Nichols, K. M.1; Vollmer, K.2; Lingenfelter, D.D.3; Wallace, J.W.4; VanGessel, M.J.5; Scott, B.6
1Ag Agent Associate, University of Maryland Extension, Frederick, MD, 21702
2Extension Weed Science Specialist, University of Maryland Extension, Queenstown, MD, 21658
3Extension Associate, Weed Science, Penn State Extension, State College, PA, 16802
4Assistant Professor and Extension Specialist of Weed Science, Penn State University, State College, PA, 16802
5Professor and Extension Specialist, Weed Science and Crop Management, University of Delaware, Georgetown, DE, 19947
6Associate Scientist, Weed Science, University of Delaware Extension, Georgetown, DE, 19947


Weed control in pumpkins is challenging for many reasons, including the production practices of wide rows, no-till which excludes the use of cultivation, long growing season, and limited number of herbicide options. These practices result in a greater reliance upon herbicides for weed control. Unfortunately, there are very few herbicides labeled for postemergence weed control in pumpkins, so novel uses of soil-applied herbicides need to be explored.

S-metolachlor is a common residual herbicide currently labeled for application between pumpkin rows, but it is not labeled for pre-emergence application in pumpkins. The objective of this research was to evaluate the potential of s-metolachlor as an overlapping residual approach for pumpkin production in Maryland, Pennsylvania, and Delaware.

S-metolachlor was applied at 2, 3, or 4 weeks after planting at a low or high rate. All plots were evaluated visually for weed control and pumpkin response. Additional plots treated with ethafloralin as a pre-emergence only were monitored weekly to document the emergence pattern of key weeds. Pumpkins did not show injury from the s-metolachlor  over-the-top applications. Both the low and high rates of s-metolachlor  provided at least 80 percent control of pigweed and large crabgrass species. However, waiting 4 weeks after application to apply s-metolachlor  resulted in weed escapes. Treatments including s-metolachlor  provided similar yield to weed free plots.

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