Pittsburgh Pest (Galinsoga parviflora)
Obviously this is not known as Pittsburgh Pest everywhere in its nearly global range, but the name seems to be well established here. It has many other names in English, including the delightful folk etymology “Gallant Soldiers.” It comes originally from South America, where it is a popular ingredient in Colombian cuisine.
The flowers are like tiny five-rayed daisies; the plant is low and hairy,and can grow from any crack in the pavement. This one grew at the edge of a sidewalk in Greenfield, where it was blooming in the middle of July. It is very much an urban weed, ubiquitous in the city of Pittsburgh, but much rarer in the near suburbs, and unknown in all the other counties of the metropolitan area but one (Washington County).
Gray describes the genus and the species:
GALINSÒGA R. & P. Heads several-flowered, radiate; rays 4-6, small, roundish, pistillate. Involucre of 4-5 ovate thin bracts. Receptacle conical, with narrow chaff. Pappus of small oblong cut-fringed chaffy scales, sometimes wanting. — Annual herbs, with opposite triple-nerved thin leaves, and small heads; disk yellow; rays white or reddish. (Named for Dr. Mariano Martinez de Galinsoga, a Spanish botanist.)
Rays white; pappus of disk-flowers about equaling the achenes.
G. parviflòra Cav. Pubescence subappressed; leaves ovate, crenate-serrate, petioled; pappus of the disk-flowers of spatulate obtusish scales.— Roadsides and waste places, from N. E. across the continent. (Adv. from Trop. Am.) Var. Híspida PC. Pubescence more copious, not appressed; pappus-scales of the disk-flowers attenuate and bristle-tipped. — Me. to Ont., Wise., and southw. (Nat. from Trop. Am.)