Ok…so how does this spell work. One of friends thinks a single bush should explode to fill the volume, while I read it to mean any vegetation already present becomes impassable (but a single tree in a dead field won’t make the field impassable). Am I too harsh?
I suppose if it was genuinely the case that the tree was the only living thing in the field then the spell might have trouble exploding to fill the volume, but usually there will be smaller plants, vines, clover, weeds, etc.
In any event, leaving aside the ecological question, certainly the intent of the spell is that if you cast it outdoors in anything except a desert or wasteland, you’ll get the full volume. The requirement of having there be plants present is to make it not useful inside dungeons and buildings.
Ok, fair enough. What about in a settlement or well trodden keep parade ground? That’s really the crux…I envision that sort of area as being effectively plant free despite the occasional bit of grass clinging to a cobblestone or at the edge of the parade ground. That’s the bone of contention. Otherwise places where any planting has been trodden to death (speaking of which…dead plants aren’t viable, right?).
Beyond that I’m all for an outside/inside divide.
I think I’d go with the intent of the keep owner. If he has purposefully said, “I’m worried about Growth of Plants”, and deliberately makes sure that no growing things exist on the parade ground, then it makes sense to forbid Growth of Plants. This would be similar to how guards will burn away all vegetation around a keep to ensure there’s no cover for enemy.
On the other hand, a typical parade ground of a typical keep is probably not so well-policed.