Present on Pacific Islands?  yes

Primarily a threat at high elevations?  no

Common name(s): [more details]

Hawaiian: oī, ōwī

Habit:  herb

Description:  "Annual or perennial herbs up to 20 dm tall, becoming somewhat woody toward base; stems dichotomously branched, younger stems moderately to densely short-pubescent.  Leaves opposite, membranous or chartaceous, elliptic to ovate or narrowly ovate, 1.5-10 cm long, 1-5 cm wide, upper surface puberulent to strigillose or scaberulous, lower surface sparsely to moderately strigillose over entire surface, margins serrate to crenate-serrate, the teeth angled forward, apex sharply acute to acuminate, petioles 0.3-1 cm long.  Spikes slender, 15-45 cm long, up to ca. 3 mm in diameter, somewhat flaccid, usually short-pubescent throughout, the furrows nearly as wide as the rachis, bracts narrowly lanceolate, ca. 5 mm long; calyx ca. 6 mm long, the teeth subequal; corolla pale blue to blue, pale violet, or lavender, occasionally white"  (Wagner et al., 1999; pp. 1321-1322).

"This species is reported to hybridize with Stachytarpheta jamaicensis" (Wagner et al., 1999; pp. 1321-1322).

Keys to the species (Smith, 2002; pp. 42-43):

S. australis:  Lower leaf surfaces, rachis and calyx hairy; flowers pale blue or almost white.
S. cayennensis:  Lower leaf surfaces, rachis and calyx hairless; rachis slender 1-3 mm diameter; leaves membranous; flowers blue to almost white.
S. jamaicensis:  Lower leaf surface, rachis and calyx hairless; rachis stout, 3-5 mm diameter; leaves fleshy; flowers mauve, blue, violet or purple.

Habitat/ecology:  "Snakeweeds invade disturbed areas, e.g. creek lines and roadsides; also monsoon vine forests where soil has been disturbed by pigs and buffalo. Can invade overgrazed pastures." (Smith, 2002; pp. 42-43). In Hawai‘i, "naturalized in usually mesic, disturbed sites, 170-1,340 m" (Wagner et al., 1999; pp. 1321-1322).

Propagation:  "Usually spread intentionally as ornamental plants.  Seeds are also dispersed on vehicles, by the movement of soil in refuse from gardens and by rainwater.  In Queensland [Australia] they have been spread in fodder, hay and in contaminated pasture seed."  (Smith, 2002; pp. 42-43).

Native range:  "Apparently native from Cuba and Mexico south to Peru and Argentina, now widely naturalized in tropical and subtropical areas" (Wagner et al., 1999; pp. 1321-1322).

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	Hawai‘i (Big) Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (pp. 1322-1323, 1896)
State of Hawaii Hawaiian Islands	Kaua‘i Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (pp. 1322-1323, 1896)
State of Hawaii Hawaiian Islands	Lāna‘i Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (pp. 1322-1323, 1896)
State of Hawaii Hawaiian Islands	Maui Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (pp. 1322-1323, 1896)
State of Hawaii Hawaiian Islands	O‘ahu Island	introduced invasive	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (pp. 1322-1323, 1896)

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)	Northern Territory	introduced invasive cultivated	Smith, Nicholas M. (2002) (pp. 42-43)
Australia Australia (continental)	Queensland	introduced invasive cultivated	Smith, Nicholas M. (2002) (pp. 42-43)
Indonesia Indonesia	Indonesia (Republic of)		Holm, Leroy/Pancho, Juan V./Herberger, James P./Plucknett, Donald L. (1979) (p. 351)

Comments:  A declared noxious weed in Western Australia (Smith, 2002; pp. 42-43).

Additional information:
Additional online information about Stachytarpheta australis is available from the Hawaiian Ecosystems at Risk project (HEAR).

Information about Stachytarpheta australis as a weed (worldwide references) may be available from the Global Compendium of Weeds (GCW).

Taxonomic information about Stachytarpheta australis may be available from the Germplasm Resources Information Network (GRIN).

References:

Holm, Leroy/Pancho, Juan V./Herberger, James P./Plucknett, Donald L. 1979. A geographical atlas of world weeds. John Wiley & Sons, New York. 391 pp.

Smith, Nicholas M. 2002. Weeds of the wet/dry tropics of Australia - a field guide. Environment Centre NT, Inc. 112 pp.

Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. 1999. Manual of the flowering plants of Hawaii. Revised edition. Bernice P. Bishop Museum special publication. University of Hawai‘i Press/Bishop Museum Press, Honolulu. 1919 pp. (two volumes).