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Palmer Amaranth and Waterhemp; New Threats To Pennsylvania Agriculture

Palmer amaranth and waterhemp are pigweed species introduced into Pennsylvania agriculture.

Palmer amaranth was first identified on seven farms in 2013, and is now on at least 30 farms across at least 14 Pennsylvania counties. Isolated populations of waterhemp have been in Pennsylvania for a number of years. Containing new infestations and preventing their spread is a critical first step to managing these new threats. The risk from these new weeds comes from their competitive growth habit, season-long emergence, prolific seed production (greater than 100,000 seeds per plant) along with potential resistance to glyphosate (e.g. Roundup) and the Group 2 herbicides (ALS-inhibitors). Some populations are also resistant to Group 3 (microtubule inhibitors), Group 5 (Photosystem II), and Group 27 (HPPD-inhibitors) herbicides.

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
Palmer amaranth - seedling; notched tip, no hairs, broad ovate shaped leaves, no waxy sheen. (A. Hager, University of Illinois)

Juvenile Palmer amaranth
Palmer amaranth - juvenile; petioles longer than leaf blade, may have red/purple watermark. (A. Hager, University of Illinois)

Waterhemp Seedling
Waterhemp seedling – egg shaped cotyledons, notched tip, no hairs, narrow lanceolate leaves with waxy sheen. (A. Hager, Univ. Illinois)

Waterhemp Juvenile
Waterhemp juvenile – egg shaped cotyledons, notched tip, no hairs, narrow lanceolate leaves with waxy sheen. (A. Hager, Univ. Illinois)

Redroot
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
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

Identification

Proper identification is the first line of defense:

  • 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).
  • Seed heads are 6 to 24 inches in length, the female flower bracts are sharp and can be painful to handle. Only the females produce seed.

If Palmer amaranth or waterhemp are identified on your farm, aggressively manage the weed to prevent seed production and its spread.

General Guidelines

  • If you discover Palmer amaranth (or waterhemp), report it to your local Penn State Extension Office and/or to a Professional Crop Advisor.
  • It is important to learn about invasive pigweeds (Palmer amaranth and Waterhemp).
  • Know where they are prevalent across the country. Know where they have been found in Pennsylvania.
  • Know the weed biology (aggressive germination, competitiveness, seed production, and herbicide resistance).
  • Know how they spread (equipment, feed, grain, hay, manure, mulch and seed).
  • Know how to reduce their impact including preventing their movement and spread.

Develop Strategies to Address Invasive Pigweeds in Commerce

  • Identify and address pathways for the movement and spread of these weeds.
  • Identify and address effective, consistent and complementary containment measures.
  • Collaborate with academia, government and industry to take effective preventative actions.

Palmer Amaranth - Hartzler
Palmer amaranth has rounded leaves and a dense leaf cluster that is poinsettia-like. (R. Hartzler, Iowa State)

Palmer amaranth petiole
Palmer amaranth petiole is often longer than the leaf blade. (W. Curran and D. Lingenfelter, Penn State)

Palmer amaranth leafblade
Palmer amaranth leaf blade. (W. Curran and D. Lingenfelter, Penn State)

Palmer amaranth leaves
Palmer amaranth leaves can have a single short hair at the tip of the leaf blade. (A. Hager, Univ. Illinois)

Spiny amaranth
Spiny amaranth has a sharp spine at the stem nodes. (R Hartzler, Iowa State)

Redroot pigweed stem
Redroot pigweed stem has fine hairs throughout. Smooth and Powell are similar. (R. Hartzler, Iowa State)

Palmer amaranth stem
Palmer amaranth stem is smooth or hairless. (W. Curran, Penn State)

Palmer amaranth male and female inflorescence
Palmer amaranth male (bottom) and female inflorescence. Female has sharp floral bracts. (W. Curran, Penn State)

Female Palmer amaranth compared to female waterhemp
Female Palmer amaranth left compared to female water-hemp. ( R. Hartzler, Iowa State)

InflorescencesStarting on the left; inflorescences of Palmer, Powell, redroot, smooth, and waterhemp. (A. Hager, Univ. Illinois)

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Article Details

Title

Palmer Amaranth and Waterhemp; New Threats To Pennsylvania Agriculture

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Contact Information

William S. Curran
  • Professor of Weed Science
Phone: 814-863-1014
Dwight Lingenfelter
  • Program Development Specialist
Email:
Phone: 814-865-2242