Well, Danny might have a better mel-way of doing that, but:

Question, is yor system of "seeds" resting on a sphere or other type of solid geometry?

If so, you could try using per-particle GOAL weights by animating a goal-weight ramp on the sphere, thus the "seeds" would blow away only when a pre-determined goal-weight/ramp value reaches them.

Lemme know if that made no sense