Yellowflag iris, native to Europe, is a rhizomatous, emergent, invasive plant found in pond margins, ditches, and other wetland sites in much of the United States. In water depths up to approximately 50 cm, it forms dense stands, which displace native sedges and rushes, reducing waterfowl habitat and water flow. The rhizomes can reach 6 m in lateral spread, making it very difficult to control by mechanical methods. In addition, conventional boom-sprayer applications are often impractical in most aquatic systems. Drizzle application is a technique for directed treatment of hard-to-reach invasive plants. It uses low volumes (26 to 104 L ha⁻¹) of concentrated herbicide solution, applied using a spray gun emitting a thin stream of solution with an effective range of 6 m. In this study, conducted along the margins of two ponds at the University of California, Davis, we compared drizzle applications of glyphosate, imazapyr, and triclopyr to applications using a conventional boom sprayer. Although both glyphosate and imazapyr provided excellent control (> 96%) of yellowflag iris with either treatment technique, only the drizzle treatments of imazapyr at 2.26 and 4.52% ae (10 and 20% product) at spray volumes of 52 and 26 L ha⁻¹, respectively, were below the maximum labeled rate and still gave > 98% control. Furthermore, a cost analysis indicated that the most-economical application for effective control of yellowflag iris was a drizzle application of imazapyr at 4.52% ae (20% product) at 26 L ha⁻¹. This study demonstrates that drizzle application with imazapyr can be a practical application method for yellowflag iris control in aquatic systems in which broadcast treatments with conventional boom sprayers may be difficult.

Nomenclature: Glyphosate; imazapyr; triclopyr; yellowflag iris, Iris pseudacorus L.

Management Implications: Yellowflag iris is an invasive, rhizomatous, perennial plant of wetland and aquatic areas in many U.S. states. In shallow ponds, it can form a perimeter band 6 to 12 m wide. Infestations are difficult to control by mechanical methods, and very little has been published on chemical control options. Most chemical application techniques, particularly conventional hand-held boom sprayers, are difficult to use in yellowflag-infested areas because of the inability to reach plants growing in deeper water and at greater distance from the shore. In this study, we used a drizzle-application technique developed at the University of Hawaii for directed treatment of hard-to-reach, invasive plants along forest trails. We compared this technique to a conventional backpack boom sprayer using three herbicides, glyphosate, imazapyr, and triclopyr. Our results showed that glyphosate and imazapyr were both effective for control of yellowflag iris using either treatment method. However, the drizzle method gave outstanding control (> 98%) with 10 and 20% imazapyr product (Habitat) at spray volumes of 52 and 26 L ha⁻¹, respectively. These were the only treatments below the maximum labeled rate for either herbicide. Moreover, the 20% rate of imazapyr at 26 L ha⁻¹ was the most-economical treatment compared with any other rate, herbicide, or application method, primarily because of reduced labor costs. In addition to its control and cost effectiveness, the drizzle application method also reduces drift potential and provides easier access to difficult-to-reach aquatic sites.