- 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.