The main issue is probably that the lots don't really advance plants if you're not on them so how long have you been in Granite Falls ? It might just not be ready.
There's a thread on the compiled list for that problem.
On the official forum, this seems to be a problem, but someone suggested the removing wild plants option from a couple of the poison fireleaf plants and seeing if a normal grows back in it's place. I will try this as soon as I can. It still doesn't explain as to why there is a normal fireleaf behind the restrooms at the welcome center when entering the woods that has to be taken care of by your active Sim. This is unlike all the other wild plants.
