This isn't meant to be a "must do combat with the indigenous plantlife" battle. It's meant to see who can create and maintain the better lawn, starting from a point of absolute nothing.
So does the potentially hazardous nature of these lawns not come into play here at all? I mean, I think these lawns are at least going to be full of bats and spiders, possibly snakes and other creepy things, too. If that's not being taken into account then that's an enormous point in Hank's favor. The more the hazards of the horror-themed environment come into play, the more our monster-tending asylum inmate is going to have an edge.
EDIT: So we are taking into account the wolfsbane, swamp plants, etc?
The point about starting with absolutely nothing is interesting. Does that mean neither has any equipment? Is there a factor of who could best improvise tools here?
Is there a time limit? Like, are we saying that once some amount of time has passed they both stop and the best lawn at that point wins? Or are they both tending to their lawn daily and we're judging who has the best lawn in the long term? I think the answer to this question decides my vote.