Pigweeds are members of the genus Amaranthus, of the family Amaranthaceae. The family comprises approximately 170 genera and 2,360 species, most being weedy herbs, although some are low or climbing shrubs. The family is cosmopolitan, with most continents claiming native members. Plants are characteristic of disturbed, arid, or saline habitats. Redroot, or rough, pigweed, A. retroflexus, a native of tropical America, is found throughout the North American continent, across Canada from British Columbia to Nova Scotia. With the exception of the northernmost countries, it has become widespread throughout Europe.

Redroot pigweed is a coarse upright annual, growing 3 to 5 feet in height. Lower stems are pink, red, or red-striped, the color extending down into the taproot, giving the plant its name. Leaves are dull dark green, have long petioles, and are broadly lance-shaped, with prominent veins. Flowers are small, green, and tightly bunched in spike-like terminal clusters that may branch. Smaller spikes may arise from leave axils. Pollination of amaranths usually occurs from July through September throughout most of the United States and Europe, although the season is longer in the southeastern states and year-round in southern Florida. Redroot pigweed is a lesser pollen producer compared with Palmer's amaranth or western waterhemp (A. palmeri and A. tamariscinus, respectively), and therefore Wodehouse felt it was less important in hay fever. However, it is ubiquitous, and several authors believe it is a locally important offender. Because of the similar appearance of amaranth and chenopod pollen grains and overlapping of weed distribution, it is often difficult to segregate the relative contributions in pollinosis.

Adapted from: Annals of Allergy, Richard Weber, August 2002.