Habitat: Hydrilla is found in the submersed plant community. The ability of this plant to adapt to a wide variety of environmental conditions has earned Hydrilla its reputation as the perfect weed. Hydrilla can grow in a variety of substrates, in waters still or flowing, low or high in nutrients. Hydrilla may also threaten estuary systems, tolerating salinities up to 10 parts per thousand. Remarkably adapted to low light conditions, Hydrilla can photosynthesize earlier and later in the day than most plants, grows well in turbid water and, when the water is clear, to depths exceeding 10 meters. Hydrilla typically occurs in dense, rooted stands, but live fragments may also be found drifting in large mats.
Description: Hydrilla is a perennial submersed aquatic plant with long, slender, branching stems emerging from horizontal underground rhizomes and above-ground stolons. The leaves are bright green, short, strap-like and pointed with sharp serrations along the outer margins (the serrations are tiny but generally visible with magnification). The leaves are typically arranged in whorls of 4 to 8. However, the lower leaves may be opposite or in whorls of 3. Small white flowers rise to the surface on slender stalks from the upper leaf axils. Hydrilla produces two types of over-wintering structures: 1) Spiny green turions (5 to 8 mm long) are produced in the leaf axils and 2) small, somewhat crescent-shaped tubers (5 to 10 mm long) that form along the rhizomes and stolons. The tubers, unique to hydrilla, have a scaly appearance under magnification and are pale cream to brownish in color. Identification of hydrilla is complicated by the fact that there are two distinct forms occurring in the United States: monoecious and dioecious.
Origin and Range: Hydrilla is native to Africa, Australia, and parts of Asia. Hydrilla has not been detected in Canada, but it has been found in neighbouring American states. The monoecious form has been confirmed in several New England states, including Maine.
Annual Cycle: Hydrilla sprouts from over-wintering rhizomes, tubers, and turions in the spring and the leafy stems grow rapidly (about 2 cm per day) toward the surface. Flowers, turions, and tubers are produced during the growing season. The turions drop to the sediments when the leafy vegetation begins to break up early in the fall. The tubers, rhizomes, and turions over-winter. The turions will sprout the following spring, but tubers may remain dormant for several years in the sediment.
Look Alikes: It may be confused with the native Canada waterweed (Elodea canadensis) and nuttall’s waterweed (Elodea nuttallii) which rarely have more than 3 leaves per whorl and also do not have noticeably toothed leaf edges. It is also very similar to the very densely leaved invasive Brazilian waterweed (Egeria densa), but this species has not yet been reported in central or eastern Canada.
Impacts: Hydrilla is considered one of the most problematic aquatic invaders. 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.