"Killing marestail in the fall is usually easier since the seedlings and rosettes are small and more susceptible to herbicides. Once weeds like marestail get through the winter, their rosettes start to bolt upwards in the spring, making them much more difficult to control," says Dwight Lingenfelter. 

In the fall, foliar-applied herbicides can be effective as long as the plants are green and appear healthy. For best activity, apply herbicides when daytime temperatures are above 50°F and nighttime temperatures are above 40°F for several days during application time.