Hydrilla (Hydrilla verticillata) is a particularly aggressive aquatic plant. Importation into the U.S. is banned as it is regulated by the USDA. Two biotypes exist, monoecious and dioecious. Monoecious hydrilla currently infests many aquatic systems throughout North Carolina from ponds to lakes and even some rivers. It is native to Korea. Hydrilla was first identified in a few lakes within Wake County around 1980. Since its introduction to Wake County hydrilla has spread across the state in all directions. The people of North Carolina have spent millions of dollars managing hydrilla infested waters. The State of North Carolina recognizes hydrilla as a noxious aquatic weed; it is illegal to culture, transport, and sell this plant.
Hydrilla grows in long strands as a submersed plant. Rooted into the bottom it will grow vertically to the surface and then horizontally just beneath the surface of the water. Mature stands tend to fill the entire water column in shallow water. Established hydrilla colonies will creep into deeper water to depths of 10 feet, sometimes deeper. Leaves are short (~1/2") and form whorls along the stems. Mature plants will produce pea-sized structures in the hydro-soil called "tubers." These propagative structures sprout in the spring after the water warms up; however, some tubers can lay dormant for many years.
For more details read our Hydrilla guide.
The loss of recreational use of waters, intake fouling, and habitat alterations are the major concerns. Watercraft get hung-up in dense stands of hydrilla to the point where docks and slips become unusable. Heavy infestations discourage or even inhibit swimming and fishing activities. Advanced infestations alter habitat and drive ecological shifts like changes in fish population dynamics. For example, largemouth bass populations shift to an increased number of small fish and decreased number of large fish as the hydrilla infestation advances.
Hydrilla has multiple reproductive strategies; it fragments (small piece of separated plant grows into a whole new plant), produces turions and tubers, and develops seeds. Boaters unintentionally spread hydrilla to new locations if they are not careful to remove fragments from their equipment and trailers at the boat ramp. Bait buckets and live wells are other "carriers" that can keep fragments from drying out and become vectors of accidental releases. Some people have intentionally spread hydrilla in the past with the notion that it would improve fishing areas. We know now that the long-term negative impacts greatly outweigh any initial benefits that may be realized.
The spread of hydrilla is best controlled through public awareness and good environmental stewardship. Once hydrilla does infest an area it can be managed with herbicides and in some cases, grass carp can be used. The grass carp (a.k.a. White Amur) is an herbivorous cyprinid indigenous to Asia. This fish has been used to control hydrilla and other aquatic weeds in the U.S. for decades. Aquaculture farms provide sterile grass carp intended for aquatic weed control. Only the sterile "triploid" grass carp can be legally released into waters of NC. Contact your local extension agent or the Division of Water Resources' Aquatic Weed Control Program for guidance on stocking grass carp.
If you've seen Hydrilla in your local water body, please fill out the Hydrilla Hunt Card.