Could be forever.
Hydrilla verticillata has been called the most problematic aquatic plant in the United States. This plant, native to Africa, Australia, and parts of Asia, was introduced to Florida in 1960 via the aquarium trade. It is a perennial plant that grows during the spring and summer, and becomes dormant in the fall and winter. In the spring, it regenerates itself by means of stolons, rhizomes, turions, and tubers, and could well cover a 30 acre lake.
I’m going to provide you with several links that explains its growth and tells how it may be controlled.
State of Washington
University of Texas
University of North Carolina
Exotic Aquatics on the Move
