Answer: Heat, light and humidity are all enemies of seeds. Store your leftover seeds in an airtight container in a cool, dark location such as the back of a cupboard or floor of a closet. Write yourself a note, though, so you don't forget where you've put them! Properly stored seeds should remain viable for several years. The longer you keep the seeds, the lower the percentage of germination. You can solve this problem by planting twice as many seeds as normal, if you have seeds that are extremely old.
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