Hygrophila polysperma T.Anders., Acanthaceae

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Present on Pacific Islands?  no

Primarily a threat at high elevations?  no

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

Common name(s): [more details]

Chinese: xiao shi zi cao

English: East Indian hygrophila, East Indian swampweed, Indian hygrophila, Indian swampweed, Miramar weed, green hygro, hygro

Habit:  aquatic herb

Description:  "A small, much branched, unarmed procumbent herb with up to 30 cm long branches rooting at the nodes and pubescent near apex. Leaves opposite, subsessile, oblong-ovate, 1-3.5 cm x 4-10 mm, mostly glabrous or puberulous, margins entire, ± obtuse at the apex. Flowers white to pale-blue or violet, c. 8 mm long, in axillary and terminal, up to 8 cm long spikes; bracts broadly elliptic, subacute; bracteoles linear, 4-5 mm long, pubescent. Calyx 5-lobed to the middle, lobes linear, c. 2 mm long, hairy, scarious-ciliate on margins, acute. Corolla pubescent, tube c. 3 mm long, limb 2-lipped, upper lip 2-lobed, patent, lower lip shortly 3-lobed. Stamens 2 fertile and 2 staminodial, anthers oblong, ± divaricate basally. Ovary oblong, many-ovuled, hairy at tip; style filiform, c. 5 mm long. Capsule narrowly oblong, glabrous except the tip, more than 15-seeded. Seeds minute, ± orbicular"  (Flora of Pakistan online).

Habitat/ecology:  "Freshwater lakes and ponds, riparian habitats.  The plant forms dense and pure stands in waters up to 3 m depth or more, impeding water flow and displacing native aquatic species.  It is adapted to low light conditions and expands rapidly.  The plant tends to grow more vigorously in flowing than in standing waters"  (Weber, 2003; p. 2005).  "A freshwater amphibious herb that was most likely introduced through the aquarium trade. It is found in warmer climates and prefers flowing streams, but may also be found in slow-moving waters and in lakes. It is a fast-growing and fast-spreading species that out-shades and out-competes other submersed plants"  (Global Invasive Species Database).

Propagation:  Seed and pieces of stems.  "Stems are brittle, and easily fragment. These fragments easily develop new stands from rooted nodes of even small fragments"  (Global Invasive Species Database).

Native range:  South-central and southeastern Asia; also cultivated; naturalized in the United States and Mexico (GRIN).

Presence:

Pacific Rim
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
China China	China (People's Republic of)	native	Zhengyi, Wu/Raven, Peter H./Deyuan, Hong (2013) Streamsides; below 600 m. Guangdong, Guangxi, Yunnan.
Mexico Mexico	Mexico (United Mexican States)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized
Taiwan Taiwan Island	Taiwan Island	native	Zhengyi, Wu/Raven, Peter H./Deyuan, Hong (2013) Streamsides; below 600 m.
Vietnam Vietnam	Vietnam (Socialist Republic of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2013)

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 invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Texas Naturalized
United States (continental except west coast) United States (other states)	USA (Florida)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2013) Naturalized

Comments:  A declared noxious weed by USDA-APHIS and by a number of states in the United States (GRIN).

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

Control:  "Hygrophila polysperma is difficult to control. Mechanical harvesting only fragments the plants and increases its distribution. Typical biological control agents for aquatic invasives do not affect this species and most registered aquatic herbicides only provide marginal control"  (Global Invasive Species Database).