The simple answer is that planting old seeds is possible and OK. Using old seeds will not cause any harm. Flowers or fruits that come from expired seeds will be of the same quality as if they were grown from fresh seeds. The answer is yes, the seeds will eventually spoil and no longer germinate, but it can take quite some time.
Those old seed packs are very likely to have a high percentage of seeds that germinate well. Most, but not all, seeds will be preserved for at least three years while maintaining a decent germination percentage. And even a group of very old seeds can have 10 or 20 percent that still sprout. The truth is that seeds don't expire.
They lose viability if improperly stored. While most seed companies will tell you to replace seeds every 2-3 years, those seeds will hold for decades and will germinate when planted if kept in a cool, dark, and dry location. The germination rate may decrease, but keep planting those seeds until they run out. I have successfully planted seeds in my collection with the date stamp “1998 over and over”.
If possible, store the seeds in a sealed plastic bag that contains a desiccant package (those small packs that often come in over-the-counter medicines), which will keep the seeds dry. If you have a seed group that you're not sure about, you can still plant them, but space them out more densely than if you had fresh seeds. If none of your seeds germinate, it would be best to start over with seeds from the new season and plant earlier. Depending on the type of seeds, the environmental conditions, and the manner in which the seeds have been stored, the germination rate of older seed packages can be greatly affected.
In many cases, these seeds are stored for safekeeping, slowly accumulating with what many in the gardening community call a “seed stash.” You can still save the seeds, and those seeds will continue to sprout into seedlings, but mature plants will likely demonstrate different characteristics than the plants from which you took the seeds. However, keep in mind that seeds collected from hybrid plants may not be realized from the seeds produced. Different seeds will take a different amount of time to germinate, but Garland said it will take five to 10 days for the seeds to germinate. All seeds will be most effectively stored in cold and dry conditions, so you need to be careful about seeds that are stored in opposite, warm and humid conditions.
Seeds expire, but expiration dates are rough guidelines; experts say it depends on the type of seeds and how they were stored. If you want to try several varieties of vegetables or flowers in a small garden without a lot of leftover seeds, consider sharing seed packs or trading with your neighbors or friends. Garland said germination rates are around forty percent (in other words, if only four of your 10 seeds germinate), just add more seeds when planting. Most seed packs indicate the year of intended use, usually on the top or bottom flap, and many also list minimum germination standards and the usual shelf life of seeds.
Like other seeds, how long grass seeds remain viable depends on the variety of seeds and how well they are stored, Kauth says.