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Hydrilla verticillata


Hydrilla colony
hydrilla in-situ

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 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. Hydrilla is considered one of the most problematic of all aquatic invaders.

Hydrilla tuber
Small, somewhat crescent-shaped
tubers (5 to 10mm long), form along the
rhizomes and stolons.

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 straplike 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 overwintering structures. Spiny green turions (5 to 8 mm long) are produced in the leaf axils. Small, somewhat crescent-shaped tubers (5 to 10 mm long), form along the rhizomes and above ground stolons. The leaves are straplike and pointed with claw- 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 forms occurring in the United States (see monetious and dioetious form information below.)

hydrilla Range Map
U.S. range map of hydrilla

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

Annual Cycle: Hydrilla sprouts from overwintering rhizomes, tubers and turions in the spring, 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 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 overwinter. The turions will sprout the following spring, but the tubers may remain dormant for several years in the sediments. Research indicates that one tubers can lead to the production of over 5,000 new tubers per square meter. The tubers and turions can withstand ice cover, drying, ingestion by waterfowl, and herbicides. Studies also indicate that the monetious form (the form found in Maine) puts more of its energy into tubers and turion production than the dioetious 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 monetious form can produce Viable seeds. Seed production and Viability is thought to be low relative to vegetative reproduction.

Monoecious hydrilla
Monoecious hydrilla

Range: Generally the form found in waters of the northern United States, including Maine, Connecticut and Massachusetts.
Flowers: Male and female flower parts appear on the same plant. Flowers have three whitish sepals and three translucent petals.
Habit: Plants branch profusely at the sediments, growing rapidly toward the surface (like a shag carpet gone berserk).
Leaves: Delicate, translucent leaves average 4 to 10 mm in length; no pronounced midrib. Leaves generally lacking midrib spines.

Dioecious hydrilla
Dioecious hydrilla

Range: Generally the form found in waters of the southern United States, including Florida, Louisiana and Texas.
Flowers: Male and female flower parts appear on separate plants. All populations found in the US are female. (All reproduction of this form in the United States is asexual.)
Habit: Stems grow upward from the sediments, branching profusely at the surface. Plants generally more robust-looking than monoecious form.
Leaves: Robust leaves average 6 to 20 mm in length with pronounced (sometimes reddish) midrib. Sharp spines may be present along the undersides of the midrib.

Look Alikes: May be confused with Brazilian waterweed, native waterweeds, water starworts, and mare's tail.

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Dioecious Hydrilla Monoecious Hydrilla Hydrilla in-situ Hydrilla tuber Hydrilla turion Hydrilla whorl Hydrilla range map Hydrilla leaf illustration Hydrilla illustration Hydrilla leaf serration

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