4 Apr 2025

Field Evaluations of Diquat for Controlling Submersed Flowering Rush in Lake Pend Oreille, Idaho

Abstract: The invasive plant flowering rush (Butomus umbellatus L.) was first observed in Lake Pend Oreille, Idaho, within the Columbia River basin in 2008, and it covered several hundred hectares by 2016. Dense infestations of this perennial weed destroy native plant communities and seriously degrade fish and wildlife habitat. Sometimes growing as an emergent on saturated soils or in shallow water, the plant also forms persistent submersed stands in deeper water, which is the typical growth form in Lake Pend Oreille. In 2016, field trials evaluated the herbicide diquat dibromide, formulated as the product Reward, for controlling submersed flowering rush. A 4 ha plot in Oden Bay was treated with diquat at a rate of 18.7 L/ha in late summer 2016 and again in 2017 using a subsurface injection method by boat. Water exchange processes were measured in treated plots in 2017 with rhodamine WT dye mixed with the herbicide. Flowering rush shoots were reduced by 87% in 2016 and 29% in 2017. No adverse effects were measured on water quality (temperature, dissolved oxygen, pH, and conductivity). Diquat should be added to the project’s operational management strategy for controlling flowering rush in Lake Pend Oreille.