This tough winter annual emerges in both fall and spring. Many university extension programs recommend controlling emerged marestail in the fall. Most fall-emerged plants will survive the winter and continue to grow in the spring, becoming more difficult to control.
Weed size matters
Herbicide effectiveness really depends on the growth stage and size of the weeds. Marestail is best controlled when small, 2-4 inches and before bolting. Because this tough weed can germinate year-round, one herbicide pass will likely not control it for the entire season, especially in a no-till situation. But a fall application can help you get a head start for the spring. Fall applications can be made even into December. Apply herbicides to actively growing weeds and during a period of mild temperatures.
See this eUpdate from Kansas State University extension for more information on fall treatments for marestail. (Dallas Peterson, Weed Management Specialist and Doug Shoup Southeast Area Crops and Soils Specialist.