Hydrilla verticillata is a freshwater plant with stems that can grow up to 25 feet long. The plant branches out horizontally at the water's surface and creates dense mats. Hydrilla has small, pointed, serrated leaves arranged around the stem, typically in whorls of five. Hydrilla grows quickly and creates dense mats that can block out sunlight and harm native plants, and its density can also kill fish by decreasing oxygen levels.
Hydrilla is native to Australia, Asia and Africa and is believed to have made its way to the U.S. as an aquarium plant.
Hydrilla is often spread by boaters, as hydrilla fragments can cling to boats and trailers. Very small fragments of hydrilla can start new populations.