Hydrilla verticillata (L. f.) Royle: Hydrilla. Hydrocharitaceae (frog's bit family).
Non-native perennial. Submersed, roots in substrate; branches freely from base. Slender rhizomes in substrate produce small starchy oval tubers at their tips. The thin stems can elongate to over 20 ft (6 m). From 2 to 8 branches emerge at each branching node; plants can grow with minimal branching to reach the surface and then branch explosively to form dense mats. Leaves occur in whorls around the stem, usually at least 4, but the number varies with position, from 2 to 3 at the base of the stem, and up to 8 per whorl at the tip. The stalkless leaf is linear oval, widest in the middle and narrowing to the base, to 0.6 in (1.5 cm) long, with a sharply pointed tip. The leaf margins are toothed; the single main vein on the underside of the leaf is slightly ridged, or keeled, and has teeth all along it. These teeth or spines are usually visible without magnification and make the plant rough to the touch. Insignificant flowers, less than 0.25 in (6 mm) wide, are carried to the surface on threadlike stalks. Vegetative reproduction occurs from stem fragments and turions, which are plump, bud-like clusters of crowded leaves produced at nodes on the stems. Colonies can cover many acres. A major noxious weed, able to out-compete all other submersed aquatic plants, hydrilla hinders aquatic wildlife and all water uses and activities.
