The effects of whitening and of dust accumulation on the optical properties of materials in use as greenhouse covers in Zimbabwe were investigated. The effect of whitening was investigated in a greenhouse by measuring the transmittances for total solar, photosynthetically active radiation (PAR) and thermal radiation before and after painting the cover with lime-based whitewash paint, as used for shading in Zimbabwe. In addition, to evaluate the effect of dust, the transmittances for total solar, PAR and thermal radiation of a plastic sample were measured when dirty, due to exposure to the environment for 6 months, and again after cleaning. Whitening reduced the transmission coefficients for PAR, total solar and thermal radiation of the greenhouse cover from 0.75 to 0.53; 0.74 to 0.55 and 0.45 to 0.43, respectively. Dust and dirt accumulation due to 6 months exposure to the environment reduced the transmittances for all wavebands of the plastic sample (relative to their initial values) by 16%, 15% and 13%, respectively. To investigate the effect of these changes in the optical properties on greenhouse microclimate and canopy behaviour, the measured changes in transmittances were used in the Gembloux Dynamic Greenhouse Climate Model (GDGCM). The model showed that whitening of the greenhouse roof led to significant reductions in the maximum air temperature, vapour pressure deficit, canopy-to-air temperature difference and transpiration rate, while the canopy stomatal resistance was not significantly affected. These reductions, together with a decrease in the crop water stress index, indicate that the crop was less stressed after whitening. The model showed similar, but smaller, changes to all the microclimate and canopy parameters due to the accumulation of dust on the greenhouse roof. (C) 2009 Elsevier Ltd. All rights reserved.