Small spots of Johnsongrass may be sprayed with 41% Glyphosate at a rate of one quart per acre. Any corn sprayed will also be killed with Roundup.
General Johnsongrass infestations can be controlled with Accent herbicide in corn that is less than 36 inches tall and exhibiting less than 10 collars. For corn more than 24 inches tall, you must use drop nozzles. Apply to rhizome Johnsongrass when the Johnsongrass is between eight inches and 18 inches tall. Sequential treatments may be made to rhizome Johnsongrass by spraying the first time with 2/3 ounces of Accent per acre when the Johnsongrass is 8-12 inches tall, followed by 2/3 ounces of Accent when the new growth is 8-10 inches tall. Accent may be tankmixed with other herbicides for control of other weeds. Rotation to other crops will be restricted after the application of Accent. Accent labels should be read before any decisions or treatments are made to determine the many restrictions.
Heavy infestations of Johnsongrass should be controlled by rotating the cropland to wheat, some other cereal grain crop, or Roundup-ready crops for a few years to get the rhizome Johnsongrass under control with treatments of 41% Glyphosate. Crop rotation also helps control other corn pests.