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If 70–90% germinated, the seed should be fine to use, but you should sow it a little thicker than you normally would.
You can’t do anything to change the life expectancy of different types of seeds. But if you save your own seed or need to store purchased seed, you can keep it fresh for the maximum amount of time by taking these steps to store it properly.
A packet of vegetable seeds may look dry, brittle, and lifeless, but in many cases, seeds are very much alive. Inside each plant seed is the embryo of a future plant. However, seeds do not remain alive forever. How long seeds remain viable depends on the type of seed and how well it is stored.
If 100% germinated, your seed is viable and you’re ready to plant.
Realistically, if less than 70% of your test seed germinated, you would be better off starting with fresh seed.
Most vegetable seeds remain good for about two to three years, but some, such as onions, deteriorate within a year and others such as lettuce, can successfully sprout after five years. The table below lists average years of viability for well-stored vegetable seeds, compiled from regional sources. There will be some variability because of the variety of seed and whether the seed was fully ripe and kept dry in storage.
There’s an easy way to determine how viable your saved seed is and what percentage of it you can expect to germinate.
There is no need to waste the seeds that have germinated; they can be planted. Don’t let them dry out and handle them very carefully so that you don’t break the roots or growing tip. It’s often easiest to just cut the paper towel between seeds and plant the seed, towel and all. If the root has grown through the towel, it is almost impossible to separate them without breaking the root. The paper towel will rot quickly enough and, in the meantime, it will help hold water near the roots.
Many vegetable seeds can be viable for years if they're stored properly. Learn how long each type of seed can survive and how to store and test them.