Most seeds that we buy from seed companies are packed in foil packets which, as well as the seeds, contain air from which the moisture has been removed. This keeps the seed fresh. When you open the packet, moist air gets in and the seed starts to deteriorate. The germination rate of some seed declines quickly, while some last much longer - but once the foil packet is opened, it’s only a matter of time. It’s worth remembering, by the way, that the “sow by” dates you see on packets refer to unopened packets.
Among those that do not last well are hellebores, heucheras, phlox, primulas, salvias, verbenas and violas along with onions and parsley; while beans, carrots, lettuce, peas, radishes and last up to five years, along with snapdragons (Antirrhinum), pot marigolds (Calendula), cosmos, sweet William and zinnias. Sweet peas, annual poppies and buddleia are unusually long lasting and tolerant of being stored in poor conditions - but damp air will finish off most seed.
So how should we store our spare seed? Well, it needs to be kept cool, dark and dry. Cool and dark is easy: they can go in the fridge (that light really does go out when you close the door!). But to keep them dry you should put them in a tightly closed plastic box with something to absorb the moisture.
The low-tech approach to keeping them dry is to put a layer of uncooked rice in the bottom of the plastic box to absorb moisture. Alternatively, you can use those little packets of desiccant that we find in so much packaging these days. Collect them, and when you have enough lay them on a baking sheet and place them in the oven for fifteen minutes on the coolest possible setting. Once cool, they can go in with your seed packets instead of rice. Do not use a microwave.
Then you will have the three necessary conditions: cool, dark, and dry.
Seed: collecting and storing
Seed: sowing indoors
Seed: sowing outdoors
More information on F1 hybrids