This cosmopolitan annual probably has a South American origin (FNA 7). It has spread into most southeastern states, especially on the Coastal Plain, and it is locally abundant on roadsides up to the New England coast (K). However, it does not seem to be well-established in the Ohio Valley. There is a recent report from Ky., but details are not yet available (J). The species has been treated in Coronpus, but that genus now appears to be unnatural and didymum is relatively close to L. virginicum (Y, FNA 7, W).