Hydrilla verticillata
Hydrilla verticillata
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Hydrilla verticillata (L.f.) Royle

Common name: hydrilla

Synonyms and Other Names: Florida elodea, waterthyme

Taxonomy: available through www.itis.govITIS logo

Identification: Submersed perennial herb. Rooted, with long stems that branch at the surface where growth becomes horizontal and dense mats form. Small, pointed leaves are arranged in whorls of 4 to 8. Leaves have serrated margins and may have one or more sharp teeth under the midrib (see Godfrey and Wooten 1979). Development of these features may vary with location, age, and water quality (Kay 1992).

The distribution of biotypes is changing rapidly, however, southern populations were predominantly dioecious female (plants having only female flowers) that overwinter as perennials (the monoecious biotype has spread south through Georgia, South Carolina, Tennessee, and Alabama). Populations north of South Carolina were often monoecious (having both male and female flowers on the same plant) (Cook and Lüönd 1982; Madeira et al. 2000). Fertile seed production was reported in the monoecious type (Langeland and Smith 1984). Both biotypes depend on tubers for overwintering, although monoecious hydrilla exhibits a more annual habit than the dioecious type, with abundant tuber/turion production around September (Owens et al. 2012).

Morphologically similar species include exotic Brazilian waterweed (Egeria densa), native western waterweed (Elodea nuttallii), and native (except Alaska and Puerto Rico) Canadian waterweed (Elodea canadensis). E. densa, E. nuttallii, and E. canadensis have 3-6 leaves per whorl, with inconspicuous leaf serration and no dentation 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).

Size: Stems grow up to 9 m in length; leaves are 6-20 mm long and 2-4 mm wide.

Native Range: The common dioecious type originates from the Indian subcontinent. Historical reports specify the island of Sri Lanka (Schmitz et al. 1991) while random amplified polymorphic DNA (RAPD) analysis points to India's southern mainland (Madeira et al. 1997). Korea appears the likely origin for the monoecious type (Madeira et al. 1997).

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Puerto Rico &
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Guam Saipan
Interactive maps: Point Distribution Maps

Nonindigenous Occurrences:
  • 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 occurr 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 waterbodies 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 waterbodies 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 Niagra 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 (Tennesse 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, Ablemarle, 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 Rappahanock 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).

Ecology: Hydrilla is found in freshwater lakes, ponds, rivers, impoundments, and canals. It mainly spreads vegetatively through dispersal of plant fragments, axillary turions, and tubers (Langeland and Sutton 1980). 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 throught the digestive tract likely renders them non-viable (Joyce et al. 1980). Sexual reproduction among and between monoecious and dioecious strains is possible (Steward 1993), but its importance is unknown (Langeland and Smith 1984). Sites such as Lake Guntersville, Alabama have large co-occurring stands of monoecious and dioecious hydrilla. Pollination occurs when pollen from free-floating male flowers disperses on the water surface (epihydrophily) to female flowers (Tanaka 2000; Tanaka 2003). It has a low salinity tolerance (Carter et al 1987; Shields et al 2012).

Means of Introduction: The dioecious strain was imported to the United States in the early 1950s for use in aquariums. It entered Florida's inland water system after plants were discarded or planted into canals in Tampa and Miami (Schmitz et al 1991). The monoecious strain was a separate introduction, first found decades later in Delaware and the Potomac Basin (Environmental Laboratory 1985; Miller 1988; Madeira et al 2000).

Hydrilla is mainly introduced to new waters as 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 break and spread hydrilla throughout the waterbody. Both biotypes 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 reintroduction for years (Van and Steward 1990).

Hydrilla may be unknowingly transplanted into private ponds as a contaminant in water garden plants. It was found spreading after extensive 2,4-D use in Tennessee Valley Authority reservoirs and Lake Seminole, Georgia, once heavily populated with Eurasian water-milfoil (Myriophyllum spicatum) (Bates and Smith 1994). This has not yet been observed in northern lakes (M. Netherland, USACE pers. comm. 2015).

Status: Currently established in 28 states of the US (AL, AZ, AR, CA, CT, DE, FL, GA, ID, IN, LA, ME, MD, MA, MS, MO, NJ, NY, NC, OH, OK, PA, SC, TN, TX, VA, WV), as well as Guam and Puerto Rico; occurrences in Iowa, Kansas, Washington, and Wisconsin were removed or controlled in isolated ponds (Sample 1972; Kansas Department of Wildlife, Parks and Tourism 2009; Herrera 2011; Asplund 2007).

Impact of Introduction: It is commonly reported that once established, hydrilla results in an array of ecosystem disruptions. Hydrilla grows aggressively and competitively, spreading through shallower areas and forming thick mats in surface waters that block sunlight penetration to native plants below (van Dijk 1985). In the southeast, hydrilla effectively displaces beneficial native vegetation (Bates and Smith 1994) such as wild-celery (Vallisneria americana) and coontail (Ceratophyllum demersum) (van Dijk 1985; Rizzo et al. 1996). However, others reported that in a community of submerged aquatic vegetation including monoecious hydrilla, other exotic and native species responded to fluctuations in weather and water quality in the fresh tidal Potomac River (Carter et al. 1994) and the Upper James River in the Chesapeake Bay watershed (Shields et al. 2012). Hydrilla does not necessarily displace native species and may be beneficial to wildlife (Rybicki and Carter 2002; Rybicki and Landwehr 2007).

Hydrilla has been shown to alter the physical and chemical characteristics of lakes. Colle and Shireman (1980) found reduced weight and size in sportfish when hydrilla occupied the majority of the water column, suggesting that foraging efficiency was reduced as open water and natural vegetation gradients were lost. Stratification of the water column (Schmitz et al. 1993; Rizzo et al. 1996), decreased oxygen levels (Pesacreta 1988), and fish kills (Rizzo et al. 1996) have been documented in waters with hydrilla. Changes in water chemistry due to hydrilla may also be implicated in zooplankton and phytoplankton declines (Schmitz and Osborne 1984; Schmitz et al. 1993). However, other studies find a lack of negative effects of hydrilla on other biota, such as plants, fish, and aquatic bird communities (Killgore et al 1989; Hoyer et al 2008).

Dense beds of hydrilla affects water flow (Rybicki et al 1997) and water use. Beds in the Mobile Delta are reducing flow in small tidal streams and creating a backwater habitat (J. Zolcynski pers. comm. 1998). Its heavy growth may obstruct boating, swimming and fishing in lakes and rivers and may block the withdrawal of water used for power generation and agricultural irrigation. However, because of the resilience of hydrilla to control efforts and its competitive success and comparative vigor in stressed systems and capacity to provide at least some beneficial services combine to suggest it may have a useful role in some systems (Herschner and Havens 2008).

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Author: C.C. Jacono, M.M. Richerson, V. Howard Morgan, and I.A. Pfingsten

Revision Date: 10/27/2015

Citation Information:
C.C. Jacono, M.M. Richerson, V. Howard Morgan, and I.A. Pfingsten. 2017. Hydrilla verticillata. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
https://nas.er.usgs.gov/queries/factsheet.aspx?speciesid=6 Revision Date: 10/27/2015

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Citation information: U.S. Geological Survey. [2017]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [3/30/2017].

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