Hydrilla

Hydrilla (waterthyme) is a genus of aquatic plant. It is usually treated as containing just one species — Hydrilla verticillata. But some botanists divide it into several species. It is native to the cool and warm waters of the Old World in Asia, Africa and Australia, with a sparse, scattered distribution; in Australia from Northern Territory, Queensland, and New South Wales.

The stems grow up to 1–2 meters long. The leaves are arranged in whorls of two to eight around the stem, each leaf 5–20 millimeters long and 0.7–2 millimeters broad, with serrations or small spines along the leaf margins; the leaf midrib is often reddish when fresh. It is monoecious (sometimes dioecious), with male and female flowers produced separately on a single plant; the flowers are small, with three sepals and three petals, the petals 3–5 millimeters long, transparent with red streaks. It reproduces primarily vegetatively by fragmentation and by rhizomes and turions (overwintering), and flowers are rarely seen. They have air spaces to keep them upright.

Hydrilla has a high resistance to salinity compared to many other freshwater aquatic plants.Hydrilla can grow up to an inch a day, producing dense mats of vegetation along the bottom of lakes and rivers. As they grow up to the water's surface, these mats can become several feet thick.