Meadow knapweed is a fertile European hybrid between black and brown knapweed that has expanded its North American distribution to 27 of the United States and four provinces in Canada. Two experiments were conducted on meadow knapweed in northwestern Washington to determine (1) whether meadow knapweed is similarly sensitive to several herbicides commonly used to control related knapweeds, (2) if herbicide application timing plays a role in control of meadow knapweed, and (3) whether mowing before, after, or instead of herbicide treatment can aid in meadow knapweed control. In the first experiment, herbicides were applied to meadow knapweed at rosette or bolting stages of growth, and again with the same herbicides in the autumn. In the summer following treatment, clopyralid alone or with 2,4-D, dicamba 2,4-D, and triclopyr ester 2,4-D ester provided 81 to 100% meadow knapweed control; the only other treatment providing similar control was glyphosate ammonium sulfate applied at bolting and in autumn in the 2004 to 2005 trial. In the second experiment, combinations of mowing and dicamba 2,4-D were applied at rosette or early flowering stages of growth. In the 2002 to 2003 trial, control when dicamba 2,4-D was used exceeded 90%, except when meadow knapweed was mowed at rosette and sprayed at early flowering (78% control). Mowing twice the previous year had only a slight effect on meadow knapweed (10% control). Grass biomass exceeded meadow knapweed biomass in all herbicide-treated plots. In the 2004 to 2005 trial, meadow knapweed control and grass biomass was maximized when plots were mowed at rosette and treated with dicamba 2,4-D at early flowering or when treated twice with these herbicides; these were the only treatments where grass biomass exceeded meadow knapweed biomass.
Nomenclature: Clopyralid, dicamba, glyphosate, imazapic, MCPP, triclopyr, 2,4-D, meadow knapweed, Centaurea debeauxii Gren. & Godr. CEDE5.
Management Implications: Meadow knapweed is a nonrhizomatous perennial hybrid of black and brown knapweed in the plant family Asteraceae that has been introduced from Europe into many of the United States and Canadian provinces. It is not as widespread in North America as certain other Centaurea species, so herbicides have not been widely tested for efficacy on this hybrid. Therefore, two experiments were conducted to determine (1) whether meadow knapweed is similarly sensitive to several herbicides commonly used to control related knapweeds, (2) if herbicide application timing plays a role in control of meadow knapweed, and (3) whether mowing before, after, or instead of herbicide treatment can aid in meadow knapweed control. Herbicides were applied either in spring autumn or in summer autumn in the first experiment; in the second experiment, dicamba 2,4-D was applied either before or after mowing and compared to one or two herbicide applications or two mowings. In the first experiment, clopyralid alone or in combination with other herbicides provided up to 100% control of meadow knapweed in the year following treatments. Triclopyr, dicamba, and glyphosate also resulted in control. These herbicides are often used at similar rates and timings to achieve control of other Centaurea species, which indicates that meadow knapweed is as susceptible to these products as other knapweeds. In the second experiment, combinations of mowing and herbicide applications at rosette or early flowering stages of growth generally reduced meadow knapweed biomass and increased grass biomass, although herbicide treatment alone achieved the same result. Mowing twice was an ineffective control strategy for this knapweed hybrid. Therefore, mowing is not recommended to augment control of meadow knapweed when using herbicides.