Wow, a very nice concept! That could really qualify for an RTS correct?
I found it a little difficult even in chill, so one must invest some time to become better.
If you intend to do more for this, consider adding short-keys for actions, as Warcraft 2 had; it speeds up the player response actions and makes it easier. Being cornered at some point, I couldn't move the whisp nor plant, so I would have wanted to be able to plant a tree to where it was standing, as there was enough place, before the whisp died.
The only scenario for instant win in chill mode is to plant a pine tree directly. If one plants a normal tree and does not spawn a whisp before the tree is cut down, is instant defeat. Maybe for these trees, they could leave a whisp behind upon cut down? Though if surrounded, not much to do anyway :-)