I've noticed that when you order seedlings to move from one asteroid to another, the infobar reports the same number of seedlings. They're only "moved" once they enter orbit around another asteroid.
What's annoying is when you have a limited number of seedlings (say 15), you dispatch 5 to other asteroids, and when another 5 have generated, you plant a tree. If those seedlings you moved haven't reached their target asteroid yet, they stand a chance of being sacrificed.
I imagine every asteroid has a unique ID, and every seedling is associated with an asteroid ID. If seedlings in motion between asteroids could be assigned a null id (like -1), it would prevent them from being counted as resources on any asteroid until they've reached their target - which only makes sense, imo.