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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.


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 grows along the bottoms of wetlands, rivers, streams, lakes, and ponds in late spring and early summer:

  • Roots or tubers in the soil grow to the surface of the water, creating a wall of vegetation.
  • Plants have many branches with each individual branch having a series of leaves growing around the stem in the same plane, like spokes on a bicycle wheel, called whorls.
  • Each whorl has 4-8 blade-like leaves with slightly toothed edges around stem.

Hydrilla may look different colors depending on the water conditions:

  • 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.

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

Distribution & Habitat

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.

Environmental Impacts

Hydrilla can grow up to an inch a day, producing dense mats of vegetation along the bottom of lakes and rivers. As they grow up to the water's surface, these mats can become several feet thick. Hydrilla mats are harmful to the ecosystem because they:

  • shade out and displace native plants that provide food and shelter to native wildlife;
  • decrease dissolved oxygen in the water, which results in fish kills;
  • interfere with waterfowl feeding areas and fish spawning sites;
  • disrupt water flow in reservoirs; disrupt drainage in irrigation canals; and

decrease the size and weight of sports fish, including largemouth bass.

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.


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 was discovered in the Croton River in October 2013 and later found in Croton Bay during a site survey in 2014 (Towns of Cortlandt and Ossining, Westchester County, NY). This survey also revealed that hydrilla is well-established in the Croton River and the New Croton Reservoir. In 2018 a control project was established to manage the infestation. Learn more about the Croton River Hydrilla Project.

How You Can Help

  • Clean, drain, and dry your watercraft and equipment thoroughly before visiting other waterbodies.
    • Inspect and remove debris and mud from boats, trailers, and equipment.
    • Dispose of all debris and bait in trash cans or at disposal stations.
    • Drain all water-holding compartments including live wells, bait wells, and bilge areas. If possible, visit a decontamination station and disinfect compartments with hot water (140°F) for at least 30 seconds.
    • Dry boats, trailers, and all equipment before use in another water body. A minimum of 5-7 days of drying time in dry, warm conditions is recommended.
  • Do not dump aquarium contents in any waterbodies, drainage ditches, or sewers.

If you think you have found hydrilla:

  • take photos of the plant(s) and note the location
    • Email the photos, noting where it was caught (coordinates preferred), to, OR
    • Submit a report through iMapInvasives, OR
    • call (518) 402-9405.

Additional Information