As a broadly defined species, tenellum has a widespread tropical and temperate American range (FNA 22, W). More northern plants in the U.S.A. have been segregated as parvulum, but variety status may be more reasonable (Y). It is an annual that typically occurs along seasonal drawdown zones in various types of marginal or periodic wetlands. In North America, records of parvulum are widely scattered over southeastern states, and it appears to be endangered or locally extinct in at least half of these states; it is more frequent only along the Gulf Coastal Plain (K, NS). In Ky. it is known only from ephemeral sinkhole ponds of CHRI (APSC) and LOGA (KSNPC), in association with Schoenoplectus hallii at both localities (Chester & Souza 1984; B. Palmer-Ball, pers. comm.).