Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
I have seen Palmer amaranth seedlings emerging in corn fields that had no preemergence herbicides applied. There are Palmer amaranth plants in DE and MD that are resistant to glyphosate and ALS herbicides (Group 2). Fields with Palmer amaranth present, or fields where you suspect they are present, need to be treated with an effective herbicide (or herbicide combination) that will provide postemergence control as well as residual control. Some considerations include atrazine, Callisto, Capreno, Impact, Armezon, or Halex GT. If you applied Lumax, Lexar, or Acuron at planting be you may be limited in how much mesotrione (Callisto or Halex GT) you can apply postemergence; refer to the label for maximum use rate per season.
Other products such as Status, DiFlexx, or Liberty can provide effective postemergence control, but will not provide residual control.
Palmer amaranth plants look very similar to smooth and redroot pigweed. However, Palmer amaranth leaves, stems, and petioles do not have hairs (smooth and redroot pigweed do have fine hairs). Palmer amaranth’s leaves have long petioles that are often as long, or longer, than the leaf blade. As a result, the leaves often droop. Occasionally, leaves will have a variegated “V” mark or watermark across the leaf blade.
Palmer amaranth seedling
Palmer amaranth on top and smooth pigweed on bottom, note presence of hairs on the smooth pigweed.