Is there any way to establish whether it is growing from the tree stump? If it is, that would presumably mean Aesculus. A City of Chicago website lists Aesculus glabra, A. hippocastanum 'Baumanii', and A. octandra. As Janet noted earlier, only the horse chestnut routinely has 7 leaflets, the other two are five with occasionally seven. It seems an odd place for a Rodgersia to pop up, whereas if it's from the tree there isn't much else it can be except an Aesculus of some kind I think?
