Non-native Invasive Freshwater Plants

Hydrilla (Hydrilla verticillata)

Technical Information


Hydrilla verticillata (common name - hydrilla) is a member of the Hydrocharitaceae family. It closely resembles other members of this family such as Elodea canadensis (native to Washington) and Egeria densa (native to South America, though established in many western Washington lakes). Several members of this family are popular with the aquarium and nursery industry because they are hardy, tolerant plants. Consequently, some have become severe pest problems where introduced to waters outside their native range.

Hydrilla verticillata is the only species in this genus, yet it shares characteristics with other genera in its family. Because of this, correct identification is imperative. Another complicating factor is that hydrilla can be either monoecious (both male and female flowers on the same plant) or dioecious (male and female flowers on different plants). Each has some unique growth characteristics. It is the monoecious type that has been found in Washington State.

Economic Importance

In areas of North America where hydrilla has become established, hydrilla has major detrimental impacts on water use. Hydrilla adversely impacts aquatic ecosystems by forming dense canopies that often shade out native vegetation. Extensive monospecific stands of hydrilla can provide poor habitat for fish and other wildlife, although hydrilla is eaten by waterfowl and is considered an important food source by some biologists. While dense vegetation may contain large numbers of fish, density levels obtained by plant species such as hydrilla may support few or no harvestable-sized sport fishes. Dense mats alter water quality by raising pH, decreasing oxygen under the mats, and increasing temperature. Stagnant water created by hydrilla mats provides good breeding grounds for mosquitoes. Hydrilla interferes with recreational activities such as swimming, boating, fishing and water skiing. In eastern Washington, hydrilla has the potential to impact power generation and irrigation by clogging dam trash racks and intake pipes.

In areas where hydrilla, Eurasian watermilfoil, and Brazilian elodea coexist, hydrilla outcompetes these two noxious species. Hydrilla has the potential to cause greater adverse impacts to aquatic ecosystems than either Eurasian watermilfoil or Brazilian elodea, both severe problem species in Washington. In states where hydrilla has become established, millions of dollars are spent each year for management activities.

Hydrilla has infested over 65,000 acres of Florida's lakes, rivers, streams, drainage and irrigation canals. Florida managers regard hydrilla as their most serious aquatic pest. Only the dioecious biotype is found in Florida, although monoecious hydrilla has been reported as far south as Macon, Georgia.

The following excerpt is taken from the Florida Aquatic Plant Survey Report 1992.

Since 1984, hydrilla has been the most abundant aquatic plant found in Florida public waters. Although hydrilla expansion throughout the state has been slow, growth within infested waters has been extensive despite nearly $50 million spent managing hydrilla in public waters during the 1980s. The percentage of waters infested by this invasive exotic plant has increased from 37 percent to 41 percent during the period 1983 to 1992. Plant acreage has expanded by a net 10,000 acres over the past 10 years.

Waters continue to be infested providing new examples of hydrilla's tremendous growth rate. The latest lesson is being learned on Lake Weohyakapka. Hydrilla was discovered in Lake Weohyakapka in 1991, but too late to contain it at the boat ramp. At its current rate of expansion, it is expected to fill this 7,500 lake by late 1993. Whole-lake management costs for a problem of this magnitude could exceed $500,000 per treatment.

Approximately 2.5 million dollars per year is spent on dioecious hydrilla control in South Carolina. State officials estimate that there are 50,000 hydrilla infested acres statewide and the infestation areas are still expanding. Management costs are expected to increase with expanding acreage.

Approximately half a million dollars is spent each year for hydrilla control activities in North Carolina. Most of the hydrilla is North Carolina is monoecious although the two biotypes do exist in Lake Gaston at the Virginia-north Carolina border.

The monoecious biotype of hydrilla became established in the Potomac River in 1981 and covered 3,600 acres by 1985. For many years, the Potomac River lacked any submersed aquatic vegetation, and the invasion of hydrilla has been viewed as mixed blessing. Hydrilla does provide an important food source for waterfowl but has proved a detriment for navigation and recreational activities.

California has a law that states:

The director [of the California Department of Agriculture] shall conduct an ongoing survey and detection program for hydrilla. Whenever and wherever hydrilla is discovered, the director shall immediately investigate the feasibility of eradication. If eradication is feasible, the director shall perform the eradication in cooperation with federal, city, county and other state agencies taking those steps and actions the director deems necessary.

California's mission is to prevent the establishment and spread of hydrilla and to protect agriculture first and urban and natural environments second.

In 1976, hydrilla was found in a 35 acre lake in Marysville, approximately 60 miles north of Sacramento, California. An intensive effort, integrating various control methods, successfully eradicated hydrilla from that lake. Since that initial infestation, hydrilla has been found infesting various aquatic sites in 15 California counties. These sites range from small, privately-owned ponds to a large 1,800 acre reservoir and from small irrigation or drainage ditches to a natural river system. Eradication has been achieved in nine counties. Eradication attempts were underway in six counties in 1993.

On June 11, 1993 the first infestation of monoecious hydrilla was reported in California. The infestation was limited to small ponds and has since been eradicated. The source of the infestation was a Maryland supplier of waterlilies. Apparently the hydrilla came in as a contaminant of waterlily shipments.

On August 1, 1994 monoecious hydrilla was discovered in Clear Lake, the largest natural lake in California with 44,000 surface acres and 100 miles of shoreline. Herbicide treatments were initiated on August 16. Eradication efforts are ongoing in Clear Lake and are still continuing in 2008.

Geographical Distribution

Hydrilla is native to parts of Asia, Africa, and Australia. A female dioecious plant was introduced into Florida in the mid-to-late 1950s and dioecious hydrilla has since spread throughout the southeastern states including Georgia, Alabama, Virginia, and South Carolina. Hydrilla is found as far west as Texas and California. Monoecious hydrilla is found in lakes in North Carolina and the Potomac River near Washington D.C. and also has been reported in Maryland, Delaware, and Maine. The monoecious plants in Washington State represent the northernmost occurrence of hydrilla in the United States. Hydrilla is not known to occur in Oregon or British Columbia. However, a infestation of dioecious hydrilla was discovered in the Bruneau River watershed in Idaho in 2008. Idaho is taking immediate action to eradicate the hydrilla in their state.


Hydrilla is a submersed, freshwater perennial herb, generally rooted on the bottom in depths of greater to 20 feet where water clarity is good. It is found in lakes, rivers, reservoirs, ponds, and ditches. It tends to form monospecific stands that can cover hundreds of acres.


Hydrilla was first introduced into North America in the mid to late fifties by the aquarium trade. California officials have also traced hydrilla infestations to shipments of mail order water lilies. Once introduced and established, hydrilla is easily spread through boating and fishing activities and by waterfowl. Hydrilla tubers are readily consumed and regurgitated tubers have been shown to be viable.

A suspected infestation of hydrilla was confirmed in Pipe Lake and Lucerne Lake in King County Washington on June 1, 1995. Tubers collected on that day, were sent to Dr. Fred Ryan for enzyme analysis. Dr. Ryan, a plant physiologist, is a specialist in determining hydrilla biotypes. Results from the analysis indicated that the hydrilla in these lakes is the monoecious variety. The Florida laboratory of the USDA has also recently (1998) confirmed Washington hydrilla as being monoecious.

Pipe and Lucerne lakes are connected by a narrow channel and are considered to be a single system (73 acres). They are located in King County in the Auburn area southeast of Seattle in the Green-Duwamish river basin. Although there is no public access to the system, there are three community areas on Pipe Lake that service a large population of recreationalists. According to lake residents, hydrilla has been present in the lakes for several years. There is a small patch of a purple-flowered exotic water lilies in Pipe Lake and we speculate that hydrilla may have been introduced along with these plants.

Divers found small hydrilla plants scattered throughout the littoral zones in Pipe Lake on June 1. By July 3, these plants were beginning to approach the water's surface. The lake system was treated with the aquatic herbicide Sonar® in 1995, 1996, 1997, 1998, 1999 and 2000. The hydrilla population thinned considerably over these years, but some tubers continued to germinate. We have seen no evidence of hydrilla sprouting from seeds or turions though. In 1999 King County wanted to control the remaining hydrilla by diver hand pulling, but the infestation was still too extensive to support this method. However, herbicide use was reduced from whole lake treatment during the first few years, to herbicide application only in the areas where the plants are sprouting. A two-year hand pulling (no herbicide) effort took place in 2001 and 2002 (due to the Talent Irrigation District Court decision), but did not prove effective. Herbicide treatments along with diver and snorkeler surveys and handpulling resumed in 2003. No hydrilla has been detected in Lucerne Lake since 2004. In 2007, no hydrilla plants were detected in either lake. Herbicide treatments have stopped in Lucerne Lake and 2009 was the last year of herbicide treatment for Pipe Lake. After that time, King County will continue survey efforts to ensure that no hydrilla plants sprout.

For the most up to date information about the status of hydrilla eradication in Washington click here.

Growth and Development

Hydrilla grows rooted to the bottom, submersed in either still or flowing water. The depth of growth depends on the water clarity and substrate type. The dioecious plants tend to form long stems until they near the water surface at which point the stems branch and form dense mats. Stem lengths of up to 30 feet have been recorded in clear Florida water. The monoecious plants tend to be more delicate and branch at the sediment instead. They then form many stems which rise to the surface and fill the entire water column.

In both types, the leaves are 2 - 4 mm wide (or as small as 1 mm wide on monoecious plants) and 6 - 20 mm long. They occur in whorls of 3 - 10 leaves along the stem. They generally have sharp spines along the leaf margin that gives the leaves a toothed appearance, visible without magnification. They also often have spines along the lower midrib of the leaf which causes the plant to feel rough (more common in the dioecious form). The midrib of each leaf is often reddish when fresh. There are small, axillary leaf scales (squamula intravaginalis) found next to the stem and inserted at the base of the leaf, a character that distinguishes hydrilla from other family members.


Monoecious hydrilla will produce female flowers with three translucent petals 10 - 50 mm long by 4 - 8 mm wide and three whitish sepals. They grow attached to the leaf axils and float on the water surface. Male flowers have three white to red narrow petals about 2 mm long and three white, red or brown sepals about 3 mm long by 2 mm wide. They are also formed in the leaf axils, but break free of the plant when mature and float to the water surface. On the surface the male flowers expel pollen which is then caught by the female flowers. In North America, all dioecious plants are female.

Hydrilla spreads via underground rhizomes and above ground stolons. It also forms vegetative propagules called tubers and turions. These propagules are a characteristic unique to hydrilla amongst the Hydrocharitaceae. 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 many years in the sediments. The monoecious form will also make tubers in the spring and will produce nondormant turions throughout the growing season.

Hydrilla has many effective means of propagation. It can sprout new plants from stem fragments containing as few as two nodes or whorls of leaves. Fragments from rhizomes and root crowns can also form new plants. The monoecious variety can set viable seed. However, the presence of seedlings appears to be rare, so seed production may be a minor means of reproduction. The most troubling traits for aquatic plant managers is tuber and turion production. 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.

Hydrilla also has several physiological and morphological adaptations which allow it to outcompete native aquatic vegetation.


Scientific research and 30 years of practical experience by aquatic plant managers using herbicides, biological agents, mechanical removal, and physical habitat manipulation have produced relatively successful management programs in Florida and other states. However, in spite of long-term intensive management efforts, hydrilla is still a major weed problem in the states where it has become well established.

Hydrilla's great reproductive potential creates the greatest problem for aquatic plant managers. The tubers are particularly troublesome, since they serve as a source of regrowth in areas where the hydrilla shoots have been controlled by chemical or mechanical methods. Hydrilla can rapidly colonize an area devoid of above ground vegetation. Within 2 months after an extensive diver-dredging project on the Potomac, hydrilla had reestablished itself at levels equal to nondredged areas either with fragments drifting in from adjacent areas or from new plants regenerated from the tubers that were missed by the divers.

 Response to Herbicides

There are two EPA-registered herbicides effective against hydrilla growth that are currently permitted for use in Washington. These are fluridone (Sonar®) and endothall (Aquathal®). 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.

Response to Cultural Methods

Localized control (in swimming areas and around docks) can be achieved by covering the sediment with a opaque fabric which blocks light from the plants. Managers of reservoirs and some lake systems may have the ability to lower the water level as a method of managing aquatic plants. This technique is sometimes successful in areas where the hydrosoil can thoroughly desiccate.

Response to Mechanical Methods

Because this plant spreads readily through fragmentation, mechanical controls such as cutting and harvesting should be used only when the extent of the infestation is such that all available niches have been filled. Using mechanical controls while the plant is still invading, will tend to enhance its rate of spread. In Florida, specially designed aquatic plant harvesters are used to cut and collect hydrilla from waterways. Hydrilla harvesting is mainly performed to open boat lanes through hydrilla beds for navigation. Because hydrilla produces more biomass per square meter than most aquatic plants, the cost of harvesting hydrilla is generally higher than for harvesting other nuisance species such as Eurasian watermilfoil. Harvesting costs on the Potomac River were about $1,200 per acre (costs for harvesting milfoil in Washington average $600 to 800 per acre).

Biological Control

Worldwide surveys for natural hydrilla enemies were begun in 1981 in a cooperative study between the University of Florida, the United States Department of Agriculture, and the U.S. Army Corps of Engineers. A number of insects were identified, quarantined and tested, and eventually released in Florida and other states. Results from these insect releases are still being evaluated. However, most of these insects were collected in tropical areas of the world and it is doubtful that populations could establish in Washington. The biological control with the most promise for the Pipe/Lucerne lake hydrilla infestation is the triploid grass carp. Grass carp, also known as the white amur, are an herbivorous Asian carp. Because they are an exotic species their introduction is tightly regulated and only the sterile triploid fish are allowed to be introduced in most states including Washington. Although they have access to many biocontrol agents, grass carp have been deemed the most effective biological control for hydrilla by Florida lake managers. Grass carp have definite food preferences and hydrilla is a preferred plant species. Grass carp have proven to be an effective tool for hydrilla eradication. Hydrilla infestation reached a maximum of 79 percent on Lake Baldwin, Florida, and was then eliminated by two successive grass carp stockings. California uses triploid grass carp in its Imperial Irrigation District as an eradication tool for hydrilla.

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