Submitted to: Proceedings of Northeastern Weed Science Society
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2008
Publication Date: 1/1/2009
Citation: Altland, J.E. 2009. Irrigation affects herbicide penetration of shrub canopies. Proceedings of Northeastern Weed Science Society.63:60
Technical Abstract: Nursery growers can apply herbicides in either granular or sprayed formulations. Despite greater risk for injury, sprayed preemergence herbicides are often less expensive than an equivalent granular formulation, easier to apply, and result in more uniform application. When herbicides are sprayed over crops, overhead irrigation is used to simultaneously wash herbicides from the foliage and incorporate the herbicide into the substrate surface. Most labels stipulate 1.25 cm of irrigation following herbicide application. Irrigation is thus an intrinsic component of herbicide applications in ornamental nursery crops, and could greatly affect how preemergence herbicides move from shrub foliage into the substrate below. It has been suggested that foliage of crops to be sprayed with herbicide should first be wetted with irrigation, then sprayed while foliage remains wet, and then irrigated soon thereafter. The logic is that herbicides will be less likely to dry on or absorb into foliage if the herbicide remains in solution after spraying. The objective of this research was to determine if irrigation before or after sprayed herbicide application affects foliar retention of the herbicide. On Sept. 24, 2008, hydrangea (Hydrangea paniculata Sieb. 'Quick Fire') growing in 10.2 L containers and approximately 31 cm tall and 36 cm wide were sprayed with pendimethalin (Pendulum 3.3 EC). Applications were made with a CO2 sprayer equipped with a three nozzle boom and calibrated to deliver 40 gal/A. Pendimethalin was applied at a rate of 4.8 qt/A. Pendimethalin was applied under each of the following three scenarios: 1) pendimethalin was applied to dry foliage, and foliage was allowed to dry after application for 30 min prior to irrigation, 2) pendimethalin was applied to dry foliage and irrigated immediately afterward, and 3) pendimethalin was applied to wet foliage and irrigated immediately afterward. In all cases, irrigation following application was run for 30 min which resulted in approximately 1.25 cm water. Leaf samples were collected following application. Samples were stored in separate glass jars that were placed in an iced cooler for transport back to the laboratory. Leaf samples were rinsed with methylene chloride and analyzed with gas chromatography-mass spectrometry to determine pendimethalin coverage on the leaf surface. There were three replications per treatment. Allowing the herbicide to dry on foliage for 30 min prior to irrigation resulted in 1.77 µg/cm2 recoverable pendimethalin on plant foliage. In contrast, irrigating immediately after application resulted in 0.53 or 0.48 µg/cm2 when applied to foliage that was either dry or wet at the time of application, respectively, but irrigated immediately afterwards. These data suggest that timing of irrigation after is critical for removing herbicide residues from plant foliage. However, this single experiment did not demonstrate significantly different levels of herbicide residue after application to either wet or dry foliage immediately followed by irrigation.