Now is the time of year when scouting for Palmer amaranth is critical. This weed’s emergence begins in May and will last through the fall. Because of its prolific growth habit and resistance to multiple herbicide modes of action, it is critical to identify and control Palmer amaranth shortly after it emerges.
It is important for growers to have the ability to identify Palmer amaranth so that it is not confused with other pigweed species. Universities have published some excellent resources for ID and control of Palmer, including this Purdue Fact Sheet and this Herbicide-Resistance Management publication. Staying on top of this aggressive weed is critical to managing it and keeping it from competing with crops.
Tips for controlling Palmer amaranth
1. Scout for and identify problem weeds early. Palmer Amaranth is a pigweed species and can easily be confused with other pigweeds, such as redroot pigweed, during early growth stages. It is critical to identify weeds correctly in order to keep them from spreading. Universities such as Ohio State, Purdue, and Michigan State have excellent fact sheets with pictures to aid in weed identification.
2. Start with a weed-free seedbed. Effective burndowns or deep tillage will help control Palmer Amaranth and will allow growers to begin the season with a weed-free seedbed. Apply herbicides when the seedlings are less than four inches tall.
3. Use residual herbicides. Use of residual herbicides will control seedlings at emergence and limit the number of plants that will need to be controlled by post-emergence applications. This is especially critical in soybean fields, where effective post-emergence options are very limited
4. Crop rotation from soybeans to corn will allow for the use of additional herbicide modes of action (herbicides used in corn) that are effective at controlling Palmer amaranth.
5. Consider hand weeding if plants escape normal management practices. Removal before they produce seeds is essential. If a seed head has formed, place a trash bag over the seed head before removing the plant from the field to eliminate the spread of seeds across the field.
6. Manage drainage ditches, field borders, etc. Regularly mowing ditches, waterways, field borders, etc. will help control the spread of this weed. Although weeds growing in ditches do not directly compete with field crops, they will produce seeds and promote the spread of Palmer amaranth.