Invasive Pigweeds: Palmer Amaranth and Waterhemp

Palmer amaranth is a species of pigweed that was recently introduced into PA and has been positively identified on more than 30 sites across the state.
Invasive Pigweeds: Palmer Amaranth and Waterhemp - Articles

Updated: September 16, 2017

In This Article
Invasive Pigweeds: Palmer Amaranth and Waterhemp

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; 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

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 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)

Appearance of weed seeds

Small dark brown-black seeds like other pigweed species (NM State Univ. and WNMU).

Lack of hairs

Both Palmer amaranth (left) and waterhemp lack hairs on the stem, petioles, and leaves compared to redroot (right) or smooth (Purdue Univ.).

Leaf shape

Seedling leaves of Palmer amaranth are broad ovate or egg-shaped vs. more narrow lanceolate-shaped leaves of waterhemp. Like other pigweeds, young leaves with notched tip. (Univ. Illinois).

Leaf blade watermark

Occasional dark red/purple or white water or "V" mark or patch on leaf blade of Palmer amaranth (Penn State).

Leaf tip hair

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

Petiole length

Palmer amaranth petiole is often longer than the leaf blade. (Purdue Univ. top and Penn State bottom). Waterhemp petiole not longer than leaf blade.

General appearance

Whorled or poinsettia type appearance of Palmer (top) and long lanceolate leaves and waxy leaf surface of waterhemp at bottom (University of Illinois and Purdue University).

Floral structures

Palmer amaranth flower head has prickly larger female and soft more narrow male on separate plants (Penn State)

Female flower heads of Palmer amaranth bottom and waterhemp top (Iowa State).

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

Agronomic Growers

  • Plant only clean crop seed that is certified. State seed laws prevent crop seed contamination.
  • Use integrated management practices to aggressively control weeds. For example, decreasing row widths results in faster canopy closure and shade formation. Palmer Amaranth does not survive well under dense crop canopies as seeds require light to germinate.
  • Manage infested fields with no-till if possible, leaving any potential seeds near the soil surface.
  • Use residual herbicides (pre and post) during the growing season to prevent new flushes.
  • Apply effective herbicides to small plants that are less than four inches tall. It’s critical that you use the full recommended rate of application. Closely monitor fields before and after herbicide application.
  • With smaller infestations, physically remove plants. Pull by hand or use a hoe. Remove plants from field so they do not re-root. Bag and bury or burn removed plants along the field’s edge.
  • Plant corn or a perennial forage instead of soybeans in fields that are known to be infested. This provides more options for effective herbicides or alternatives for mowing and mechanical control.
  • Do not combine harvest mature pigweeds. If combine harvest cannot be avoided, harvest infested fields last to avoid moving seeds away from the infested fields.
  • Clean tillage and harvest equipment before leaving infested fields.
  • Ensure that used equipment, custom machinery, imported feed or hay, imported manure and compost are not contaminated with noxious pigweed.
  • Monitor field edges, ditches and fencerows for noxious pigweed plants. It’s important to scout after harvest, especially after silage harvest.
  • Aggressively control plants to prevent seed production and spread.

Fruit and Vegetable Growers

  • Plant only clean crop seed that is certified. State seed laws prevent crop seed contamination.
  • Herbicide options for Palmer Amaranth is most vegetable crops are limited. Residual herbicides for grasses and small broadleaves will provide some control, but often not enough for full-season control.
  • Cultivation in combination with herbicide application can increase the overall level of control. Cultivation is effective before Palmer Amaranth is three inches tall.
  • Utilize crop rotations. Rotations should include early-season crops that are harvested before pigweed seeds are produced.
  • Physically remove plants. Pull by hand or use a hoe. Remove plants from field so they do not re-root. Bag and bury or burn removed plants along the field’s edge.
  • Plants that are six inches or taller and are present after early harvest should be mowed or disked to prevent seed production. Palmer Amaranth can regrow and repeated mowing may be necessary.
  • Decreasing row widths results in faster canopy closure and shade formation. Palmer Amaranth does not survive well under dense crop canopies as seeds require light to germinate.

Organic Growers

  • Plant only clean crop seed that is certified. State seed laws prevent crop seed contamination.
  • Utilize diverse crop rotations. Rotations should include early-season crops that are harvested before pigweed seeds are produced. Consider winter cereals, cover crops and perennial hay crops that can be mowed.
  • Crop competition is an important component of cultural weed control. Aim for quick crop establishment, optimum soil fertility and crops planted in narrow rows to prevent the competitiveness of weeds.
  • Use mixed tillage practices such as deep tillage, shallow tillage and no-till when possible. Burying pigweed seed at least four inches deep in the soil with plowing will provide approximately 50% control.
  • Physically remove plants. Pull by hand or use a hoe. Remove plants from field so they do not re-root. Bag and bury or burn removed plants along the field’s edge.
  • Plants that are six inches or taller and are present after early harvest should be mowed or disked to prevent seed production. Palmer Amaranth can regrow and repeated mowing may be necessary.

Equipment and Transportation

  1. Evaluate used equipment for contamination before use. Know where the equipment has been and whether or not the weeds are prevalent in that region.
  2. Clean all farm equipment that comes in contact with noxious weeds. Clean tillage and harvest equipment for leaving infested fields.
  3. For combines, compressed air and vacuum are recommended for clean out. This is particularly important for combines with a platform header.
  • Start at the front of the machine with the header, feeder and rock trap.
  • Next move to the grain tank, unload auger and sump.
  • Finally, move the cleaning shoe and back of the machine.

This process can take minutes to several hours, depending on the equipment and where it has been.

Animal Production

  • Compost manure to destroy weed seeds that may remain in the manure.
  • Ensile crops that contain noxious weed parts.
  • Do not feed animals grain or hay contaminated with weed seeds without first destroying by grinding, roasting or ensiling.
  • Do not purchase feedstuffs or hay from noxious pigweed infested regions. Inspect feed and hay for evidence of noxious weed plant parts and seed.

Instructors

Integrated Weed Management Herbicide Resistance

More by Annie Klodd 

Managing weedy plants in agroecosystems Conservation tillage and cover crops Herbicide use Integrated weed management Weed management in organic cropping systems

More by William S. Curran, Ph.D.