Weed control of paraquat can be erratic and may be attributable to differing species sensitivity and/or environmental factors, of which, minor guidance is available in commercial labels. Therefore, the objectives of this research were to quantify selectivity of paraquat across select weed species and the influence of environmental factors. Experiments were performed under controlled conditions in the greenhouse and growth chamber. Compared to purple deadnettle (GR50 = 39 g ai ha-1), waterhemp, Palmer amaranth, giant ragweed, and horseweed were 4.9, 3.3, 1.9, and 1.3 times more sensitive to paraquat, respectively. The injury progression rate over 3 d after treatment (DAT) was a more accurate predictor of final efficacy at 14 DAT than the lag phase until symptoms first appeared. For example, at the 17.5 g ha−1 dose, the injury rate of waterhemp and Palmer amaranth was, on average, 3.6 times greater than horseweed and purple deadnettle. The influence of various environmental factors on paraquat efficacy was weed-specific. Applications made at sunrise improved control of purple deadnettle over applications at solar noon or sunset. Lower light intensities (200 or 600 μmol m-2 s-1) surrounding the time of application improved control of waterhemp and horseweed over 1,000 μmol m-2 s-1. Day/night temperatures of 27/16 C improved horseweed and purple deadnettle control over 18/13 C. Though control was positively associated with injury rates in the application time of day and temperature experiments, a negative relationship was observed for waterhemp in the light intensity experiment. Thus, while there are conditions that enhance paraquat efficacy, the specific target species must also be considered. These results advocate paraquat dose recommendations, currently based on weed height, be expanded to address sensitivity differences among weeds. Moreover, these findings contrast with paraquat labels stating temperatures of 13 C or less do not reduce paraquat efficacy.