The main aim of the study was to model heat and mass transfer in a greenhouse in order to simulate the microclimate in a greenhouse. A dynamic greenhouse climate model, the Gembloux Dynamic Greenhouse Climate Model (GDGCM) was adapted, calibrated and validated to predict the microclimate in a naturally ventilated, plastic greenhouse typical in Zimbabwe with a rose crop. The main sub-models modified were the ventilation and vegetation sub-models. The modified GDGCM was then used to (i) evaluate the effects of cover properties, different climate management strategies and ventilation on the inside greenhouse climate and crop transpiration; (ii) design an optimal greenhouse based on the selection of the greenhouse covering material properties and construction details (covering material, floor area, volume, vent configurations and dimensions, etc); and (iii) estimate leaf wetness duration for use in disease control. This information can be used by growers to help them decide on appropriate greenhouse designs, covering materials, climate management strategies and the precautions to take to prevent possible diseases epidemics.