Efficacy and rates of diquat for reducing invasive knotweeds and potential for sequential use with imazapyr

Abstract

Invasive knotweeds (Reynoutria spp.) pose an ongoing challenge for weed management programs worldwide, requiring years of persistent treatment. In this study, we tested efficacy of diquat applied as foliar spray early in the growing season for short-term suppression, and for potential to boost long-term control when applied sequentially with a mid-season treatment of imazapyr. We evaluated responses of knotweeds (R. japonica, R. Â bohemica, and R. sachelinensis) treated with one of three diquat concentrations (0.25%, 0.50%, or 1.0%, v/v), using multiple metrics to capture changes in aboveground plant density and abundance. Canopy cover and biomass were significantly reduced by all diquat concentrations and resembled pre-treatment conditions at 8WAT, reflecting the shift toward reduced stem heights in treated compared with untreated plots. Stem counts at 8WAT, however, did not differ in response to diquat treatments. Sequentially applied early-season diquat and mid-season imazapyr treatments reduced canopy cover (66%-82%), biomass (65%-91%), and stem density (57%-79%) when evaluated 1 year after treatment; reductions were greatest in plots treated with medium and high diquat concentrations, suggesting that diquat treatments may have an additive effect. Overall, our study establishes rates of diquat to reduce biomass of invasive knotweed, and supports additional testing to assess improvement in long-term control when used sequentially with a systemic herbicide.