water thyme, Florida elodea

Development of these features may vary with location, age, and water quality (Kay 1992). Southern populations are predominantly dioecious female (plants having only female flowers) that overwinter as perennials. Populations north of South Carolina are essentially monoecious (having both male and female flowers on the same plant) (Cook and Lüönd 1982, Madiera et al. 2000). These set some fertile seed, but both biotypes depend on tubers for overwintering (Langeland and Smith 1984). Monoecious hydrilla exhibits a more annual habit than the dioecious type, with abundant tuber/turion production around September (Owens et al. 2012).

Female flowers consist of three white sepals and three translucent petals, which float on the water surface (Langeland 1996). The petals and sepals are 10-50 mm long, 4-8 mm wide, attached to leaf axils, and clustered toward the tips of the stems. Male flowers have three whitish red or brown sepals that are up to 3 mm long and 2 mm wide. They have three whitish or reddish linear petals that are about 2 mm long and have three stamens formed in the leaf axils. Male flowers may be free-floating.

Morphologically similar species include exotic Brazilian waterweed (Egeria densa Planch.), native western waterweed (Elodea nuttallii (Planch.) H. St. John), and native (except Alaska and Puerto Rico) Canadian waterweed (Elodea canadensis Michx.). E. densa, E. nuttallii, and E. canadensis have 3-6 leaves per whorl, with inconspicuous leaf serration and no dentition on midrib, but E. densa leaves are 2-3 cm long, and both E. nuttallii and E. canadensis usually has 3 leaves per whorl near stem base (Langeland et al. 2008, Wunderlin and Hansen 2011, Rybicki et al. 2013).

Recent research into molecular techniques for identifying hydrilla and its biotypes has proven successful (Verkleij 1983; Ryan et al. 1995; Madeira et al. 2004). An early method used isoenzyme patterns in hydrilla to distinguish origin and biotype (Verkleij 1983). A later method used a random amplified polymorphic DNA (RAPD) procedure to find DNA markers in hydrilla samples (Ryan et al. 1995; Les et al. 1997; Madeira et al. 1997, 2000). A relatively inexpensive alternative method used “universal primers” to sequence hydrilla DNA (Madeira et al 2004; Benoit and Les 2013; Rybicki et al. 2013).

• Alabama – First recorded in Coffeeville Lake in 1978 (Bayne 1979). Centrally located in Shades Creek (Alabama Plant Atlas Editorial Committee 2015) and Oak Mountain State Park lakes (D. Powell, Alabama Power, pers. comm. 1996); east along the Chattahoochee River (Alabama Plant Atlas Editorial Committee 2015; Owen 2015), and in Chattahoochee and Lakepoint Resort State Parks (Thomas M. Pullen Herbarium 2005); south in Lake Jackson (J. Zolcynski, AL DCNR, pers. comm. 1993), W.F. Jackson State Park (Alabama Plant Atlas Editorial Committee 2015), and the Mobile Delta (Zolczynski and Shearer 1997); west along the Tombigbee River (Alabama Plant Atlas Editorial Committee 2015; Madsen et al. 2010); and north along the Tennessee River (Bates and Smith 1994; D. Webb, TVA Muscle Shoals, pers. comm. 1997; Alabama Plant Atlas Editorial Committee 2015). The monoecious biotype was confirmed (Rybicki et al. 2013) in Wheeler Lake in 2007.
• Arizona – Few reports since 1984 along the Agua Fri River near Litchfield Park and the Santa Cruz River and its tributaries near Tucson (Thomas and Guertin 2007).
• Arkansas – First occurrence in Lake Ouachita in 1999 (R. Stokes, pers. comm. 1999); later in De Gray Lake (R. Stokes, pers. comm. 2003), Millwood Lake, the Ouachita River near Jack Lee and Calion Lakes, and the Arkansas River (USACE 2013).
• California – Previously and currently occurring in multiple drainages. The dioecious biotype was first recorded in 1976 within Ellis Lake of Marysville (Kratville 2013). The monoecious biotype was later found in Clear Lake, within the Upper Cache sub-basin in 1994 (Kratville 2013). Populations were reported as far north as the Clear Creek-Sacramento River sub-basin to as far south as Lower Colorado, San Diego, and Salton Sea sub-basins. Populations were found along the Chowchilla and Sacramento Rivers, the All-American Canal system (R. O’Connell, pers. comm. 1981), and in Lake Murray. Presently only found in Clear Lake in the Upper Cache sub-basin, and in small ponds in the Upper Yuba and Upper Bear sub-basins (P. Akers, pers. comm. 1997; Kratville 2013). The CA DFA still manages an hydrilla eradication program (Kratville 2013).
• Connecticut – Discovered on Mason Island in a pond in 1987 within the Long Island Sound sub-basin (Les at al. 1997); also found in the Mystic Seaport along the Mystic River (Benoit and Les 2013). Found also in Domenicks and Held Ponds, and along the Silvermine River near Deering Pond in the Saugatuck sub-basin (Benoit and Les 2013).
• District of Columbia – First reported in 1982 in ditches off the Anacostia River near Kenilworth Gardens (Steward et al. 1984); later along the Potomac River upstream of Theodore Roosevelt Island (Rybicki et al. 2013).
• Delaware – First occurred in Ingrams Pond in 1976 of the Chincoteague sub-basin (Miller 1988), also one of the first sightings of the monoecious biotype (also sighted in Maryland in 1976) (Haller 1982; Steward 1984); has since been reported throughout the state in the Nanticoke, Broadkill-Smyrna, and Brandywine-Christina sub-basins (E. Stetzar, pers. comm. 1995; C. Martin, pers. comm. 1995).
• Florida – Found in nearly half the lakes (Hoyer et al. 1996) and 77% of sub-basins of the state. First nationwide occurrence was in 1953 in a Tampa canal, followed by 1955 in South Miami canals (Schmitz et al. 1991). All known populations in Florida are dioecious. Hydrilla is currently absent from the following sub-basins: Blackwater, Escambia, Nassau, New, Pensacola Bay, Perdido, Perdido Bay, St. Mary’s, and Vero Beach.
• Georgia – First occurrence was in 1967 in Lake Seminole (Gholson II, A.K. Institute for Botanical Exploration: IBE-004496) on the Florida/Georgia border. Currently in Lake Sinclair, Lake Juliette, Lake Harding, Lake Oliver, Walter F. George Reservoir, Reed Bingham State Park Lake, Twin Lakes, Long Pond, Lake Worth, Little Ocmulgee Lake, Evans County Public Fishing Area, Radium Springs, and J. Strom Thurmond Lake (Thomaston 1984; R. Ober, GA DNR, pers. comm. 1994; M. Geihsler, GA DNR, pers. comm. 1998; T. Broadwell, GA Power Company, pers. comm. 2003; Benoit and Les 2013; Owen 2015; USDA 2008). Both biotypes occur in Clarks Hill (Strom Thurmond) Reservoir, Lake Oliver, Lake Harding, and Eufaula (Walter F. George) Reservoir (M. Netherland, USACE pers. comm. 2015)
• Guam – First found in 1962 in Fena Valley Reservoir (Bernice Pauahi Bishop Museum 2015); later found in Agana Springs (Smithsonian 2015) and the Pago, Chaot, Lonfit, Ajayan, Tolaeyuus, Atantano, and Agaga Rivers (Walsh 2010).
• Idaho – First found along the Bruneau River downstream from the C.J. Strike Reservoir in 2007 and in a drainage ditch in Boise in 2008 (Rybicki et al. 2013). Both populations are dioecious.
• Indiana – Reported in 2006 in Lake Manitou, but likely present in 2003 (Stout 2006). Populations were also discovered in the Ohio River at a marina near Madison and a boat ramp near New Albany, as well as in a drainage ditch north of Evansville (Central Hardwoods Invasive Plant Network 2010; Rybicki et al. 2013).
• Iowa – A single occurrence was found in 1971 in an ornamental pond near Davenport and the Mississippi River (Sample 1972).
• Kansas – Previously occurred in two locations: first in a small ornamental pond in Lawrence in 2006 (Associated Press 2009), then in a pond at Black Bob Park near Olathe in 2008 (Kansas Department of Wildlife, Parks and Tourism 2009).
• Kentucky – Discovered in Kentucky Lake in 1999, along the Tennessee River (Oster et al. 2008); later found in Carr Creek, Paintsville, Dewey, and Grayson Lakes (Oster et al. 2008; F. Howes, pers. comm. 2010).
• Louisiana – Originally sighted in 1973 in Spanish and Sibley Lakes in the Iberia and Natchitoches Parishes, respectively (Johnson and Manning 1974). Populations have since been discovered in 32 of the 48 sub-basins of the state, with many occurring in water bodies along the Red River, south of Baton Rouge along the Mississippi River, and throughout southeastern Louisiana (Montz 1980; LA DWF 1998).
• Maine – Discovered in 2002 in Pickerel Pond and eradicated since 2014 (Netherland and Greer 2014). Newly found in Damariscotta Lake in 2009 (Gregory 2009; Invasive Aquatic Species Program ME DEP 2015).
• Maryland – Along with Delaware, the first monoecious population was found in 1976 (Haller 1982); located in the Potomac River at River Bend Park (Rybicki et al. 2013). Currently in drainages along the Potomac River, including as far west as Deep Creek Lake (M. Naylor, MD DNR, pers. comm. 1997); since found in the Upper Chesapeake Bay, Gunpowder-Patapsco, Chester-Sassafras, Lower Susquehanna, Patuxent, and Nanticoke sub-basins (M. Naylor, MD DNR, pers. comm. 1997; Moore et al 2000; Orth et al. 2001; Rybicki et al. 2013).
• Massachusetts – Found in 2001 in Long Pond of the Cape Cod sub-basin (Baker 2002); later sighted in Hobomock Pond and Mystic Lake also in the Cape Cod sub-basin (Annear 2008), in a backyard pond in Charles sub-basin, and in South Meadow Pond in the Nashua sub-basin (Bertin, R. Harvard University Herbaria: NEBC-00579928; Bellaud 2010).
• Mississippi – Since 1980, widespread in water bodies and drainages along the Tennessee-Tombigbee Waterway (D. Blount, USACE, pers. comm. 1980; Kight 1988; Madsen et al. 2006); also reported in Bluff, Loakfoma, Spring, and Arkabutla Lakes, and the Ross R. Barnett Reservoir (Madsen et al. 2006; USACE 2008; Cox et al. 2010).
• Missouri – Found in 2006 in Harry S. Truman Dam and Reservoir (USACE 2013) and then in a recreational pond off Walnut Hollow Lane and county highway DD (Tvedt, K. Dunn-Palmer Herbarium: UMO-195882). Distinct populations found in Stockton Lake (USACE 2013) and in two artificial ponds, east of the Niangua River and Roberts Road (Stamper, A. Missouri Botanical Garden: MO-2442810) and west of Smith Creek off Mason Road (Yatskievych, G. Missouri Botanical Garden: MO-2442844).
• New Jersey – First reported in Tamarack Lakes in Lower Delaware sub-basin (G. Sullivan, pers. comm. 2003; Rybicki et al. 2013); since found in two more sub-basins: Crosswicks-Neshaminy in Burlington Country Club ponds and Sandy Hook-Staten Island in Franklin Lake (G. Sullivan, pers. comm. 2003; USACE 2013).
• New York – First sighted at Creamery Pond in Sugar Loaf in 2008 (L. Surprenant, NY DEC, pers. comm. 2008; King 2008). Also found in the Erie Canal in the Niagara sub-basin, Cayuga Lake in the Seneca sub-basin, the Croton River in the Lower Hudson sub-basin, and Ronkonkoma, Sans Souci, and Lotus Lakes in the Southern Long Island sub-basin (L. Surprenant, NY DEC, pers. comm. 2008; NAPMS 2009; iMapInvasives 2015).
• North Carolina – First occurrence was in 1980 in Big Lake at William B. Umstead State Park near Raleigh (Falk and Bryant 1995). Found in 23 of the 58 sub-basins within North Carolina, most heavily in the Upper Neuse sub-basin (NC Division of Water Resources 1996); northern populations along the Roanoke River near Lake Gaston, the Eno River near Eno River State Park, the John H. Kerr, Hyco, and Mayo Reservoirs, Belews Lake, and Catherine Creek off the Chowan River; southern populations in Shearon Harris Reservoir, Cape Fear River, Crane Creek north of Fort Bragg, Burnt Mill Creek near Wrightsville Beach, Lake Wylie, and Lake Waccamaw; and western populations along the Catawba River in James, Norman, and Mountain Island Lakes (NC Division of Water Resources 1996; K. Manuel, Duke Power, pers. comm. 1999; NCSC 2009; R. Westbrook, pers. comm. 2012; B. Tracy, NC DENR, pers. comm. 2014; Manuel et al. 2015).
• Ohio – Populations were recorded in 2010 along the Ohio River from Toronto to Manchester, and later in the West Creek Reservation in Parma and in a sanctuary marsh in North Chagrin Metropolitan Park (Central Hardwoods Invasive Plant Network 2010; J. Hillmer, Cleveland Metroparks, pers. comm. 2011; iNaturalist.org 2015).
• Oklahoma – Found between 2005 and 2007 in Lake Murray, Lake of the Arbuckles, and Sooner Lake (Foster et al. 2009).
• Pennsylvania – Reported in 1996 in the Schuylkill River near Fairmont Park (P. Madeira, USDA/ARS, pers. comm. 1996); later in the lower Susquehanna River near Drumore, in Marsh Creek Reservoir, and in Highland, Harveys, Nockamixon, and Pymatuning Lakes (Benoit, L.K. George Safford Torrey Herbarium: CONN 00177372; Colangelo 1998; Pennsylvania Flora Database 2011; URS Corp. 2012; Erie-Times 2015; Skrapits 2015).
• Puerto Rico – Noted in three locations since 2015; Canovanas River, El Paterre Park, and Bahia Beach Resort (F. Grana, PR DNER, pers. comm. 2007; GAEI 2015).
• South Carolina – First occurrence in 1982 at Lake Marion of Lake Marion sub-basin (Johnson 1982; Roach et al. 2003); later in Lake Keowee of Seneca sub-basin, Craig and Sedalia Lakes of Tyger sub-basin, Lake Wylie of Kings Mountain sub-basin, Wateree Lake of Wateree sub-basin, Lakes Murray and Greenwood of Saluda sub-basin, J. Strom Thurmond Lake of Upper Savannah sub-basin, and Lake Moultrie and various creeks and rivers near the Charleston Naval Weapons Station of Cooper sub-basin (Roach et al. 2003; SC DNR 2007; Benoit and Les 2013; Manuel et al. 2015).
• Tennessee – Discovered in 1988 within the Chickamauga Reservoir, near Chattanooga, then a year later in the Nickajack Reservoir within the same sub-basin (Tennessee Valley Authority 1990). Not found again until 2006 at the Old Hickory and Cordell Hull Reservoirs (USACE 2013). Hydrilla last spotted along the Obed Wild and Scenic River (National Park Service 2012), which contains both biotypes (Simmons 2007)
• Texas – First reported in Lake Conroe in 1980 (Johnson et al. 1991). Mostly in eastern Texas in every drainage basin (HUC 6) east of Wichita Falls to Del Rio, except for Central Texas Coastal, San Bernard Coastal, and Galveston Bay-Sabine Lake basins; with western extents at Amistad Reservoir, Lake Nasworthy, and O.H. Ivie Reservoir, and southerly from Falcon Reservoir, along the Rio Grande River to Brownsville (Helton and Hartmann 1997; Texas Invasives 2015).
• Virginia – Reported first in 1982 in the Potomac River near Dyke Marsh (Steward et al. 1984; Carter and Rybicki 1994), and later spread throughout the tidal Potomac River and is also present in the Chesapeake and Ohio Canal National Historic Park (Carter and Rybicki 1994; E. Steinkoenig, VA Game and Inland Fisheries, pers. comm. 1995; Ruhl and Rybicki 2010; NPS 2012). It occurs in many lakes and reservoirs such as Fairfax, Barcroft, Braddock, Burke, Breckinridge, Motts Run, Orange, Louisa, Anna, Chris Greene, Albemarle, Powhatan, and Swift Creek (E. Steinkoenig, VA Game and Inland Fisheries, pers. comm. 1995; Terlizzi 1996; McConnell 2010; NPS 2012; Rybicki et al. 2013). Recent populations were discovered in the Kanawha Basin in Claytor Lake (Heineck 2011) and the Roanoke basin at Philpott, Smith Mountain, and Leesville Lakes as well as older reports from John H. Kerr Reservoir and Lake Gaston (USACE 2013; Tarbell and Assoc. 2007; Lynchburg News & Advance 2010). Observations in Chesapeake Bay between 1985 to 1996 showed hydrilla present in the fresh to oligohaline portions of the Potomac  and York Rivers (Moore et al. 2000).  More recently it was observed in the upper James (2000) and Rappahannock Rivers (1999) (web page is http://web.vims.edu/bio/sav).
• Washington – Discovered in Duwamish sub-basin in the connected Pipe and Lucerne Lakes in 1995 (Parsons 1996).
• West Virginia – First occurrence in 2003 in the north along the Cacapon River of the Cacapon-Town sub-basin and a year later in Deckers Creek near Marilla Park of the Upper Monongahela sub-basin (D. David, WV Dept. of Ag., pers. comm. 2004). Populations also found along the Shenandoah River of the Shenandoah sub-basin, just south of Harpers Ferry, and along the Kanawha and New Rivers of the Upper Kanawha, and the Lower and Middle New sub-basins, from the Ohio River to Claytor Lake in Virginia (D. David, WV Dept. of Ag., pers. comm. 2013).
• Wisconsin – Discovered in 2007 in a privately-owned 1-1.5 acre pond not connected to any natural water body, near Athelstane of the Menominee sub-basin; likely hitchhiked from aquaculture and present since 2005 (Asplund 2007).

The dioecious biotype (plants with either male or female flowers) were introduced to Florida in the 1950s, and the monoecious biotype (plants with both male and female flowers) were introduced in the late 1970s (Cooke et al. 2005). Monoecious biotypes may be better adapted than dioecious biotypes to cool temperate climates due to its ability to form overwintering tubers quickly when photoperiods are short (Langeland 1996). Hydrilla verticillata can reproduce both asexually through vegetative fragmentation and tubers, and sexually where female and male plants occur in the same location. Sexual reproduction among and between monoecious and dioecious strains is possible (Steward 1993), but its importance is unknown (Langeland and Smith 1984). This plant mainly spreads vegetatively through dispersal of plant fragments, axillary turions, and tubers (Langeland and Sutton 1980).  Vegetative fragmentation is likely the most common method of reproduction for this species.

Tubers remain viable out of water for several days (Basiouny et al. 1978) and in undisturbed sediment for over 4 years (Van and Steward 1990). Viability remains after ingestion and regurgitation by waterfowl, although passage of vegetative propagules through the digestive tract likely renders them non-viable (Joyce et al. 1980).

Hydrilla is mainly introduced to new waters as castaway fragments on recreational boats, their motors and trailers, and in live wells. Stem pieces root in the substrate and develop into new colonies, commonly beginning near boat ramps. Once established, boat traffic continues to shatter and spread hydrilla throughout the waterbody. Both types propagate primarily by stem fragmentation, although axillary buds (turions) and subterranean tubers are also important. Tubers are resistant to most control techniques (Schardt 1994) and may be viable as a source of reinfestation for years (Van and Steward 1990).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

This species has been found inside the Great Lakes basin at Erie Canal (Tonawanda Creek), Cayuga Lake, and Tinker Nature Park pond, NY, and in marshes and ponds around Cleveland, OH (Pfingsten, 2018, pers. comm).
Hydrilla verticillata was first introduced to the United States as an aquarium plant in the 1960s and has since spread through recreational activities (Langeland 1996). Hydrilla verticillata spreads through vegetative fragmentation (Clayton et al. 2006). Hydrilla verticillata can potentially enter the Great Lakes region through fragmentation and subsequent dispersal from its current range. Hydrilla verticillata can be dispersed by plant fragments attached to boats or trailers, or tubers that are consumed and excreted by waterfowl (Clayton et al. 2006).
2012).

This species can be purchased and introduced to the Great Lakes via unauthorized intentional release. Although H. verticillata is listed on the United States Federal Noxious Weed list, it is still sold over the internet as an aquarium plant and can be obtained from Ontario’s aquarium trade (Marson et al. 2009). Moreover, this species has been found as a contaminant in aquarium orders of other plant species (Maki and Galatowitsch 2004).

Hydrilla verticillata has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).

Hydrilla verticillata has physiological tolerances and adaptive attributes that make it the “perfect aquatic weed” (Langeland 1996). Hydrilla verticillata can survive fresh and brackish waters (0-7 ppt salinity) (Langeland 1996). This species can grow in ogliotrophic to eutrophic conditions and in low-light levels (Ramey 2001). It is likely that the Great Lakes has appropriate light and nutrient levels for H. verticillata. It is possible that Hydrilla verticillata can overwinter in the Great Lakes.The monoecious biotype is somewhat winter-hardy (Ramey 2001), and produces turions, which are overwintering vegetative propagules (Maki and Galatowitsch 2004). This species occurs on every continent except Antarctica. It occurs in inland waters near the Great Lakes region such the Erie Canal in New York (NY DEC 2012), which has similar climatic and abiotic conditions as the Great Lakes region. This species inhabits fresh waters at depths up to 15 m (Langeland 1996): such habitats are available in the Great Lakes. The predicted effects of climate change in the Great Lakes may benefit H. verticillata. Increased carbon dioxide levels and elevated water temperatures may increase the growth rate of H. verticillata (Chen et al. 1994), and climate change may enhance its northward spread (Cooke et al. 2005).

Hydrilla verticillata has a high rate of productivity. It can produce an average of 6,046 tubers per season (Sutton et al. 1992). Experiments show that H. verticillata can produce up to 46 axillary tubers per gram dry weight (Thullen 1990). Small amounts of Hydrilla verticillata can be moved and develop into new populations. About 50% of the fragments with a single whorl can sprout to form a new plant, while those with a greater number of whorls have an even higher chance of sprouting (Langeland and Sutton 1980). Its ability of vegetative fragmentation may aid its establishment in new environments. It spreads especially quickly in fast-flowing waters, as these conditions efficiently disperse fragments (Ramey 2001).

Hydrilla verticillata has the potential for high environmental impact if introduced to the Great Lakes.

Hydrilla verticillata can potentially be detrimental to native species and the ecosystem. This species grows aggressively and competitively as dense mats that can displace or shade out native submersed plants. In the southeast U.S., H. verticillata effectively displaces beneficial native vegetation, such pondweeds (Potamogeton sp.) (Langeland 1996), eelgrass (Vallisneria americana), and coontail (Ceratophyllum demersum) (Rizzo et al. 1996, Van Dijk 1985). The frequency of occurrence of southern naiad in Florida was reduced from 56% to 4% after the establishment of H. verticillata (Estes et al. 1990). Infestations may reduce seed production of native aquatic plants, which may reduce the number of native species in the community (De Winton and Clayton 1996). Infestations of this species may shift phytoplankton compositions and alter chlorophyll content (Schmitz et al. 1993). Experimental evidence suggests that H. verticillata has high allelopathy potential and can inhibit the growth of lettuce seedling and duckweed (Elakovich and Wooten 1989).

Infestations of H. verticillata may alter water chemistry, decrease oxygen levels, increase pH, and increase water temperature (Woodward and Quinn 2011). Abnormal stratification of the water column (Rizzo et al. 1996, Schmitz et al. 1993), decreased oxygen levels (Pesacreta 1988), and fish kills (Rizzo et al. 1996) have been documented in H. verticillata infestations.
Sportfish exhibited lower weight and size when H. verticillata occupied the majority of the water column, which suggests that foraging efficiency was reduced as open water space and natural vegetation gradients were lost (Colle and Shireman 1980).

Hydrilla verticillata has the potential for high socio-economic impact if introduced to the Great Lakes.

Hydrilla verticillata is among the worst aquatic plants in the southeastern U.S., causing costly damage to irrigation and hydroelectric power projects, and recreation (Cooke et al. 2005). This species causes major impacts on infrastructure. Hydrilla verticillata can reduce the flow in drainage canals, which can result in flooding and damage to canal banks and structures (Langeland 1996). This species can clog intake pumps used for irrigation. In 1994-1995, Florida spent $14.5 million controlling hydrilla (Woodward and Quinn 2011). During September 1989 in South Carolina, heavy rainfall and consequent flood discharge caused large mats of H. verticillata to break loose and clog intake screens, shutting down two hydroelectric turbines at Guntersville Dam (North Carolina Agricultural Extension Service 1992). This species is a nuisance for navigation of recreational and commercial waters and interferes with swimming (Langeland 1996). The economic value of Orange Lake, Florida ($11 million) was lost during the infestation of Hydrilla verticillata (Milon et al. 1986). The dense mats of H. verticillata create stagnant waters that can be used as mosquito breeding habitat (Kerr Lake Guide 2013), making this species a risk for human health.

Hydrilla verticillata has the potential for moderate beneficial impact if introduced to the Great Lakes.

Hydrilla verticillata may increase water clarity by reducing sediment resuspension and reducing phytoplankton populations (Langeland 1996).Hydrilla verticillata may benefit some species as a food source, but only when its coverage is below 30% (Cole and Shireman 1980, Estes et al. 1990).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control:

Biological
The control of Hydrilla verticillata often involves sterile grass carp or another biocontrol agent (Cooke et al. 2005). Biocontrol was attempted using weevils Bagous affinis and B. hydrillae (Cooke et al. 2005). Bagous affinis and B. hydrillae were released in Florida in 1987 and 1991, respectively, but both species were unsuccessful in controlling Hydrilla infestation. The fly Hydrellia pakistanae was released in the U.S. and reduced infestations of Hydrilla, resulting in a recovery of native plants (Cooke et al. 2005). Efforts to control Hydrilla verticillata may be enhanced by utilizing insects and pathogenic fungus, such as Fusarium culmorum (Shabana et al. 2003).

Physical
Mechanical harvesting is costly and may aid the spread of Hydrilla verticillata by vegetative fragmentation (Woodward and Quinn 2011). Small populations may be controlled by mechanical harvesting as long as they are closely monitored for regrowth. Drawdown appears to have minimal effects on fragments of Hydrilla verticillata buried in sediment, which exhibited unimpaired regrowth after being buried for short time periods and then reintroduced to water: as a result, tilling or other methods that would expose buried vegetative fragments to desiccation in air may be more successful in preventing the growth of vegetative fragments (Pickman and Barnes 2017).

Chemical
Herbicides approved for water use may help control infestations. Fluridone is slow to act on Hydrilla verticillata and will not completely eliminate it (Woodward and Quinn 2011). Endothall works more quickly than Fluridone. Copper and diquat are also fast-acting herbicides (Langeland 1996).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

For more information on management of Hydrilla in the Great Lakes region, please visit the Great Lakes Hydrilla Collaborative.