Any of several American plants containing a bitter substance: as either of two ragweeds: (1); a low-growing common ragweed (Ambrosia artemissiifolia) (2); horseweed; an annual yellow-flowered sneezeweed (Helenium amarum syn. h. tenuifolium); an erect annual composite herb (Hymenoxys odorata) of the southwestern United States that has yellow flowers and is poisonous to livestock