Hydrilla verticillata (L.f.) Royle

Hydrilla, Water Thyme

Threat Level	Category
Established	Aquatic

Similar Species May be confused with the following native and/or non-native species. Landscape Alternatives lists native horticultural substitutes	Egeria densa Elodea canadensis

Description

Height

Hydrilla is variable in height depending primarily on the depth of the water.

Leaves

The leaves are in whorls of 4 to 8. Individual leaves measure 1/4 to 3/4 inch (0.6-2 cm) long and 1/16 to 1/8 inch (2-4 mm) wide. The margins have 11 to 39 sharp teeth per centimeter and spines on the underside along the reddish midrib.

Flowers

The axial pistillate flowers are 0.5 to 2 inches (10-50 mm) long and 1/8 to 5/16 inch (4-8 mm) wide. The three sepals are whitish in color with three translucent petals. They arise from the tip of the stem and float on the surface of the water. The female flower resembles an inverted bell filled with a bubble. The free-floating staminate flowers have three whitish to reddish sepals and petals. Both types of flowers are produced singly surrounded by a bract or spathe.

Turions

Hydrilla primarily reproduces through turions (compact dormant buds) and tubers. Turions are produced in the axils of the leaves and the tubers on the rhizomes or stolons. The turions are condensed shoots 3/16 to 5/16 inch to (5-8 mm) long with 12 to 15 internodes surrounded by alternating whorls of fleshy leaves. They are dark green in color and are spiny in appearance. The tubers are terminally produced on the rhizomes or stolons. The whitish to yellow tubers are 3/16 to 3/8 inch (5-10 mm) long and may be submerged in 12 inches (30 cm) of sediment.

Images

Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
More images of Hydrilla verticillata

Life History

Hydrilla is an aquatic plant and member of the Hydrocharitaceae or Frog’s-bit family. Its appearance is variable depending on the growing conditions. It may be monoecious or dioecious. It continues to be sold as an aquarium plant, which may have been the origin of its initial introduction into natural water systems.

Hydrilla is an obligate aquatic plant that attaches itself to the substrate by rhizomes or stolons. Fragments may break off and continue to live in a free-floating state. It can grow very rapidly (up to one inch [2.5 cm] per day) to reach the water surface. Once it reaches the surface, it produces numerous branches, effectively shading-out other aquatic plants. Reproduction occurs from stem fragmentation, seed, axillary buds (turions), and tubers. Small fragments from one to three whorls are capable of establishing new colonies. Although seeds are produced, viability is relatively low and probably does not play a major role hydrilla reproduction or rate of spread. The turions and tubers are the primarily means of propagation. Turion production begins in September, continues until December, and then increases again in late spring. Floating plants have significantly higher turion production than rooted plants. Tubers are the most prolific means of vegetative reproduction. They are produced primarily during shorter days in the spring and fall. One tuber may produce as many as 6000 new tubers per square meter under ideal conditions. The tubers can remain viable for several days out of water and for over four years submerged in sediment.

Habitat

Hydrilla is an obligate submerged perennial aquatic plant and can be found in a variety of aquatic habitats such as reservoirs, lakes, ponds, springs, rivers, and tidal zones. It can tolerate a wide range of water chemistry conditions including lakes and ponds of high and low nutrient concentrations. It is rarely found in fast moving water systems. Hydrilla will tolerate salinities as high as 7% and a wide range of acidity, although optimum growth is seen at a pH of 7. It will grow in very low light conditions, 1% of full sunlight, allowing it to colonize deeper depths than many native plants.

Origin and Distribution

It is believed that Hydrilla is native to Asia and/or India. It was discovered in Florida in the 1960’s and by 1988 it had spread to an estimated 20,000 hectares of water. It is believed that two separate introductions occurred since two different forms of hydrilla occur. Boats and trailers are the primary means it is spread to new water systems. Current distribution includes all of the southeastern states, Connecticut, Delaware, Maryland, Virginia, Alabama, Texas, Arizona, California, and Washington. It has been listed as a Federal Noxious Weed and is prohibited or restricted in eleven states.

Other states where invasive: FL, WA, OR, TX, GA, NC, VA, MD, DE, VT, NH, CT. Federal or state listed as noxious weed, prohibited, invasive or banned: US, AL, AZ, CA, CO, CT, FL, ME, MA, MS, NV, NM, NC, OR, SC, TX, VT, WA.

Sources: Information on this plant's page is derived primarily from Southeast Exotic Pest Plant Council's Exotic Plant Management Plan.

Management Recommendations

Mechanical Control

Harvesting: Harvesting may control small initial populations. Where possible, remove all of the plant parts from the water and dispose of properly. Since plant fragments as small as one whorl can potentially start a new infestation, care must be taken not to break plants. Consistent monitoring for several growing seasons is required to control new or missed plants. For larger infestations in high value areas, specialized harvesters are available, although the cost may be prohibitive at as much or more than $1000 or more per acre. Up to six harvests per year may be necessary due to hydrilla’s rapid growth.

Drawdowns: When growing in bodies of water that have control structures, hydrilla may be controlled by seasonal drawdowns. Drawdowns are most effective while the tubers are developing in the fall and before regrowth occurs in the spring. Tubers may remain dormant and viable in the soil even after the lake or pond has been drained.

Herbicidal Control

Fluridone: Fluridone is a selective herbicide depending on application rates, contact times, and timing of application. Application rates depend on site-specific factors such as infestation size, water depth and chemistry, and water flow rates. It is intended for use on whole ponds and large-scale infestations (greater than 2 ha). It has been used successfully with minimal long-term effects to native plants. Fluridone is intended to reduce but not eliminate Hydrilla. Refer to manufacturer's label for specific information and restrictions regarding use.

Biological Control

Triploid Grass Carp: Sterile grass carp or white amur (Ctenopharyngodon idella) may be an option in areas with adequate control structures to ensure retention of released fish. Grass carp will eat a variety of vegetation including native species. This method is an option only in areas where impact to all of the vegetation is acceptable. The stocking rates for grass carp have not been established. Local and state laws should be checked before release.

Since 1981, the U. S. Department of Agriculture, U. S. Army Corps of Engineers and the University of Florida-IFAS have conducted worldwide surveys for other biological control agents. Over forty insect species have been identified as feeding on hydrilla and are currently being evaluated. The larvae of a weevil (Bagous affinis) discovered in India and Pakistan feeds on the subterranean tubers of hydrilla. This insect is only effective in areas of intermittent wet and dry periods or where periodic drawdowns are possible. A leaf-mining fly (Hydrellia pakistanae) has been released in Florida but its impact has yet to be determined. Hydrellia balciunasi was released in 1989 but had limited success due to variation in hydrilla populations and parasitism by native wasps among other factors. The accidental release of an aquatic moth (Parapoynx diminutalis) was shown to defoliate plants but leave viable stems.