Leaves are opposite at the base of the plant, becoming alternate further up the stem. Leaf blades are deeply divided. The uppermost leaves usually smaller. All leaves are covered in hairs.
Male flower heads (more numerous) droop from branching spike-like inflorescences borne at the tips of the stems. Female flower-heads (fewer) consist of a single floret.
Very similar to parthenium weed Parthenium hysterophorus when in the vegetative stage of growth. However, parthenium weed has ribbed stems and masses of small white, bisexual flower-heads.
SOURCE: Suburban and Environmental Weeds, Sheldon Navie et al.
Weeds Australia notes: “Major cause of allergy with pollen causing flu-like symptoms; contact with the plant can also cause skin allergies. The stem-galling moth Epiblema strenuana, introduced for control of Parthenium Weed Parthenium hysterophorus, reduces Annual Ragweed populations in warmer areas.” – www.weeds.org.au
Photo: Robert Whyte