Palmer amaranth

Identifying Palmer amaranth

  1. Palmer amaranth has hairless stems. Redroot pigweed and smooth pigweed, on the other hand, have tiny, fuzzy hairs on the stems. On newer stem growth, these hairs often feel and appear similar to peach fuzz. However, as redroot pigweed plants grow, their hairs become less evident on the woody stems. So, find some young growth toward the top of the plant or the ends of branches to determine if the plant indeed does have hairless stems.

    Smooth, hairless stem of Palmer amaranth. Photo: Annie Klodd, Penn State

  2. If the plant has hairless stems, you can further confirm it’s identity as Palmer amaranth by examining the leaves. On Palmer amaranth, the petioles are often longer than the leaf blade itself. The petiole is the thin stalk that connects the leaf blade to the stem.

    Palmer amaranth leaf blade and petiole. Photo: Penn State.

    3. If the plant has produced seed heads and flowers (called influorescences), you can use them to readily distinguish Palmer. Palmer’s influorescences get quite long compared to those of other pigweeds. Redroot and smooth pigweed influorescences are small and clumped.

    Pigweed seed heads. From left: Palmer amaranth, Powell amaranth, redroot pigweed, smooth pigweed, and waterhemp. Photo: Aaron Hager.

    4. Sometimes, Palmer amaranth leaves have a single small hair at the tip of the their leaves.

    The single short hair at the tip of a Palmer amaranth leaf blade. Photo: Annie Klodd

    5. Palmer amaranth leaves look quite similar to leaves of redroot and smooth pigweed. However, they can be distinguished from waterhemp by their shape. Palmer leaves are more rounded, while waterhemp leaves are relatively longer and slim.

Herbicide resistant populations:

  • Group 2 + 9
  • Group 2
  • Group 3
  • Group 5
  • Group 9
  • Group 14
  • Group 27
  • Group 9 + Group 14
  • Group 5 + Group 27
  • Group 5 + Group 9

Locations of resistant populations:

Alabama, Arizona, Arkansas, California, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Minnesota, Mississippi, Missouri, Nebraska, New Jersey, New Mexico, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Texas, Wisconsin, Virginia

Resources:

VIDEO: Palmer Amaranth and Waterhemp Identification – From Pigweed Management Video Series – Penn State

8 Key Points to Palmer Amaranth and Waterhemp Identification

Palmer amaranth Pest Alert – Regional IPM Centers

Palmer amaranth National Fact Sheet – USDA

The Grim reality of ‘Pigweed Patrol’ – Farm Journal

Video – Palmer amaranth ID and management – Penn State

Palmer amaranth Management in Illinois – University of Illinois

Palmer Amaranth Identification, Biology, and Management – Purdue University

Palmer Amaranth Control – Kansas State University

Palmer amaranth could affect Illinois soybean yield

Up-to-date info from U of Minnesota on the management and spread of Palmer Amaranth in the state

Pictorial Guide for Aid in Identifying Palmer Amaranth and Waterhemp – University of Minnesota

Palmer Amaranth and Waterhemp – New Threats to Pennsylvania Agriculture – Penn State

Detailed Palmer amaranth information – ID and management – Penn State