The higher rates are needed for aerial applications. When dead erect stems are present from previous year's infestation, control can still be achieved with labeled use rates. Higher rates may provide measurable control for a second season. In the rosette and bolting stage, higher rates of dicamba, 2,4-D, triclopyr, clopyralid, and glyphosate will control yellow starthistle. The best time to treat with glyphosate is after annual grasses or forbs have died but prior to yellow starthistle seed production.
Control is less effective when mature plants show physical signs of drought stress.
Roundup is not recommended when desirable perennial grasses or broadleaf species are present, except when used as a spot application. Any control program should be continued for at least three years to reduce the yellow starthistle seedbank. Whenever possible, make every effort to expose an infested site to high light during the germination period of yellow starthistle germination. This will deplete the seedbank more rapidly by increasing the rate of germination. Fall or winter grazing, burning, or mowing will provide increased soil surface light during the germination period.