Ben Beale, Senior Agriculture Agent
University of Maryland Extension, St. Mary’s County
We observed Palmer amaranth germinating in fields in Southern Maryland the first week of May. These plants are now exceeding the optimal height for control. Plants are now 8-10 inches tall in fields that did not receive any burn down treatment or tillage this spring. The optimal size for effective control of palmer is 3 to 4 inches.
In fields that received a burn down herbicide application in May, expect to see additional plants emerging, often before the soybean crop is planted. Be sure to kill these plants before the soybeans emerge. Paraquat (Gramoxone® or generic alternatives) is an effective burn down treatment for smaller plants provided it is applied with adequate water and spray coverage is adequate.
Other options include products like 2,4-D, saflufenacil (Sharpen®), or dicamba. Each of these products have plant-back restrictions. Glyphosate, while not effective on Palmer amaranth, can still be added to control other weeds present. Integrating several strategies into your weed management plan will help avoid resistance and increase control. Effective strategies include crop rotation, tillage, seed bank management, preventing seed movement, use of cover crops, choosing a residual herbicide program with at least two effective modes of action and use of timely post-emergent herbicide applications.
More information on management of palmer amaranth is available from University of Maryland Fact Sheet 1086, Palmer Amaranth in Maryland.
