Myriophyllum spicatum L., Haloragaceae

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Present on Pacific Islands?  no

Primarily a threat at high elevations?  no

Risk assessment results:  Reject, score: 25 (Go to the risk assessment).

Common name(s): [more details]

Chinese: sui zhuang hu wei zao

English: Eurasian water-milfoil, Eurasian watermilfoil, spike watermilfoil

Japanese: hozakinofusamo, kingyomo

Habit:  aquatic herb

Description:  "A perennial or biennial, submersed, aquatic herb; stems flexible, sparsely or much branched, slender, becoming leafless toward base, 1 to 4 m length, with extreme of 12 m reported from western Himalayas in Asia; adventitious roots on lower side of stem when on or in mud with additional at the upper surface prior to fragmentation; leaves dimorphic, grayish-green, in whorls of 4 to 5, usually with 14 to 24 pairs of filiform obtuse segments, 6 to 12 mm in length, extending outward from axis to give feather-like appearance; bracts inconspicuous, shorter than stem internodes; inflorescence borne on terminal spike of main stem and branches, 5 to 20 cm long, usually plink, below the spike are 5 to 20 nodes with diameter twice that of remainder of stem, rigid, lying parallel to water surface, spike erect at anthesis, parallel to water surface at fruit set; flowers, staminate flowers on upper axils; pistillate flowers below, often with bisexual flowers in the transition zone, flowers verticillate in whorls of 4, 2-ranked, lower 2 to 4 whorls of floral bracts usually comb-like, often longer than flowers, upper bracts entire, broader than long and shorter than flowers; staminate flower sepals ovate to triangular, 0.5 by 0.3 mm, erect, petals wine red, auriculate, 2.5 mm long, falling at anthesis, stamens 8; pistillate flower sepals strongly reduced, semiorbicular, erect, 0.2 to 0.3 mm, petals spreading, 0.3 to 0.5 mm, falling soon after anthesis; bisexual flower sepals and petals as in pistillate flowers, petals reddish; fruit muricate (a rough surface with sharp projections), globoseovoid, about 2.5 mm long, 4-grooved with 2 somewhat wrinkled ridges adjacent to lines of dehiscence, separating into 4 nut-like fruits"  (Holm et al., 1997; p. 482).

"Perennial aquatic herb with submerged stems to 2 m long.  Leaves in whorls of four, sessile, feathery; 2-3.5 cm long.  Inflorescence an aerial, terminal spike 3-8 cm long with 4 flowers at each node.  Female flowers in lower whorls with petals to 1 mm long; male flowers in upper whorls, with 4 reddish petals to 2.5 mm long, and 8 stamens.  Fruit ovoid, to 2.5 mm long, with 4 dry, 1-seeded nutlets."  (Waterhouse & Mitchell, 1998; pp 45-46).

Habitat/ecology:  "Freshwater wetlands, lakes, coastal estuaries.  This aquatic is most common in waters of 1-3 m depth, but can invasde waters up to 10 m deep.  It forms dense stands that shade out other species and alter the temperature profile of the water body.  It flourishes in lakes with nutrient rich water, and the spread of the plant is promoted by eutrophication"  (Weber, 2003; p. 278).  "Lakes, canals and reservoirs. Tolerates a wide pH range and brackish water. Can survive stranding, and depths up to 17 m. It chokes lakes and water storages, affecting fish and shellfish production, recreation and boat traffic." (Waterhouse & Mitchell, 1998; pp 45-46).

Propagation:  "Vegetative fragments and seed spread by moving water and water birds" (Waterhouse & Mitchell, 1998; pp 45-46).

Native range:  Eurasia (Waterhouse & Mitchell, 1998; pp 45-46).

Presence:

Pacific
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
State of Hawaii Hawaiian Islands	Maui Island	introduced	Wester, Lyndon (1992) (p. 141)
Philippines Philippine Islands	Philippine Islands		Waterhouse, B. M./Mitchell, A. A. (1998) (p. 46)
Philippines Philippine Islands	Philippine Islands		Merrill, Elmer D. (1923) (p. 221) In shallow water of Lake Lanao, altitude 670 m.

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)	Australia (continental)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized
British Columbia (province of Canada) Province of British Columbia	Canada (British Columbia)		Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 492)
Cambodia Cambodia	Cambodia (Kingdom of)		Waterhouse, B. M./Mitchell, A. A. (1998) (p. 46)
China China	China (People's Republic of)	native	Zhengyi, Wu/Raven, Peter H./Deyuan, Hong (2013) Stagnant waters, lakes, ditches, slow streams, springs; near sea level to 4200 m, rarely to 5200 m in springs in Xizang. Throughout China.
China China	Hong Kong	native	Wu, Te-lin (2001) (p. 158)
Indonesia Indonesia	Indonesia (Republic of)		Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 480)
Japan Japan	Japan	native	U.S. Dept. Agr., Agr. Res. Serv. (2013) Hokkaido, Honshu, Kyushu, Shikoku
South Korea South Korea	South Korea (Republic of)		Holm, Leroy/Pancho, Juan V./Herberger, James P./Plucknett, Donald L. (1979) (p. 243)
Taiwan Taiwan Island	Taiwan Island	native	Zhengyi, Wu/Raven, Peter H./Deyuan, Hong (2013)
Thailand Thailand	Thailand (Kingdom of)		Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 480)
United States (west coast) United States (west coast states)	USA (Oregon)	introduced	U.S. Dept. Agr., Nat. Res. Cons. Serv. (2013)
United States (west coast) United States (west coast states)	USA (California)	introduced	U.S. Dept. Agr., Nat. Res. Cons. Serv. (2013)
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)		Waterhouse, B. M./Mitchell, A. A. (1998) (p. 46)

Also reported from
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Canada Canada	Canada	introduced	U.S. Dept. Agr., Nat. Res. Cons. Serv. (2013)
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)	United States (other states)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized
United States (continental except west coast) United States (other states)	USA (Florida)	introduced	U.S. Dept. Agr., Nat. Res. Cons. Serv. (2013)

Comments:  "A serious weed in South Africa and USA, and a principal weed in India" (Waterhouse & Mitchell, 1998; pp 45-46).

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

Physical:  "Plants can be removed by mechanical harvesters, although this may enhance stem fragmentation".

Chemical: "The plant is highly susceptible to 2,4-D.  Regrowth is fast if plants are not killed, and repeated control is necessary"  (Weber, 2003; p. 278).