A dynamic misting control strategy for poinsettia cutting propagation was developed using a set point for canopy surface resistance to water loss. Evapotranspiration and canopy surface resistance from poinsettia cuttings was estimated using a modified Penman-Monteith equation. The model was developed from parameters obtained in carefully controlled growth chamber experiments and validated in greenhouse studies using computer controls. The system requires continual computer acquisition and control with inputs for air temperature, relative humidity and light irradiance. Mist events were activated when predicted evaporation reduced stored water in the canopy to below a threshold quantity. Compared to static mist control (5 sec every 10 min) there was a 38% reduction in misting using the evapotranspiration determined system. Method of control did not effect the number of roots per cutting, but there was an increase in overall root length and production of finer (less diameter) roots in cuttings misted based on the evapotranspiration model.