
Kurt Vollmer, Weed Management Extension Specialist|kvollmer@umd.edu
University of Maryland Extension
Palmer amaranth and other pigweeds have emerged and are thriving. These weeds can quickly become too large and dense to control with many herbicides, so fields should be scouted and control measures implemented immediately. A contact herbicide, such as paraquat, applied at a spray volume of 20 gal/A can be used to control small plants prior to planting.
Soil-applied, residual herbicides may also be included as part of the preplant application. Flumioxazin or sulfentrazone-based products plus metribuzin or pyroxasulfone or S-metolachlor plus metribuzin have proven to provide effective control of Palmer amaranth and other pigweeds. However, planting should not be delayed too long after application, as herbicide efficacy will wane over time. If possible, incorporate with at least 0.5 inches if irrigation to activate the herbicides. Consult the label before mixing any herbicides, and be aware of any rotational restrictions before applying these herbicides, especially to double-crop soybean. Applicators should also be aware of potential drift issues to sensitive areas.
Cultivation can be effective on small seedlings (<3 inches), but larger plants are likely to re-grow if roots or stem remain in contact with the soil. Since Palmer amaranth can emerge throughout the growing season, repeated cultivation is necessary for control. However, local research has shown that spring tillage can result in higher Palmer amaranth densities compared to no-till systems. Other practices such as spring disking, chisel plowing, or vertical tillage will not provide adequate control.
Regardless of the control method used, be sure to scout in a timely manner after each operation to determine if additional control is needed. Tank mixing at least two effective modes of action, such as Enlist One + Liberty, and an additional residual herbicide, is recommended for both postemergence control and herbicide resistance management. Reducing soybean row spacing from 30 inches to 15 inches will also allow for faster canopy development, which will help negate the need for additional weed control.