This annual is widespread across southeastern states and Central America; it is most common on calcareous or other base-rich soils. Original habitats are not clear, but it has become locally abundant in crop fields, gardens, roadsides and similar sites with regularly disturbed soil. It may have increased much since settlement; the first records from Ky. were in the 1880s (Linney 1882; Pr). It has become resistant to some common herbicides, and it may have been somewhat resistant even before the use of these chemicals (Horak et al. 1998).