Print Fact SheetParthenium hysterophorus

Taxonomy

Parthenium hysterophorus L., Sp. Pl. 2: 988. 1753.

Common name

Parthenium weed, barley flower, bastard feverfew, broomweed, congress grass/weed, dog flea weed, whiteheads, wormwood

Description

Propagule or dispersal unit is the fruit with attached bracts and florets. Fertile part 1.5-2.5 mm long, 0.8-2(-2.5) mm wide, in side view widest in upper part (obovoid), +/- straight, the upper (apical) end narrowing, in cross-section flattened or angular (prismatic), basal scar (carpopodium) inconspicuous and undifferentiated, central or off to the side (oblique) or at least asymmetric, beak (=thinner sterile stalk between seed and pappus) absent, wings absent or wings present (by misinterpretation), 0-0.5 mm wide, propagule surface light brown or straw (but fruit surface black), smooth (except at cellular level) or with round protrusions or bumps, with papillae (small protuberances), thickened margin present, thick margin hairs no hairs (glabrous), longitudinal ribs absent or present, 1(-3), their surfaces smooth, with papillae (small protuberances).

Pappus type scales, pappus elements all +/- similar, up to 0.5-1 mm long, in one row, number of pappus elements two, persistent, the individual scales triangular to acute.

Notes: The unsual propagule of this species consists of the black fruit itself, two pappus scales, the dried corolla of the female flower, and two male flowers and their subtending receptacle bracts attached laterally. Note that e.g. the surface traits used in this key refer to the fruit itself, not to the covering female flower, although it has been scored to accommodate misinterpretation of the surface colour. The fruit has a keel on the inner side which was here scored so that a user calling it a longitudinal rib does not exclude the species. Likewise, calling the fruit winged (because of the female flower) does not exclude the species.

Ecology

Annual herb, fruits dispersed passively via wind or sticking to animals and machinery, with seeds noted to be long-lived. Tropical to subtropical, humid areas, but able to colonise any soil type and a range of habitats. Rail- and roadsides, riverbanks, agricultural areas, plantations, orchards and managed forests, urban areas. Outcompetes more palatable fodder plants on pasture.

Native range

Unclear but assumed to be somewhere between the southern United States and central South America.

Introduced range

United States, Mexico, Bermuda, Central America, Carribean, eastern and southern Africa including Madagascar, Israel, Yemen, Oman, Pakistan, India, Sri Lanka, Buthan, Nepal, Bangladesh, China, Taiwan, Japan, Vietnam, Malaysia, Australia, Pacific Islands.

Particularly weedy in Australia, India and southern Africa, range still increasing.

Weed status within Australia

Weed of National Significance (WONS); Qld Restricted Invasive Plant Cat. 3; NT Class A (to be eradicated); NSW Class 1 State Prohibited Weed; Vic State Prohibited Weed; SA Category 1 Declared Weed; WA Declared Pest s12 (C1 Prohibited); Tas Declared.

Past interceptions (Australia): origins

India.

Past interceptions (Australia): commodities

Agricultural machinery.