Purple nutsedge is a troublesome C4 weed, characterized by highphotosynthetic efficiency, compared to C3 weeds. As its dispersalis based on vegetative growth, accurate prediction of its growth could helpin arriving at favorable management decisions. This article details thedevelopment and validation of predictive models of purple nutsedge spatialgrowth, based on temperature (thermal model), and temperature and radiation(photothermal model) measurements. Plants were grown in six experiments inthe summers of 2008, 2009, and 2010, under different temperature andradiation conditions. Results indicate that under optimal temperatures,radiation becomes the main growth-limiting factor, and is highly related tothe final leaf-cover area (R2 = 0.89). Comparison of the thermal and photothermal modelsshowed that under all conditions, including varied temperature andradiation, the photothermal model performs significantly better, withdifferences in root-mean-square error values reaching up to 0.073, comparedto 0.195 with the thermal model. Validation experiments confirmed theability of the photothermal model to predict purple nutsedge spatial growthaccurately.