Japanese knotweed is an invasive plant that occurs along waterways, highways, abandoned agricultural land, and other disturbed areas. It reduces plant diversity and can increase shoreline erosion. An experiment was conducted in Bible Hill and Antigonish, NS, Canada to evaluate early POST aminopyralid at 120 g ae ha^-1 and POST imazapyr applications at 720 g ae ha^-1 at maximum shoot height, flowering, senescence, maximum height flowering, maximum height senescence, flowering senescence, and maximum height flowering senescence. Early POST aminopyralid only provided 10% to 15% control at 52 wk after treatment (WAT) whereas 83% to 100% control occurred following imazapyr applications at all application timings. Percent control at 2, 4, and 8 WAT tended to be higher where imazapyr followed aminopyralid. By 52 WAT, equivalent damage ratings and reductions in stem density occurred at both sites in all plots where imazapyr was applied. The use of aminopyralid or multiple imazapyr applications provided no additional benefit over a single imazapyr application. We conclude that early POST aminopyralid suppresses knotweed growth, which should facilitate late-season imazapyr applications, especially in large stands.

Nomenclature: Aminopyralid; imazapyr; Japanese knotweed, Polygonum cuspidatum Sieb. & Zucc., syn. Fallopia japonica (Houtt.) Dcne. POLCU.