Native to tropical America, Billy Goat Weed is an annual or short-lived perennial herb growing to 1m. It is a widespread weed of disturbed areas. A common weed but not a particularly damaging one.
Stems and leaves are softly hairy.
Mostly opposite but sometimes (upper) alternate leaves. Bluntly toothed (crenate) almost triangular to egg-shaped (ovate), with either blunt or pointed tips.
Flowers are light lavender blue florets to 3 mm grouped into fluffy flower-heads to 7 mm across. It does not have petals (ray florets) as sucht. Flowers thoughout the year.
Bracts at the base of the flower-head (involucral) obviously hairy. Flowering occurs throughout most of the year.
Image: Flower closeup
Often confused with another much less common, but very similar species Ageratum conyzoides.
Ageratum houstonianum has a pappus that is shorter than the floret and glandular hairs on its involucral bracts.
Ageratum conyzoides has a pappus which is longer than the floret and its involucral bracts are only sparsely hairy and never have glandular hairs.