Weed Habitat

  • Native to Central and South America, Siam weed is a rapid invader of wet/dry tropical areas. It threatens native vegetation, pastures and many crops, and is toxic to stock.
  • Shade intolerant, produces chemicals that inhibit the growth of other plants.


  • An erect or sprawling, fast growing shrub.
  • Yellowish slender stems, sparsely hairy, much branched, lateral branches in pairs, forms a dense tangled bush and can scramble up to 20 m over trees.
  • Leavers are soft green, slightly hairy on both sides, roughly triangular and toothed on edges, 3 distinctive pitchfork veins, pungent smell when crushed.
  • Flowers are pale lilac, maturing to pink, tubular with protruding 2-branched stigmas, in terminal clusters.
  • Masses of small brown seeds, each with a tuft of hairs white at first and brown when dry.

Siam weed seed - Chromolaena odorata

Siam weed plant - Chromolaena odorata

Siam weed - Chromolaena odorata


This species is declared Restricted Matter under the Biosecurity Act 2014 for further information on declaration refer to your Local Government or the Department of Agriculture and Fisheries.