Palmer Amaranth, a New Threat to Pennsylvania Agriculture
Containing new infestations and preventing its spread is a critical first step to managing this new threat. The risk from this new weed comes from its competitive growth habit, prolific seed production (greater than 100,000 seeds per plant) along with its potential resistance to glyphosate (e.g. Roundup) and the Group 2 herbicides (ALS-inhibitors). Resistance to other herbicide classes has also been documented in this species (Groups 3 and 5).
Palmer amaranth (Amaranthus palmeri) is a summer annual broadleaf weed that is native to the southwestern US and Mexico. It is also known as Palmer pigweed. Palmer amaranth is related to other pigweeds in our region including redroot, smooth, Powell, and spiny, but unlike these other pigweeds, Palmer amaranth grows faster and is dioecious, meaning that plants are either male or female. Pollen from male plants can travel with the wind to susceptible female plants and if the male is herbicide resistant, a portion of the offspring will also be resistant. Waterhemp (Amaranthus tuberculatus), another dioecious herbicide resistant pigweed species common in the Midwest is also getting a foothold in Pennsylvania. Although not as great a competitive threat as Palmer amaranth, it too should be aggressively managed to prevent its spread.
Palmer amaranth - seedling; notched tip, no hairs, broad ovate shaped leaves, no waxy sheen. (A. Hager, University of Illinois)
Palmer amaranth - juvenile; petioles longer than leaf blade, may have red/purple watermark. (A. Hager, University of Illinois)
Waterhemp seedling – egg shaped cotyledons, notched tip, no hairs, narrow lanceolate leaves with waxy sheen. (A. Hager, Univ. Illinois)
Waterhemp juvenile – egg shaped cotyledons, notched tip, no hairs, narrow lanceolate leaves with waxy sheen. (A. Hager, Univ. Illinois)
Redroot – notched tip, small fine hairs, ovate shaped leaves. Powell amaranth very similar. (P. Westra, Colorado State Univ., left and B. Ackley, Ohio State
Smooth pigweed – notched tip, small fine hairs, ovate shaped leaves. Powell amaranth very similar. (P. Westra, Colorado State Univ., left and B. Ackley, Ohio State
Proper identification is the first line of defense and plants that survive glyphosate application are the first clue. Here are some key characteristics:
- Palmer amaranth plants look similar to other pigweeds and especially as seedlings.
- Palmer leaves, stems, and petioles are hairless and petioles are usually longer than the leaf blade.
- Sometimes, Palmer amaranth leaves will also have a “V” mark or dark red/purple patch (watermark) on the leaf blade (spiny as well as the other pigweeds can also sometimes have this mark).
- Palmer amaranth seed heads are 6 to 24 inches in length, the female flower bracts are sharp and can be painful to handle (the males are not), and only the females produce seed.
If Palmer amaranth is identified on your farm, aggressively manage the weed to prevent seed production and its spread. Here are some suggested management tactics.
- If you discover Palmer amaranth, report it to your local Penn State Extension Office and/or to a Professional Crop Advisor.
- Use an integrated management approach that includes soil residual herbicides and effective postemergence control. Small plants that are less than 4 inches tall are easier to manage.
- With smaller infestations, physically remove plants from the field prior to flowering.
- For flowering plants, determine if the plants have set viable seed. This will be important for determining the future of the problem. If no viable seed is yet present (darkened seeds that shatter), hand rogue smaller infestations and remove plants from within field. Bury or burn removed plants along field edge. Manage the field with no-till if possible leaving any potential seeds near the soil surface. This should enhance seed predation and mortality.
- If the problem is too large to handle by hand or if viable seed are present, please consult with Penn State Extension and/or a Professional Crop Advisor to determine the best management strategy.
Palmer amaranth has rounded leaves and a dense leaf cluster that is poinsettia-like. (R. Hartzler, Iowa State)
Palmer amaranth petiole is often longer than the leaf blade. (W. Curran and D. Lingenfelter, Penn State)
Palmer amaranth leaf blade. (W. Curran and D. Lingenfelter, Penn State)
Palmer amaranth leaves can have a single short hair at the tip of the leaf blade. (A. Hager, Univ. Illinois)
Spiny amaranth has a sharp spine at the stem nodes. (R Hartzler, Iowa State)
Redroot pigweed stem has fine hairs throughout. Smooth and Powell are similar. (R. Hartzler, Iowa State)
Palmer amaranth stem is smooth or hairless. (W. Curran, Penn State)
Palmer amaranth male (bottom) and female inflorescence. Female has sharp floral bracts. (W. Curran, Penn State)
Female Palmer amaranth left compared to female water-hemp. ( R. Hartzler, Iowa State)
Starting on the left; inflorescences of Palmer, Powell, redroot, smooth, and waterhemp. (A. Hager, Univ. Illinois)
Prepared by William S. Curran, Professor of Weed Science and Dwight D. Lingenfelter, Extension Associate