This is widespread across the Americans and Eurasia, but segregates are not recognized. It is known to form hybrids with several other tetraploids (2n = 52; FNA 22). The closely related northern species, P. natans L., is not verified for Ky. but appears to have been confused with nodosus in some earlier records (M). Leaves of nodosus are almost as large as those of illinoensis; see notes under that species.