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Scientific Name:

Common Name:

Hydrilla verticillata (L.f.) Royle
Hydrilla
Family: Hydrocharitaceae

USDA Plant Code: HYVE3

Habitat: Aquatic

Growth Habit: Perennial Forb/herb

Native Environment: Old World

AKA: water thyme

Introduction


Problems Caused
Hydrilla (Hydrilla verticillata (L.F.) Royle) is an invasive aquatic macrophyte that was introduced into the United States (US) by tropical fish and plant dealers. It was first seen in the US at the west coast of Florida in 1960 and was confused with Elodea canadensis. It grows in ponds, canals, ditches, lakes and rivers. Considered a federal noxious weed since 1974, economic losses and environmental impact are related to hydrilla worldwide. Hydrilla decreases the recreational use of rivers and lakes (fishing, swimming and boat traffic). It chokes waterways, clogs on irrigation pumps, changes nutrient cycles and alters endemic flora.

Regulations
Hydrilla has been listed as a federal noxious weed since 1974. It is listed as a class A noxious weed in Alabama and Mississippi, and recognized throughout the US states as a potentially problematic species.

Description



Hydrilla Stem Showing a Spiny Mid-Rib in a Serrate Margin Leaf
Vegetative Growth
Hydrilla is a completely submersed plant with erect stems that are rooted on the bottom of water. Its lance-shaped leaves are about 0.1” (2-4 mm) wide and 0.25”- 1.0” (6-20 mm) long arranged in whorls. Hydrilla has small teeth on the leaf margins, and small spines on the leaf midrib. It reproduces by vegetative (asexual) means. Asexual reproduction occurs by means of turions and tubers, although stem fragments may also develop into new plants. The tuber is an enlargement part in the terminal node of the rhizome growing underground the sediment. Its color is white to black with 0.2-0.6” (4-15) mm in length. The turion is a dormant spiny green bud of 0.1”-0.5” (3-12 mm) in length that arise from the leaf axils or branches. Hydrilla is the only species in this family found in the United States that forms tubers and axillary turions.

Flowering
Two biotypes of this nuisance aquatic weed occur in the US. The first one is a dioecious biotype with staminate (pollen-forming) and pistillate (ovule or seed forming) flowers on different plants and the second one is the monoecious biotype with staminate and pistillate flowers on the same plant. Flowers are composed by three sepals and three petals that arise from the spathe. It floats on the surface promoting pollen transport by the wind. Viable seeds are produced by the monoecious biotype. The seed are fusiform and brown in color and about 0.1” (2-3 mm) in length. However, seeds rarely initiate new colonies in the wild. Water and light are the requirements to germinate. The dioecious biotype does not produce viable seeds in the US. Only the dioecious biotype is currently found in the MidSouth.


Hydrilla Tubers (left), Found Buried in the Sediment, Resist Both Desiccation and Herbicides. Hydrilla Turions (right) are Formed
in the Axils of Leaves.
Dispersal
Dispersal can be from tubers, turions, or stem fragments. Wave action, boating activity, and currents can all cause the formation of stem fragments from existing colonies of plants. Tubers are resistant to dry periods and most control methods.

Spread By
While hydrilla was likely introduced into the US by the aquarium trade, it has spread predominantly by boat trailering from one water body to another. Boats should be cleaned thoroughly, removing all plant fragments before moving from one water body to another.



Habitat


Freshwater ecosystems such as lakes and rivers are susceptible to hydrilla colonization. Hydrilla is able to grow in water with different chemical composition, including salinities of up to 7 parts per thousand (slightly brackish), a wide range of pH, and trophic states from oligotrophic to eutrophic lakes. Physical factors within the water body such as water depth and low intensity of light allows hydrilla growth as well. Hydrilla often grows to a depth of 15’, but may grow in even deeper water if water is very clear.

Distribution


United States
This submersed aquatic weed is native to the warmer regions of Asia and is widely distributed in the continents of Europe, Africa, Australia, South America and North America. Dioecious biotype (staminate and pistillate flowers on different plants) occurs in AL, AR, AZ, FL, GA, LA, MO, MS, OK, PR, SC, TN and TX. The monoecious biotype (staminate and pistillate flowers on the same plant) occurs in CT, DE, ID, IN, MA, MD, ME, NY, PA, WA, and WI. CA and GA have both biotypes. In NC and VA, both biotypes occur on the same water body.

MidSouth
The dioecious biotype is found in AL, LA, MS, TN, and AR.

IPAMS Surveys:

Control Methods


Biological
Insects, such as Hydrellia balciunas, Bagous hydrillae, and Hydrellia pakistanae, can be used as a biocontrol agent to reduce photosynthetic capacity, biomass, and tuber number. Require constant releases. Of these, Hydrellia pakistanae is the most successful insect biocontrol agent but requires large and often-repeated releases to impact the long-term growth of hydrilla. A fungal pathogen, Mycoleptodiscus terrestris or Mt, is under development to collapse and disrupt the cells. The grass carp (Ctenopharyngodon idella) has been successfully to control hydrilla in isolated water bodies, typically as a fish stock rate of 10 fish/acre. Numerous environmental concerns surround the use of grass carp, even when sterile triploid fish are used. Individual states may have stocking restrictions; contact your state’s natural resource agency for information.

Herbicides Recommended for Managing Hydrilla
Chemical Trade Name Formulation Concentration in Water Rate of Formulation Notes
Diquat Reward Liquid 0.36 ppm a.i. 2 gal/acre Broad spectrum contact
Endothall Aquathol K Liquid 2 - 4 ppm a.i. 1.3 - 2.6 gal/acre-foot
Aquathol Super K Granular 8.8 - 17.6 lbs/acre-foot
Hydrothol 191 Liquid 1 - 2 ppm a.i. 1.4 - 2.7 gal/acre-foot
Hydrothol 191 Granular 54 - 108 lbs/acre-foot
Copper Complex Cutrine-plus Liquid 0.4 - 1.0 ppm a.i. 1.2 - 18.0 gal/acre-foot
Cutrine-ultra Emulsified
Clearigate Emulsified 3.6 - 8.7 gal/acre-foot
Fluridone Sonar AS Liquid 5 - 45 ppb a.i. 0.42 - 3.8 oz/acre-foot Broad spectrum systemic
Sonar SRP
Sonar PR
Sonar Q
Granular 0.27 - 2.5 lbs/acre-foot
Chemical
Chemical control techniques have been successful for managing hydrilla. Fluridone is a systemic aquatic herbicide widely used to control hydrilla, although some populations in Florida have developed a tolerance to this product. Contact herbicides that are successful in controlling hydrilla include chelated copper, diquat, and endothall. While the industry standards are listed in the table, generic products are widely available. New systemic herbicides are being developed and undergoing the aquatic label registration process, so new products may soon be available for hydrilla control. Carefully read all herbicide labels before use, and check with your local natural resource or regulatory agency for any additional permits or restrictions.

Mechanical
Harvesting has been used to control hydrilla nuisance growth, but does not provide long-term control. Hand picking has also been effective for scattered individual plants by removing roots and stems from the bottom. However, reinfestation is possible from tubers and turions.

Physical
Drawdown has been used to control hydrilla growth, but will not affect hydrilla tubers, so it does not provide long-term control. Benthic barriers may be effective for small colonies.

References


Langeland, K. A. 1996. Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), “The perfect aquatic weed”. Castanea 61(3):293-304.

Aquatic Ecosystem Restoration Foundation (AERF) 2004. Best Management Practices Handbook for Aquatic Plant Management in Support of Fish and Wildlife Habitat. Aquatic Ecosystem Restoration Foundation, Flint, MI. www.aquatics.org/aquatic_bmp.pdf.

More Information


Aquatic Ecosystem Restoration Foundation (Herbicide Information)
http://www.aquatics.org/

Geosystems Research Institute, Mississippi State University
http://www.gri.msstate.edu

Sea Grant Nonindigenous Species Site
http://www.sgnis.org/

University of Florida’s Center for Aquatic and Invasive Species
http://aquat1.ifas.ufl.edu/

USACE Aquatic Plant Control Research Program
http://www.wes.army.mil/el/aqua/

US Geological Survey Nonindigenous Aquatic Species
http://nas.er.usgs.gov/

Contributing Authors


Wilfredo Robles, Geosystems Research Institute, Mississippi State University

Contact Info


Dr. John D. Madsen
Mississippi State University
Geosystems Research Institute
Box 9652
Mississippi State, MS 39762-9652
Ph. (662)325-2428
jmadsen@gri.msstate.edu

Geosystems Research Institute
Contact: John D. Madsen, Ph.D.  •  WebMaster