2018 ASHS Annual Conference

Recirculation of irrigation water bears the risk of introducing residual concentrations of chemicals from overspray, runoff, and substrate leaching into water, which can reduce commercial quality of subsequent crops. Paclobutrazol is commonly used in the ornamental industry to regulate plant growth. Label concentrations for paclobutrazol application are lower for drench applications than foliar applications mainly because paclobutrazol moves in the plant through the xylem (bottom-up) and not through the phloem. Low concentrations of paclobutrazol in water applied via sub-irrigation systems can significantly reduce crop size. The objective of this project was to quantify residual concentrations of paclobutrazol in recirculated water in commercial greenhouses and determine potential risks of recirculating irrigation water. We collected water samples from irrigation catchment tanks or ponds in commercial greenhouses dedicated to ornamental plant production and measured paclobutrazol using gas chromatography-mass spectrometry. Paclobutrazol levels were 30% higher in spring compared with autumn. Paclobutrazol levels in recirculated water across operations reached up to 77.2 ppb and 4.3 ppb in spring and autumn, respectively. Only one sampling time and location exceeded these values in spring (841 ppb). Our results suggest that there is a risk of undesired and uncontrolled growth reduction caused by residual concentrations of paclobutrazol in recirculated water in ornamental greenhouses.