Hydrilla plant close-up. Photo:Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
This plant is a perennial that grows rapidly, covering the surface of the water, restricting boating, fishing and swimming among other recreational uses. Nuisance weeds like Hydrilla are characterized by their ability to form dense thick mats at the water surface. These mats prevent the penetration of sunlight into the water and effectively shade out other species growing beneath, threatening biodiversity. Under ideal conditions Hydrilla can grow up to an inch per day! As the mats die and decay, bacteria consume the oxygen in the water that can lead to fish kills.
Hydrilla was imported to the United States as an aquarium plant. Releases from aquaria, along with transport by recreational boaters, helped it spread across the United States. Hydrilla reproduces primarily through plant fragments; even very small fragments can float downstream and form a new plant. This makes it easy for whorls of Hydrilla stuck on boat motors, trailers or in bait buckets to start new infestations.
The stems are long and branching, forming intertwined mats at the water's surface. Small sharp teeth on the edges of the leaves and sometimes spines on the midvein make the plant rough to the touch. Leaves grow along the stem in whorls of 3-6, but often 5 leaflets. Plants are usually rooted to the lake bottom, growing to the surface in water up to 12 feet deep. During the late growing season, small white tubers form on the plants roots that are used for food storage and allow the plant to over-winter.
Hydrilla (Hydrilla verticillata) closely resembles invasive Brazilian elodea (Egeria densa) and the native North American eldoea (Elodea canadensis). Brazilian elodea typically has whorls of 3-6, leaves and has smooth leaves. North American elodea has leaves in whorls of three and is usually a much small plant. Neither North America nor Brazilian elodeas produce tubers. Hydrilla grows in a variety of still and flowing water settings such as freshwater, ponds, rivers and canals. It tolerates a wide range of pH, nutrient, and light levels. Hydrilla is somewhat winter-hardy; however, the optimum temperature for growth is 68-81°F.
Physical, chemical, and biological methods have been used to control Hydrilla. Mechanical aquatic weed harvesters open boating lanes temporarily, but risk spreading vegetation faster. Herbicides can provide temporary control, but require a permit. Herbivorous fish can be used for Hydrilla but also require a permit.
To prevent the spread of Hydrilla, always remove all visible mud and plants before leaving a water body. Drain water from all equipment. Clean gear and equipment with either hot water (104°F), or salt water OR let dry thoroughly for five days before entering a new water body.
Hydrilla is listed as a federal noxious weed, making it illegal to possess, distribute, or transport in Pennsylvania.
It is spreading rapidly throughout the state and efforts are underway to map the locations and slow the spread within the state.
Visit Pennsylvania Sea Grant's website to access fact sheets, AIS field guides, and the PA Rapid Response Plan. You can also report a sighting of hydrilla or other aquatic invasive species.