Hydrilla in Lake Austin

 by Casey Oliver and Eric Watters



Table of Contents




        Competitive Edge

        Economic Damage

        Problems associated with hydrilla growth

        Hydrilla Management


        GIS Focus

        Defining Eradication and Prevention Zones


        GIS Processes

        Future research on the topic may include






Since its introduction, hydrilla (also known as Indian star vine) has infested and degraded the foreign ecosystems it inhabits.  Hydrilla has been listed on the “1979 federal noxious weed list and also is identified in the noxious weed laws of Florida (FDEP, 2000), Louisiana (LDWF, 2000) and Texas (TPWD, 2000)” to mention a few. (information gathered from Driesche) Through different initiatives, we have been able to combat and reduce the distribution and the impact hydrilla has on an ecosystem.  In our study, we focused on the effectiveness of the procedures used to hinder or eliminate hydrilla from Lake Austin.  We investigated the different levels of infestation throughout the lake and by cross referencing several data inputs we are able to identify areas that require the most attention.  Once certain areas are identified, procedures can go into effect to eliminate the hydrilla.



Hydrilla verticillata is native to parts of Asia, Africa, and Australia. A female dioecious plant was introduced into Florida in the mid-to-late 1950s by the aquarium trade and has since spread throughout the southeastern states including Georgia, Alabama, Virginia, and South Carolina. Hydrilla is found as far west as Texas and California.  California officials have also traced hydrilla infestations to shipments of mail order waterlilies.  Once introduced and established, hydrilla is easily spread through boating and fishing activities and by waterfowl.  There are two species of hydrilla that inhabit the waters of the United States.  Both were introduced in the same manner. (information gathered from WAMPS)


Hydrilla is a submersed, freshwater perennial herb, generally rooted in depths up to 20 feet where water clarity is good.  It is found in lakes, rivers, reservoirs, ponds, and ditches.  Hydrilla is commonly confused with Brazilian elodea (Egeria densa) and native American waterweed (Elodea canadensis)  It tends to form monospecific stands that can cover hundreds of acres. (information gathered from WAMPS)


Rooted plants require little nutrients and can grow up to 2 cm a day. For the most part, the plant is submersed, except when branches have reached and grown across the waters surface. On occasion, the surface mat becomes detached from the plant and forms free floating mats. The appearance of hydrilla can vary substantially depending on the growth conditions.  This means that hydrilla have different appearances from region to region.  The stems of the plants are slender (about 1/32 in. thick) and sinewy. The plant is also very long in length and can grow up to 25ft. The leaves are small (5/8 in. long), strap like, pointed tips, with a conspicuous midrib. They are arranged in whorls of 4 to 8, joined directly to the stem, and the whorl internodes are 1/8 to 2 in. long.  The leaf margins are distinctly visible saw-toothed; often with one or sharper teeth along the underside midrib.  Leaf color varies depending on the location of the leaf.  A normal submerged leaf is green in color and if it is clean, it is translucent.  The leaves toward the surface are usually bleached by the sun and are attacked by fungus and bacteria which gives them a yellowish brown green color.  Both sexes of hydrilla flower and you can distinguish one from the other by the color of their flower.  The female has a solitary, tiny, white flower that reaches the surface on a long (to 4 in.) threadlike stalk.  The flower opens at the surface and forms a funnel into the water.  The pedal rims hold the flower to the surface and prevent water from entering the flower.  The male flower on the other hand is a tiny green flower that breaks away from the plant and floats around the surface.  The male flower fertilizes the female flower by randomly bumping into them.  Once fertilization occurs seeds are produced. Though the monoecious variety of hydrilla can set viable seeds, the presence of seedlings appears to be rare, so seed production may be a minor means of reproduction.  Fragmentation is another means by which hydrilla reproduces.  From small fragments, hydrilla is capable of regenerating new plants.  This feature allows for the plant to spread very easily.  Hydrilla has two other closely related means of reproduction. (information gathered from Ramey)


There is an above ground form and a below ground form.  The above ground form is an over wintering bud called a turion.  Turions are cylindrical with a length of ¼ in. and are dark green in color.  Turions are compact buds produced in the leaf axils or, in the case of monoecious plants, on stem tips.  They break off the parent plant and drift or settle to the bottom to start a new plant.  They are 5 - 8 mm long, dark green and appear spiny.  Tubers are underground turions which form at the end of rhizomes.  They are 5 - 10 mm long, and are usually white or yellowish.  Both types of hydrilla produce tubers and turions in abundance in the fall as dormant overwintering structures.  Tubers may remain dormant yet viable for several years in the sediment, especially in the case of dioecious plants.  The monoecious form will also make tubers in the spring and will produce nondormant turions throughout the growing season.  Although these tubers seem small, they provide very viable hydrilla plants.  “It has been shown 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. The monoecious form apparently puts more of its energy into tuber and turion production than the dioecious form, and so has a greater potential for spread by these means.”  All of these reproductive methods produce a plant that is very hardy and very difficult to get ride of. (information gathered from WAMPS) 


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Competitive Edge

It can grow at lower light intensities than many other plants. This makes it difficult to shade out, and allows it to grow for longer periods during the day.  It can absorb carbon from the water more efficiently than other plants, so can continue to thrive during the summer when carbon can become limiting.  It can also store extra phosphorus, so when lack of this nutrient limits the growth of other plants, hydrilla can use what it has stored.  It is tolerant of a wide range of water conditions, though water quality and sediment density can influence tuber production and growth.  It will thrive in flowing water as well as still water. Studies have shown that it actually grows faster in flowing water.  It will tolerate salinity of up to 9 -10 parts per thousand, so could encroach upon the outer limits of estuaries. (information gathered from WAMPS)


Economic Damage


“In the United States, hydrilla often dominates aquatic habitats causing significant economic damage. Hydrilla interferes with a wide variety of commercial operations. Thick mats hinder irrigation operations by reducing flow rates by as much as 90% (CDFA, 2000a) and impede the operation of irrigation structures (Godfrey et al., 1996). Hydroelectric power generation also is hindered by fragmented plant material that builds up on trash racks and clogs intakes. During 1991, hydrilla at Lake Moultrie, South Carolina shut down the St. Stephen powerhouse operations for seven weeks resulting in $2,650,000 of expenses due to repairs, dredging, and fish loss. In addition, during this repair period, there was an estimated $2,000,000 loss in power generation for the plant (letter from Charleston District Engineer to Commander, South Atlantic Division, dated March 8, 1993).”  The thick surface mats of hydrilla hinder boat traffic by clogging the boat’s motor.  “The fragmented plant material removed from the propellers can easily colonize new areas.”  This fragmentation is a dominating factor in the spread of hydrilla through an aquatic ecosystem and once an aquatic site is infested, eradication of hydrilla is very difficult.  In Texas there is a two thousand dollar fine given to any person with hydrilla fragments attached to their boat.  Hydrilla also poses a severe risk to swimmers.  (information gathered from Driesche)


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Summary of problems associated with hydrilla growth (information gathered from Gilroy)

Hydrilla Management:

There are three major methods of hydrilla eradication; biological, chemical, and mechanical.  The biological method uses the introduction of natural consumers and competitors to control the growth of hydrilla.  The most effective biological method is the introduction of grass carp.  Grass carp’s “preferred food is hydrilla.”  Biological methods also include the introduction of insects that consume different parts of the plant.  The three insects are; hydrilla flies (which consume the leaves), tuber weevils and stem weevils (the tuber weevil consumes the tuber and the stem weevil consumes the stem). One other biological attempt is the introduction of native plants used to stop the spread of hydrilla to bare habitat. The chemical method uses approved herbicides to control and eradicate the hydrilla.  “There are three EPA-registered herbicides effective against hydrilla growth. These include; fluridone (Sonar®), endothall (Aquathal®), and copper compounds. Fluridone is a systemic herbicide that has proven effective against hydrilla in Florida and other states. The drawbacks to using fluridone include its high cost, slow-action, and non-selectivity toward other macrophyte species. However, it is the herbicide of choice. Endothall, a fast-acting contact herbicide, is used when immediate control of vegetation is needed. Copper compounds are often used in conjunction with endothall applications, although copper by itself exhibits herbicidal action against hydrilla. Copper is also used for its algicidal properties when heavy periphytic growth on the hydrilla may interfere with herbicide uptake. These herbicides do not affect hydrilla seeds, tubers, and turions.  Repeated applications are needed to control hydrilla regrowth.”  The final method is the mechanical which involves “cutting and harvesting.” This process is labor intensive and expensive to do. (information gathered from WAMPS)

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“Hydrilla was first reported in California in 1976, and at that time, the state established an eradication management plan. This program has eradicated hydrilla from various sites in ten counties. At some sites, treatment of hydrilla continued for six to eight years before eradication was achieved. Funding for this program has gradually increased over time, and during the last three years, California has spent more than $5.39 million (nearly $1.8 million annually) to eradicate hydrilla infestations in that state (CDFA, 2000a).”  It is currently still a major problem in the state of Texas.  (information gathered from Driesche)

GIS Focus

Determining Optimal Areas for Eradication and Prevention


With the use of GIS, we attempted to locate areas of Lake Austin we felt crucial to focus city and community eradication and prevention efforts.  These crucial areas consisted of boat ramps, community parks along the lake, dams and densely populated residential lake front property.  Reasons for the importance of these areas include:


1.  Boat ramps are a major source of contention due to the ability of boats to fragment hydrilla.  As boats come in and out of the area, their propeller cuts through the dense mats of hydrilla, increasing the fragmentation that ultimately leads to an increasing population.  On busy days, boats will be lined up or circling at the boats ramps, making multiple passes through the dense vegetation.  This not only increases the fragmentation, but also poses a nuisance to boaters and raises safety issues.  Due to observed boat traffic, a 200 meter buffer was placed around the ramps.  This distance was determined by personal observation as being an area in which boats would come into contact with hydrilla.


2.  Lake front recreational areas create another location the community should pay close attention to.  From a visitation standpoint, numbers will decline if the water front is matted with hydrilla and other aquatic vegetation.  Dense vegetation causes a safety hazard to swimmers.  Swimmers will increase the fragmentation.  These parks also consist of high boat traffic.


3.  Hydrilla has a major effect on local dams and power stations.  Hydrilla may lead to a decrease in efficiency, as well as create a major expense for companies when dealing with the exportation of hydrilla from the surrounding area and equipment. 


4.  Densely populated residential lake front property poses many of the same hazards as boat ramps and local parks.  Many residences have boats they keep in their personal slips.  This concentration in boat traffic leads to an increase in fragmentation.  Also, the LCRA has been drawing the lake down multiple feet to allow the seasonal weather and freezes to combat the vegetation.  When this drawdown occurs, many individuals attempt to cut the hydrilla down.  Instead of discarding the plants in the trash, they throw them back into the lake.  This extreme plant has the ability to survive and grow in free floating mats or root itself down stream.  A population density of 300 or more was considered densely populated.  Through examination of the DOQQ and census track outlines, I visually selected those census tracks with a visible concentration of residential homes.  I then examined the population density of these tracks and found the population density to be consistently above 300 persons per square kilometer.


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Defining Eradication and Prevention Zones

In order to locate and map these areas, we first defined the criteria necessary for labeling eradication and prevention zones.


Level 1 Eradication:  priority areas of Lake Austin that leaders should focus immediate eradication measures

Defined by:  any area containing hydrilla within 200 meters of a lake access point or within 1000 meters of a dam


Level 2 Eradication:  areas of Lake Austin that leaders should focus immediate eradication measures

Defined by:  any area containing hydrilla along the bank of a census block with a population density greater than 300 persons per square kilometer


Level 1 Prevention:  areas of Lake Austin that leaders should focus prevention measures

Defined by:  any area not containing hydrilla and within 200 meters of a lake access point or 1000 meters of a dam



    Locating these areas began by collecting data from a compilation of sources.


        Texas Parks & Wildlife (TPWD)

                -  hydrilla coverage shapfiles

                -  Lake Austin outline shapefiles

     City of Austin              

                - 2004 census data

                - four year hydrilla maps

                - high recreation locations

                - Lake Austin access points

                - Dam locations

        Texas Natural Resource Information Systems (TNRIS)

                - 1 meter DOQQ


GIS Processes

                               Levels of Eradication                                                                       Level 1 Prevention



Future research on the topic may include:

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Driesche, Van R. “Hydrilla.” Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication. 2002-2004

http://www.invasive.org/eastern/biocontrol/7Hydrilla.html  accessed on 4/15/04

Gilroy, Mary. “Hydrilla.”  City of Austin: 1995.                                                                                                                         http://www.ci.austin.tx.us/watershed/hydrilla_default.htm accessed on 3/20/04

Ramey, Victor.  “Non-Native Invasive Aquatic Plants in the United States Center  for Aquatic and Invasive Plants, University of Florida and Sea Grant.” http://plants.ifas.ufl.edu/seagrant/hydver2.html. accessed on 4/10/04

The Western Aquatic Plant Management Society (WAPMS). “Hydrilla verticillata Hydrilla.”                                                              http://www.wapms.org/plants/hydrilla.htmlaccessed on 420/04

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