Irrigation scheduling for container grown Spiraea japonica based on cumulative vapor pressure deficit

The increasing scarcity of water globally is a prominent issue facing the nursery industry, one that must be dealt with if the industry is to remain viable as climate change continues to escalate. Current irrigation practices involve nurseries watering plants as they see fit, usually based off agronomic indicators (e.g., plant height, visual signs of wilting). These are not suitable in providing timely data for interpreting the need to irrigate and leads to very subjective watering schedules that lean to the side of over-watering. Past work in this lab has quantified the relationship between cumulative plant water stress (cWS) and cumulative vapor pressure deficit (cVPD), identifying species-specific water stress thresholds in relation to cVPD. The objective of this study was to use irrigation schedules based on cVPD correlated to the predicted cWS of the popular containerized ornamental plant Spiraea japonica 'Goldflame'. The aim was to generate irrigation schedules which decrease water use while not impacting market quality. A split-plot field trial with irrigation as the main-plot factor and mycorrhizae as the split-plot factor was employed. Three irrigation treatment/schedules were applied via overhead irrigation: 1) conventional practice (nursery control), 2) moderate water stress, and 3) high water stress. Three mycorrhizae treatments were examined in combination; 1) inoculation at time of potting, 2) inoculation at time of potting and transplant, and 3) control (no inoculation). The growth and quality of the plants were recorded throughout a growing season. The results indicate that water use can be cut by up to 64% compared to conventional practices while maintaining acceptable (sellable) plant quality. This demonstrates that irrigation schedules based off of cVPD significantly reduce water use without impacting the growth and quality of Spiraea japonica 'Goldflame'.