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A greenhouse irrigation control system using direct plant water status feedback was developed. The control system used a crop water stress index (CWSI) to turn irrigation on/off and an evapotranspiration (ET) model to determine the water volume to be delivered during irrigation events. CWSI-IRT was based on infrared thermometry (IRT) measurement of plant canopy temperature. The ET model estimated water loss rate for a mildly drought-stressed crop. Experiments were conducted to compare closed-loop proportional irrigation control to open-loop control in the form of timed irrigation. Two experiments using closed-loop proportional control delivered 52% and 43% of the water used for open-loop irrigation (timer), which served as the baseline for each experiment. The closed-loop proportional control system had less than 2 L m-2 per event error between amount of crop water use by 25 plants and the amount of water delivered. The improved performance of the closed-loop proportional control was due to including information about the dynamic response of the plant system to changes in water supply and more accurate ET estimates. The proportional control system produced plants whose height, fresh mass, and dry mass were not significantly different (95% confidence interval) from a control crop (open-loop irrigation), although photographs suggested there was some reduction of plant quality.