Usually in ponds and lakes, including the Great Lakes, in deep to shallow water; sometimes plants with “floating” leaves stranded on a wet shore.

An extremely variable species. The distinction between P. illinoensis and larger-leaved plants of P. gramineus sometimes seems quite arbitrary, especially in the absence of fruit and floating leaves. Ordinarily, the smaller stipules and more slender submersed leaves of P. gramineus will distinguish it.