I was just thinking that you could have only a single copy of a map, but it contained more than one map. If that makes sense.
I shall explain: You start with a very basic map, perhaps only two asteroids, one called EASY, one called HARD, and 10 seedlings, no trees. If you plant a tree on either of the asteroids, a map is set up, changed according to which 'roid was planted. I'll demonstrate sometime (got less time now I'm starting my A-levels), but for now, here's some pseudo-code as a stop-gap.
If <tree is planted on EASY> then
generate <map1>
else
generate <map2>
end if
You coud potentially make a whole campaign like this, it would just be a bit of a marathon, since you can't save levels. (hint, hint)