Hydrilla verticillata (L.) Hoyle, Hydrocharitaceae

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Present on Pacific Islands?  yes

Primarily a threat at high elevations?  no

Risk assessment results:  Reject, score: 20 (Go to the risk assessment)

Common name(s): [more details]

Chinese: hei zao

English: Florida elodea, Indian stargrass, hydrilla, water thyme, water weed

Japanese: kuromo

Tagalog: ináta, lomolomótan

Habit:  aquatic herb

Description:  "A wholly submerged branching perennial herb...  Stems:  Pale green, vertical, slender, branching above and rooting freely from the lower nodes, to 8 m long depending on water depth; stolons, which are short slender lateral stems, form on or slightly below the surface of the bottom mud.  Vegetative reproductive organs, tubers and turions are produced, respectively, on the stolon and stem apices and as short lateral branches along the upper stems.  Leaves:  Pale to bright green, often red-streaked on the prominent midrib and margins, sessile; upper leaves in distinct whorls of 4 to 6, sometimes 3 to 8, linear-lanceolate, 1 to 3 cm long, margins with strong forward-pointing teeth; lower leaves smaller and fewer, often in opposite pairs only.  Flowers:  Solitary, inconspicuous, 3 mm across, exserted from spathes in leaf axils and carried to the surface on thread-like perianth tubes (hypanthia), perianth segments 6; female flower, spathe sessile, usually one in each leaf whorl, 3 filamentous stigmas, male flower, exserted from spathes on short stems, usually several to each leaf whorl, stamens 3.  Fruit:  Cylindrical, 5 to 15 mm long, 3 to 6 mm diameter, develops within spathe and contains 0 to 6 seeds.  Seed:  Green to dark brown, shiny, 2 to 2.5 mm long, seedcoat extended at each end.  Root:  Filamentous, rising from the lower stem nodes"  (Parsons and Cuthbertson, 1992; pp. 567-570).

Habitat/ecology:  Warm freshwater ponds and slow-moving streams.  Can grow in cool and brackish water.  The plant is normally firmly rooted in the bottom mud but occasionally breaks free and forms free-floating mats.

In Fiji, "near sea level, forming extensive mats in ditches and pools, and also in freshwater rivers and streams, and carried into saltwater bays by floods.  The species has become a very serious weed of many rivers on Viti Levu, especially those of the Rewa system, where it sometimes fouls the propellers of launches and outboard motors"  (Smith, 1979; pp. 125-127). In New Caledonia, "localement abondant dans des mares et des ruisseaux à courant faible" (MacKee, 1994; p. 66).

Propagation:  Reproduces both sexually and asexually. Male and female flowers are usually borne on separate plants but occasionally plants are bisexual. Local extension may occur from tubers and stolons. Most commonly, pieces break free and float to new locations.

Native range:  Europe, Africa, Asia, Australia; exact native range obscure; also cultivated and naturalized (GRIN).

Presence:

Pacific
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Fiji Fiji Islands	Viti Levu Island	introduced invasive	Smith, Albert C. (1979) (pp. 125-127) Vouchers cited: G. Dennis 955, DA 16032, DA 16033, DA 13566, DA 16629, DA 16610, DA 16608, DA 16612, DA 16645, DA 16627, DA 16604, DA 16609, DA 16603, DA 16657, DA L.15601
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1968) (voucher ID: BISH 33986) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1968) (voucher ID: BISH 33987) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1969) (voucher ID: BISH 33989) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1969) (voucher ID: BISH 33990) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1968) (voucher ID: BISH 33991) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1968) (voucher ID: BISH 33992) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1942) (voucher ID: BISH 112348) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Fiji Fiji Islands	Viti Levu Island		Bishop Museum (Honolulu) (1970) (voucher ID: BISH 699483) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Guam Guam Island	Guam Island	native	Stone, Benjamin C. (1970) (p. 102) Also as Egeria densa sensu Stone (Fosberg, 1987). Vouchers cited: Stone 4468 (GUAM), Stone 4305 (GUAM)
Guam Guam Island	Guam Island	introduced	Fosberg, F. R./Sachet, Marie-Hélène/Oliver, Royce (1987) (p. 29)
Guam Guam Island	Guam Island		Bishop Museum (Honolulu) (1962) (voucher ID: BISH 112263) Taxon name on voucher: Egeria densa Planch.
Guam Guam Island	Guam Island		Bishop Museum (Honolulu) (1962) (voucher ID: BISH 112341) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Guam Guam Island	Guam Island		Bishop Museum (Honolulu) (1978) (voucher ID: BISH 438117) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Guam Guam Island	Guam Island		Bishop Museum (Honolulu) (1983) (voucher ID: BISH 649480) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
Guam Guam Island	Guam Island		Bishop Museum (Honolulu) (1983) (voucher ID: BISH 1000193) Taxon name on voucher: Hydrilla verticillata (L.) Hoyle
State of Hawaii Hawaiian Islands	Hawaiian Islands	introduced cultivated	Staples, George W./Herbst, Derral/Imada, Clyde T. (2000) (p. 22)
New Caledonia New Caledonia	New Caledonia Islands		Swarbrick, John T. (1997) (p. 92)
New Caledonia New Caledonia	New Caledonia Islands	introduced	Meyer, Jean-Yves (2000) (p. 100) "Potential invader".
New Caledonia New Caledonia Archipelago	Île Grande Terre	introduced invasive	MacKee, H. S. (1994) (p. 66) Vouchers cited: MacKee 14782, Veillon 1139 (NOU), MacKee 22542, MacKee 34796, MacKee 38205
Papua New Guinea Papua New Guinea (eastern New Guinea Island)	Papua New Guinea (eastern New Guinea Island)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Philippines Philippine Islands	Philippine Islands	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Philippines Philippine Islands	Philippine Islands		Merrill, Elmer D. (1925) (p. 26) In shallow fresh water at low and medium altitudes; often abundant.

Pacific Rim
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia Australia (continental)	New South Wales	native	National Herbarium of New South Wales (2013)
Australia Australia (continental)	Australia (continental)	native	National Herbarium of New South Wales (2013) South Australia, Victoria, Western Australia
Australia Australia (continental)	Northern Territory	native	National Herbarium of New South Wales (2013)
Australia Australia (continental)	Queensland	native	National Herbarium of New South Wales (2013)
Cambodia Cambodia	Cambodia (Kingdom of)		Waterhouse, D. F. (1993) (pp. 66, 77)
Central America Central America (Pacific rim)	Panama (Republic of)	introduced invasive	Cronk, Q. C. B./Fuller, J. L. (2001) (p. 162)
China China	China (People's Republic of)	native	Zhengyi, Wu/Raven, Peter H./Deyuan, Hong (2013) Ponds, running water. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Liaoning, Shaanxi, Shandong, Sichuan, Xizang, Yunnan, Zhejiang.
China China	Hong Kong	introduced cultivated	Wu, Te-lin (2001) (p. 292)
Indonesia Indonesia	Indonesia (Republic of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Japan Japan	Japan	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
North Korea North Korea	North Korea (Democratic People's Republic of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
South Korea South Korea	South Korea (Republic of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Malaysia Malaysia	Malaysia (country of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Mexico Mexico	Mexico (United Mexican States)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized
Negara Brunei Darussalam Brunei	Brunei (Negara Brunei Darussalam)		Waterhouse, D. F. (1993) (pp. 66, 77)
New Zealand New Zealand	New Zealand (country)	introduced invasive	Owen, S. J. (1997)
Singapore Singapore	Singapore (Republic of)		Waterhouse, D. F. (1993) (pp. 66, 77)
Singapore Singapore	Singapore (Republic of)	uncertain if introduced invasive	Chong, Kwek Yan/Tan, Hugh T. W./Corlett, Richard T. (2009) (p. 49) Weed of uncertain origin
South America (Pacific rim) South America (Pacific rim)	Colombia		Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 393)
Taiwan Taiwan Island	Taiwan Island	native	Zhengyi, Wu/Raven, Peter H./Deyuan, Hong (2013) Ponds, running water.
Thailand Thailand	Thailand (Kingdom of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
United States (west coast) United States (west coast states)	USA (California)	introduced invasive	Cronk, Q. C. B./Fuller, J. L. (2001) (p. 162)
United States (west coast) United States (west coast states)	USA (Washington)	introduced	U.S. Dept. Agr., Nat. Res. Cons. Serv. (2013)
Vietnam Vietnam	Vietnam (Socialist Republic of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)

Indian Ocean
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
La Réunion (France) La Réunion Island	La Réunion Island	native	Comité français de l'Union Internationale pour la Conservation de la Nature en France (2013)
Mascarene Islands Mascarene Islands (Mauritius, La Reunion, Rodrigues)	Mascarene Islands	introduced	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Mauritius Mautitius Islands (Mauritius and Rodrigues)	Mauritius Island	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Seychelles Seychelles Islands	Seychelles Islands	introduced	U.S. Dept. Agr., Agr. Res. Serv. (2013)
Seychelles Seychelles Islands	Seychelles Islands	introduced	Invasive Species Specialist Group (2017)

Also reported from
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
United States (continental except west coast) United States (other states)	United States (other states)	introduced	U.S. Dept. Agr., Nat. Res. Cons. Serv. (2013)
United States (continental except west coast) United States (other states)	USA (Florida)	introduced invasive	Cronk, Q. C. B./Fuller, J. L. (2001) (p. 162)

Comments:  Planting of this species in the State of Florida (U.S.) is prohibited by Florida Department of Environmental Protection (Hunsberger, 2001).

Control:  See "Biology and control of aquatic plants: A best management practices handbook"  for control information (large file, PDF format).

Additional control information from the Bugwood Wiki.

Physical: Mechanical methods are usually only temporarily effective, and may serve to spread the plant.

Chemical: "In flowing water acrolein is used extensively, and controlled delivery of fluridone, as the newly developed fibre formulations, has been effective in the United states.  In ponded waters overseas, chelated copper compounds, diquat, paraquat, diuron and granular formulations of endothal and 2,4-D have given good control.  The length of exposure required to give effective control varies with the herbicide used, the bipyridyls, diquat and paraquat, usually giving the quickest kill" (Parsons and Cuthbertson, 1992; pp. 567-570).

Biological: The white amur or Chinese grass carp has been used, but this species of fish is invasive in some instances. OK if sterile (triploid) fish are used. Several insects are under investigation in the US (Julien, 1992; pp. 69-70).  "In the USA, Paraponyx diminutalis (Lpidoptera:Pyralidae) was introduced accidentally; it causes some damage but does not result in full control" (Cronk & Fuller, 2001: p. 162).

Biological control information from the publication "Biological control of invasive plants in the eastern United States".