It really depends on how the seed is stored. I keep mine in the refrigerator. I've had delphinium seed sprout that is way older than 2 years. And ageratum seed can remain viable in the ground for many years. I was renting a house and had the backyard aerated. I got a huge stand of ageratum that I never planted that summer! My neighbor told me several years earlier another renter had planted ageratum there. My worst luck is with pansy/viola seeds but I think it might be because I try to germinate them in a room that is too warm.
