Hydrilla verticillata

Hydrilla

Habitat: Hydrilla is found in the submersed plant community. The adaptability of this plant 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 can 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 strap-like and pointed with claw-like serrations along the outer margins. (The serrations are tiny but generally visible without magnification.) The leaves are typically arranged in whorls of 4 to 8. (Note: 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. Spiny green turions (5 to 8 mm long) are produced in the leaf axils. Small, somewhat crescent-shaped tubers (5 – 10 mm long), form along the rhizomes and stolons. The tubers 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 varieties of hydrilla occurring in the US. Below is a summary of characteristics that help differentiate the two varieties. (NOTE: The bold text indicates the variety that has been identified in Maine.)

Origin and Range: Native to Africa, Australia, and parts of Asia. The dioecious variety, found primarily in more southern latitudes, was first introduced to the US through the aquarium trade during the mid-1900s. The monoecious variety, found primarily in northern latitudes, was introduced some time later and has now been confirmed in several New England states, including Maine, Connecticut and Massachusetts.

Annual Cycle: Hydrilla sprouts from over-wintering rhizomes, tubers and turions in the spring, the leafy stems growing 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 in the fall. The plants die back completely to the sediments by early winter, a remarkable “vanishing act” given the amount of biomass involved. The rhizomes, tubers and turions over-winter. The turions will sprout the following spring, but the tubers may remain dormant for several years in the sediments. Research indicates that one tuber can lead to the production of over 5,000 new tubers per square meter. The tubers and turions can withstand ice cover, drying, ingestion and regurgitation by waterfowl, and herbicides. Studies also indicate that the monoecious form (the variety found in Maine) puts more of its energy into tuber and turion production than the dioecious form, and may have a greater potential for spread by these means. In addition to reproducing vegetatively by way of tubers and turions, hydrilla propagates readily from stem or root fragment. New plants can sprout from stem fragments containing as few as two nodes or whorls of leaves. The monoecious variety can produce viable seeds. Seed production and viability is thought to be low relative to vegetative reproduction.

Impacts: Hydrilla is considered one of the most problematic of all aquatic invaders. Once established in a waterbody, hydrilla causes substantial ecological, economic and aesthetic damage. Infestations interfere with navigation, recreation and other water uses. Vast monotypic stands displace native aquatic plant communities, and adversely impact the habitats of aquatic wildlife including fish and waterfowl.

Look Alikes: May be confused with Brazilian elodea, American waterweed, slender waterweed, common water starwort and mare's tail.