Evapotranspiration of container ornamental shrubs: modelling crop-specific factors for a diverse range of crops

Crop coefficients, which define the relationship between estimated evapotranspiration from a standard reference crop and actual evapotranspiration of a specified crop, are generally not known for hardy ornamental nursery stock (HONS) crops. This severely limits the application of a sensor, known as the ‘Evaposensor’, which generates an electrical signal proportional to potential evapotranspiration, for irrigation scheduling of HONS. We therefore attempted to develop a method for the rapid estimation on the nursery of appropriate crop-specific calibration factors for use with the Evaposensor. Calibration factors were calculated for a range of HONS crops through the growing season, in a series of experiments under plastic tunnels or outdoors. Across all crops and over a growing season, 76% of the variation in calibration factors under a plastic tunnel could be explained by a combination of plant height, leaf area and stomatal conductance. For the same crops grown outdoors, 77% of the variation could be explained by these variables along with percentage cover. Plant height and percentage cover, which can both be easily and rapidly measured on commercial nurseries, alone explained 64 and 57% of the variation under plastic and outdoors, respectively.