Kurt Vollmer, Wee Management Specialist | kvollmer@umd.edu
University of Maryland Extension
Despite a less than ideal Memorial Day weekend, I’m sure many of us are grateful for the rain. While corn was planted several weeks ago, many places went for weeks without significant rainfall. As a result, many fields lacked adequate moisture to properly activate any preemergence herbicides that were applied. The amount of moisture required to activate a particular herbicide depends on its water solubility (Table 1). The lower the water solubility the more rainfall or irrigation is needed to activate the herbicide and move it into the root zone. Soil-active herbicides such as atrazine, Princep, Balance Flexx, and Prowl need more than 0.75 inches of water to become activated. Under ideal conditions (good activation, no excessive moisture, and sensitive weed species), most preemergence products will provide control for about 3 to 4 weeks after application. Therefore, it’s a good time to start scouting fields to see if an early postemergence application is needed. Remember, preemergence herbicides will not control weeds once they’re up!
Along with products you are considering for postemergence weed control, be sure to include a soil-active herbicide in the tank mix to help extend residual weed control. Group 15 herbicides such as Dual and Outlook can extend control of grasses, pigweeds, and other small-seeded broadleaf weeds. The addition of atrazine can help to improve the efficacy of certain postemergence herbicides as well. Finally, always be sure to consult the label for the proper application rates and tank mix requirements for your crop and soil type.
Table1. The Relative Amount of Water Needed to Activate Common Herbicides and the Duration of Residual Weed Control.
| Trade Name | Amount of Water Required to Activate (inches) | Duration of Residual Weed Control (weeks) |
| Atrazine | > 0.75 | 4-5 |
| Balance Flexx | > 0.75 | 2-4 |
| Callisto | 0.33 | 2-4 |
| Dual II Magnum | 0.33 – 0.5 | 4-5 |
| Harness | 0.5 – 0.75 | 2-4 |
| metribuzin | 0.33 | 2-4 |
| Outlook | 0.33 | 2-4 |
| Princep | > 0.75 | 4-5 |
| Prowl | > 0.75 | 4-5 |
| Valor | > 0.75 | 4-5 |
| Zidua | > 0.75 | 4-5 |
*Table adapted from the 2021 Mid-Atlantic Weed Management Guide.
