• The type of seed because some seeds are inherently viable for longer; parsley can be very fickle and may not be good after a year by contrast, lettuce may last up to 6 years.
• How the seed has been stored. Naturally, the better the storage conditions, the longer the seed will last, cool and dark is good. I put my left over seed in the fridge at the end of the season and it keeps well over winter despite having been kept in the greenhouse most of the summer. I keep the seeds in a dry sealed tin and transfer that to the fridge when I have finished with it.

Seed is expensive and if you are growing a small  but diverse number of vegetables and flowers there is often too many seeds to use in one season, unless by F1 hybrids when there seem to be a pitiful few in the packet. Once seed was a cheap option but now it's easy to spend £30-40 on seeds so the longer the seed can be kept, the more money saved.

Very often old seed will germinate fine and I would recommend you try it out.

Love lies bleeding on the left is an annual, easy to grow and great in hanging baskets. It prefers sun and in common with many other exotic annuals such as  Ipomoea (Morning glory)  a cool summer will not suit it,  but then it will not suit the gardeners either.