Technical Abstract: Density of horseweed is increasing in no-till cropping systems because horseweed seed is transported by wind and seedlings can establish from seeds lying on the soil surface. Also, horseweed has developed resistance to commonly-used herbicides, thus control tactics are often ineffective. This report describes characteristics of horseweed seedling emergence to gain insight for possible control tactics. Horseweed began emerging in early April and continued through October. Approximately 40% of seasonal emergence occurred in between April 18 and May 31, with the remainder of seedlings emerging between August 8 and October 31. Apparently, horseweed has a wide tolerance to environmental conditions related to emergence, as seedlings were observed in 5 of the 7 months of the growing season. Horseweed invasion of the no-till corn-soybean rotation may be related to the wide window of horseweed seedling recruitment in the fall. A possible control tactic may be including a 3-year interval of alfalfa in the rotation. Horseweed is not able to complete its life cycle in alfalfa due to frequent mowing and crop competition. Seedling density was reduced more than 95% if seed addition to the land area was prevented for 3 years.