Photo credit: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
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Species at a Glance
Hydrilla is a submerged aquatic perennial that many consider nature’s “perfect weed.” It comes in two forms: dioecious and monoecious. Both forms grow and spread at a very fast rate, covering the surface of water bodies and restricting boating, fishing, swimming, and other recreational uses.
Leaves: While morphological characteristics can vary, leaves are typically strap-like and pointed with small sharp teeth on the edges that are difficult to see with the naked eye. Spines or conical bumps are sometimes found on the midrib on the underside of the leaf; however, these are not always present. The underside of the midrib can also be red. Leaves are generally 2-4 mm (0.08-0.2 in) wide, 6-20 mm (0.2-0.8 in) long, and occur in whorls of 3-8.
Flowers: Small (10-50 mm [0.4-2 in] long) white flowers float on the water’s surface, are attached at the leaf axils, and are clustered towards the tips of the stems.
Stems/Roots: Long branching stems form intertwined mats at the water’s surface. Plants are usually rooted to the lake bottom, growing upward from the substrate in water up to 3.7 m (12 ft) deep. During the late growing season, small white tubers, which are used for food storage, are formed on the plant’s roots, allowing it to overwinter.
Hydrilla closely resembles Brazilian elodea (Egeria densa) and North American elodea (Elodea canadensis). Brazilian elodea typically has whorls of 3-6 leaves, is usually 2-3 cm (0.8-1.2 in) long, and has tiny teeth on the margins with no conical bumps on the midrib below. The native North American elodea has leaves that occur in whorls of three and is usually a much smaller plant. Neither species of elodea produces the tubers or turions found on Hydrilla.
Hydrilla grows in a wide variety of still and flowing waters including freshwater lakes, ponds, rivers, impoundments, and canals. It tolerates a wide range of pH, nutrient, and light levels and is somewhat winter-hardy, but optimum temperature for growth is 20-27°C (68-81°F).
Because Hydrilla reproduces primarily vegetatively, even the smallest living plant fragment can float downstream and form a new plant. While it was imported to the United States as an aquarium plant, recreational activities now help it spread.
While it is unknown where Hydrilla originated, possible native ranges include Asia, Africa, and Australia. It continues to spread and is listed as a federal noxious weed in the United States. While the dioecious form appears to spread from South Carolina south, the monoecious form is spreading both north and south and is typically the form found north of North Carolina and in all the Mid-Atlantic states.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
Hydrilla’s dense thick mats interfere with commercial activities by clogging water intake pipes and filters and hindering irrigation. It also restricts recreational uses and prevents sunlight from reaching other species growing beneath it. As the mats die and decay, bacteria deplete oxygen from the water, impacting fish and other aquatic organisms.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).