Glyphosate-resistant horseweed populations have become more widespread in the state and are now a significant management issue in western PA and the Northern Tier. In addition to glyphosate- or glyphosate and ALS-inhibitor resistance, horseweed is challenging to control because of variable emergence patterns.

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.