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Species of the Month: Monoecious Hydrilla

Apr 2023

Monoecious hydrilla is one of the most invasive aquatic weed species affecting the U.S., negatively impacting fishing areas, recreation and native plant communities. The resulting decrease in property values and tourism is also a concern.

Monoecious hydrilla and dioecious hydrilla are the two hydrilla biotypes found in the United States. Monoecious hydrilla was first confirmed in North Carolina in the 1980s, and has spread north — the invasive plant can now be found throughout the mid-Atlantic and northeastern United States.

Monoecious hydrilla is a perennial plant, resprouting from tubers and turions left in the sediment each spring. The plant is very shade tolerant.

The plants can grow rapidly, with the thin stems growing more than 20 feet long. On average, two to eight branches emerge at each branching node; plants can grow with minimal branching to reach the surface and then branch explosively to form dense mats. Leaves occur in whorls around the stem, usually at least 4, but the number varies with position, from 2 to 3 at the base of the stem, and up to 8 per whorl at the tip.

It’s naturally prolific too, with tubers able to remain viable in water sediment for more than seven years.

Control measures
At this time, the most effective strategy for management is maintenance control which means maintaining the lowest feasible level of the plant so as to support native plants in each region.

Monocieous hydrilla can be effectively managed with both the AQUATHOL® and HYDROTHOL® formulations, AQUASTRIKE® and CURRENT® herbicide.

To learn more, reach out your UPL Aquatics representative by emailing