The aquarium trade introduced hydrilla (Hydrilla verticillata) to the United States. There are two types: the dioecious strain imported from India in the 1950s, and the monoecious strain from Korea that appeared several decades later. They are similar in appearance, and both survive winter by underground tubers or rhizomes. Because of its aggressive growth habit and adaptability, hydrilla has become a major pest in the United States, where it interferes with wildlife and recreation. For example, in Florida, hydrilla has invaded over 70 percent of the waterways, and control methods cost millions of dollars each year.
Hydrilla grows as a long, soft stem with groups of leaves evenly spaced along the stem in whorls. The leaves are pointed, 1/4-inch long and bright green. It remains rooted in the bottom of both shallow and deep waterways, where few plants grow. The long stems reach toward the light at the surface of the water. Once it nears the surface, it grows horizontally, forming thick, impenetrable mats. Hydrilla grows as much as 1 inch a day. The hydrilla stem breaks easily in moving water or if disturbed by boats or wildlife. Once the stem breaks, it sinks to the bottom of the waterway, where it quickly roots to form a new plant.
Hydrilla is currently found throughout most of the coastal states in the United States and as far inland as Arizona, Idaho and Colorado. Because it can survive winters rooted in the bottom of waterways, it has the potential to spread anywhere there is permanent water. Even a tiny piece of hydrilla carried from one body of water to another can take root, multiply and ruin a pristine waterway in only a few years.
Hydrilla is considered an invasive noxious weed because it grows faster and larger than competing vegetation and quickly overwhelms an ecosystem such as a pond or lake. It grows so thickly, it crowds out aquatic life, such as fish and turtles, that cannot penetrate the thick mats. The dense shade that the thick mats create do not allow competing plant species to grow, and interferes with all recreational uses, especially fishing, boating and swimming. Hydrilla is a major problem around hydro-electric dams and anywhere water is used for industrial purposes.
Methods to control hydrilla all have their drawbacks. Herbicides can be toxic to wildlife and restrict recreational use. Large amounts of dying vegetation consume valuable oxygen that fish need to survive. Grass carp, specially bred to consume noxious weeds, can escape and eat desirable plants and mechanical controls are time-consuming and expensive.
Responsibility of the control of hydrilla lies with the recreational user. Boats and other water-recreational vehicles, including boat trailers, must be inspected carefully when they leave a hydrilla infested waterway. Also, aquarium hobbyists should never release aquarium plants or fishes into open water.
Another, less common method of control is used where water levels can be lowered in the winter. This allows cold weather to dry and freeze the rhizomes rooted in the mud around boat docks, fishing piers and swimming areas.