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Hydrilla or "water thyme" (Hydrilla verticillata) is an aquatic plant from Asia that is one of the most difficult aquatic invasive species to control and eradicate in the United States. Infestations can have negative impacts on recreation, tourism, and aquatic ecosystems. It is a federally listed noxious weed (leaves DEC website) which means that its movement between states and in foreign commerce is prohibited. It is a popular aquarium plant, but it was recently prohibited from sale or possession in New York State.

2016 Distribution map of Hydrilla in New York State
Current known distribution of hydrilla in New York State

New York Locations

Hydrilla was first discovered in 2008 in a small pond in Orange County and has since been discovered in Broome, Cayuga, Erie, Kings, Monroe, Nassau, Niagara, Suffolk, Tioga,Tompkins, and Westchester counties.

DEC and partners are actively controlling hydrilla in the Croton River using annual herbicide treatments.

Impacts to Lakes, Rivers, and Wetlands

Hydrilla can grow up to an inch a day, producing dense mats of vegetation that initially grow along the bottom of lakes and rivers. As they grow up to the water's surface, these mats can become several feet thick, shading out and displacing native plants that provide food and shelter to native wildlife. They interfere with waterfowl feeding areas and fish spawning sites. Hydrilla disrupts water flow in reservoirs, hampers drainage in irrigation canals, and decreases dissolved oxygen in the water, which results in fish kills. The size and weight of sport fish such as large mouth bass are also reduced in areas infested with hydrilla.

Hydrilla forming a dense mat in water
Hydrilla forms dense mats in water

Economic Impacts

Hydrilla's dense mats of vegetation can interfere with boating, swimming, and fishing. Municipalities that rely on tourist dollars from recreational use of lakes and ponds can suffer serious losses in income due to an infestation. Waterfront property values can be greatly reduced, and property owners may incur some of the costs of management, which is expensive and long-term.

Methods of Spread

In addition to producing seed, hydrilla has green overwintering buds called turions and tubers that grow at the end of the roots and store energy. New populations of hydrilla can sprout from any of these, as well as from plant fragments that easily break off from the main plant. Turions, tubers, and plant fragments can be carried by currents, boats, boat trailers, and fishing gear to new locations.


Hydrilla has whorls of 3 or more leaves
Hydrilla has whorls of 4-8 blade-like leaves
Leaves have serrated edges and mid-veins
Leaves have serrated edges and mid-veins

Hydrilla generally grows along the bottoms of wetlands, rivers, streams, lakes, and ponds in late spring and early summer. As the days grow longer and the water temperature becomes warmer it will deposit tubers in the soil and grow to the surface of the water, creating a wall of vegetation. Plants branch profusely with each individual branch having a series of whorls of leaves (leaves growing around the stem in the same plane). Each whorl has 4-8 blade-like leaves, 5 leaves being most common, with slightly toothed edges around stem. In brackish water hydrilla will turn brown in early fall and in hard water (water high in dissolved calcium and magnesium) it may develop a whitish coating of calcium carbonate.

Lookalike Plants

Brazilian elodea is a native of South America that has whorls of more than three leaves. Its whorls of leaves are closer together on the stem which give the plant a bushy appearance. Individual leaves have smooth edges and a smooth mid-vein.

Native elodea has whorls of three leaves that have smooth edges and smooth mid-veins.

How You Can Help

  • Inspect and remove plant fragments and mud from boats, trailers, and equipment before and after each use.
  • Dispose of all debris in trash cans or above the waterline on dry land. Note: tubers and turions can easily be transported in sediment.
  • Clean and dry your equipment thoroughly before visiting other waterbodies.
  • Do not dispose of unwanted aquarium plants in waterbodies, ditches, or canals.
  • Monitor recently acquired aquatic plants because hydrilla tubers can be transported in the attached soil/growing material.

If you think you have found hydrilla, take photos of the plant(s) and note the location. Then report the infestation to, email or call (518) 402-9405.

Learn More about Hydrilla in New York and the U.S.