinating in soil.

The first thing to check to see if your grass seed will germinate is the expiry date. Most grass seed retailers will include an expiration date, or at least a rough idea of the shelf life, on the original packaging.

If you don’t have the original packaging to check the shelf life, there are other ways to see if your seeds will germinate when planted.

  • Inspect the grass seeds. Look for any signs of damp, mould, or fungi. This means that the seed has become too damp and is unlikely to germinate.
  • Do a test to see the grass seed germination rate. Get a small plant pot, fill it with dirt, and push some of your grass seeds just below the surface. Water them twice daily, ensuring they get some sunlight, and see if you notice any signs of grass seed germination over the course of a week. Remember to count how many seeds you planted, and then note how many germinated, to find the germination rate. If only half of the seeds germinated, it may be worth buying fresh seed, otherwise you’ll end up with patchy grass when you plant them.
  • Do a test on a small patch of grass. If you’re planning on using the seed for overseeding, you can do a test on a small area of grass to see if the seeds germinate on your lawn.

How to store grass seed

If you do end up buying a new bag of grass seed, here’s how you can store leftover seed to ensure that they last as long as possible.

  • Provided the original packaging is no longer sealed, it’s best to store the grass seed in something breathable. This ensures that there is some air flow, and helps to prevent mould buildup. A sack is a good option if you have one, otherwith you could use a cloth shopping bag.
  • Find a cool, dark location to store the grass seed. Your garage or basement is best – it’s a good idea to avoid the shed because the temperature can vary quite a lot, and the grass seed is more likely to be exposed to sunlight. Although you want to store your seed bag somewhere cool, ensure that it’s not left in an area that will freeze. A fridge is a good place to store grass seed if you have one in the garage.
  • Check the seed every few months if you can to make sure it’s not becoming damp over time. If you have issues with dampness, you could use a moisture absorber product such as DampRid to remove moisture from the air.

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