Illinois Pondweed has lance–shaped submerged leaves eight inches long that come to a point at the end.
Illinois pondweed has thin submersed leaves that are usually curved somewhat and elliptical or sword-shaped. The leaves are eight inches long, nearly two inches wide, have a sharp tip, and are attached directly to the stem or have a short stalk. Floating leaves are often not present. If they are, floating leaves are leathery, elliptical-shaped, gradually become submersed, and are about seven inches long and two and a half inches wide.
The flower spikes of Illinois pondweed rise one to three inches above the water, are thicker than the stem, and have tiny green flowers whorled along the spike that is one to three inches long. Horizontal rhizomes underground have red spots and are up to two-tenths of an inch in diameter. Illinois pondweed stems grow up to six feet tall and less than a half inch wide.
Large-leaf pondweed can hybridize with Illinois pondweed. Long-leaf (or American) pondweed’s floating leaves look similar, but Illinois pondweed does not usually produce floating leaves and if it does, the leaves are either on a short stalk or directly attached to the stem. When growing in mineral-rich water, a coating of calcium or magnesium carbonate can deposit itself on the leaves, making the leaves feel crusty. As is the case with most types of pondweed, Illinois pondweed seeds are eaten by ducks and geese; and muskrats, beavers, deer, and moose eat other parts of the plant.
Illinois pondweed is common throughout United States.