Predictions of long-distance pollen dispersal require a priori knowledge regarding the size and shape of pollen grains. Our objectives were to (1) determine the mean size and settling velocity (Vs) of mature Amaranthus palmeri (AMAPA) pollen grains, and (2) estimate the theoretical maximum flight distance for pollen grains. Pollen diameters of four AMAPA populations (FL, GA, NC, TN) ranged from 20-38um. Assuming a mean diameter of 28.4um, we determined that the theoretical Vs of AMAPA pollen grains to be 2.9-3.9cm/s. Results from laboratory studies suggest that the mean Vs is 5.0cm/s. The theoretical value for Vs was established using an equation that assumes particles maintain a constant shape, size and density. In reality, AMAPA pollen grains lose water and become more dense over time. Assuming that pollen flight is unimpeded, AMAPA pollen carried by convective mixing to a height of 3500m could travel between 300-500km with a wind moving 4.5m/s.