Determining plant-available water of woody ornamentals in containers in situ during production

Abstract

In nursery production, small root balls are transplanted into larger containers and grown for sale or further transplanting into still larger containers. When a root ball is smaller than a container, the amount of plant-available water (PAW) is initially limited to that of the original root ball. With growth, roots colonize new substrate and thereby increase the volume of water available to a shoot. Because of hydraulic gradients in container substrates, PAW is not linearly proportional to the volume of substrate occupied by roots. To practice precision irrigation in nursery production, it is important to know the extent of PAW and how it changes with growth. A method is detailed that calculates in situ PAW in containers based on changes in actual evapotranspiration while irrigation is withheld. The method is applied under field conditions and requires only daily mass loss measurements and corresponding reference evapotranspiration. An example of how PAW changes during production from rooted cuttings to marketable size plants is provided.