Treatment with postemergence herbicides generally works best during seedling and early rosette stages. They are most effective when temperatures are warm, soil moisture is high, and plants are actively growing. Treatment during cold or wet conditions often fails due to poor herbicide absorption and removal by rain. Applications at later stages of growth allow a greater number of plants to be treated but may not control larger plants or may require higher rates. Glyphospate is nonselective and controls yellow starthistle at a 1 percent solution.
Control is effective even at the early flowering stage.
Because glyphosphate is nonselective, treatment should occur after desirable species have completely gone to seed for the season. Because ester formulations are very volatile, amine forms are recommended to reduce the chance of off-target movement. Manual Control Manual control of yellow starthistle is very effective in dealing with small, localized infestations. Small populations can be controlled and greatly reduced by hand pulling, hoeing, or cutting with scythes or weedeaters. Manual methods are also useful for spot-weeding plants that survive from other control methods.