Bermuda grass (Cynodon dactylon) is a stoloniferous and rhizomatous prostrate weed displaying high growth rates under non-limiting conditions. Radiation has been considered the major factor affecting C. dactylon patch extension and biomass growth, but little is known about the influence of water availability on those processes. C. dactylon green area and biomass growth were analysed in response to different soil water conditions. Four experiments comprising eight drought treatments were carried out during 2 years at different dates, to generate a wide range of soil water and evaporative demand conditions. Significant reductions in weed green area and biomass growth were found between treatments when low soil water conditions were imposed. Reductions in green area were of greater magnitude than reductions in biomass, suggesting a higher effect of drought on green area increment. There were significant (P < 0.05) relationships between green area increment and cumulative incident radiation, although slopes [relative green area growth rates (RGAGR)] differed among treatments. A strong logistic relationship was found between RGAGR and the ratio of plant-available soil water to potential evapotranspiration. The approach and parameters generated in this work can be used to predict C. dactylon growth changes under rain-fed conditions.