This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Annual branching herbs, with opposite, mostly petioled, dentate or entire leaves, and small peduncled heads of both tubular and radiate flowers, terminal and in the upper axils. Involucre hemispheric or broadly campanulate, its bracts in 2 series, ovate, obtuse, membranous, striate, nearly equal, or the outer shorter. Receptacle conic or elongated, its thin chaff subtending the disk-flowers. Ray-flowers white or red, pistillate, fertile, the rays 4 or 5, short. Disk-flowers yellow, perfect, the corolla 5-toothed. Anthers minutely sagittate at the base. Style-branches tipped with acute appendages. Achenes angled, or the outer ones flat. Pappus of the disk-flowers of several short laciniate or fimbriate scales, that of the ray-flowers of several or few short slender bristles, or none. [Named in honor of M. M. Galinsoga, superintendent of the Botanic Gardens at Madrid.]
About 5 species, natives of tropical and warm temperate America, the following typical.
Galinsoga parviflora Cav. Icon. 3: 41. pl. 281. 1794. Galinsoga parviflora hispida DC. Prodr. 5: 677. 1836.
Slightly appressed-pubescent or hirsute, 1°-3° high. Leaves thin, ovate or deltoid-ovate, 3-nerved, 1' - 3' long, acute at the apex, mostly obtuse at the base, dentate, the lower slender-petioled, the upper short-petioled or sessile, and sometimes nearly or quite entire; heads usually numerous, 2"- 3" broad, slender-peduncled; bracts of the involucre glabrous or nearly so, the outer shorter; pappus of the disk-flowers 4-16, oblong to spatulate, fimbriate obtusish or bristle-tipped scales, somewhat shorter than or equalling the finely pubescent obpyramidal achene.
In door-yards and waste places, Maine to Ontario, Oregon, North Carolina, Missouri, Arizona, California and Mexico. Bermuda; Jamaica. Naturalized from tropical America. Introduced into Europe as a weed. June-Nov.
Galinsoga caracasana (DC.) Sch. Bip., similar to this species but with reddish rays, and the pappus of the disk-flowers only half as long as the achene, also tropical America, has been found in waste grounds in New Jersey and Maryland.