Description: Annual monoecious herbs.
Leaves alternate, variously lobed and/or toothed. Male heads terminal or in upper leaf axils, falling after flowering; heads ± globose; involucral bracts numerous, 1–3-seriate; receptacle hemispherical with scales; florets numerous, corollas 5-toothed. Female heads axillary below male heads, ovoid, involucral bracts 2-seriate, the inner 2 fused and completely enclosing the florets; florets 2, corollas absent.
Heads forming a hard burr, the tips of the involucral bracts becoming hooked spines, 2-beaked at the summit.
Achenes 2, retained inside the burr, the burr shed as a unit; pappus absent.
Distribution and occurrence: World: c. 30 species, temperate & tropical regions. Australia: 6 species (naturalized), all States.
The four species X. cavanillesii, X. italicum, X. occidentale and X. orientale, are sometimes included in the one variable species, X. strumarium L. s. lat All species of Xanthium are declared noxious weeds in NSW..
Text by L. Murray
Taxon concept:
Taxa not yet included in identification key
Xanthium strumarium
| | Key to the species | |
| 1 | Stems with 3-pronged spines at base of leaves | 2 |
| Stems without spines at base of leaves | 3 |
| 2 | Leaves dark green, upper surface slightly pubescent, lower surface white and felted, 3–8 cm long; widespread in most regions | Xanthium spinosum |
| Leaves grey-green, both surfaces very finely pubescent, 2–4 cm long; SWP only
Back to 1 | Xanthium ambrosioides |
| 3 | Burrs with the 2 apical spines divergent, but straight | 4 |
| Burrs with the 2 apical spines either incurved or incurved at tips
Back to 1 | 5 |
| 4 | Leaves with definite lobes, usually 3–5-lobed; burrs 16–18 mm long | Xanthium occidentale |
| Leaves shallowly and unevenly lobed; burrs 15–30 mm long
Back to 3 | Xanthium cavanillesii |
| 5 | Spines stout and incurved; burrs 15–20 mm long; grows in SWP & SFWP | Xanthium orientale |
| Spines incurved at tips only; burrs 20–30 mm long; grows in NC, CWS & NWP
Back to 3 | Xanthium italicum |
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