Palmer Amaranth Identification & Management

Common Name: Palmer amaranth 
Alternate Names: Palmer pigweed, careless weed 
Scientific Name: Amaranthus palmeri  
Legal Status: Prohibited- Eradicate

Palmer amaranth was added to the Maryland Noxious Weed Law in 2020. The law prohibits the import and transport of the noxious weed throughout the state and requires infested lands be managed for the eradication of the species. 

What is Palmer Amaranth?

Palmer amaranth is a summer annual weed in the pigweed family that has become a major challenge for farmers across the U.S. for much of the last decade. Native to the Sonora Desert of Arizona and Northern Mexico, Palmer amaranth is well adapted to the hot Maryland summers. In Maryland, Palmer amaranth has widespread resistance to Group 9 herbicides (Glyphosate) and Group 2-ALS herbicides. It was first discovered in Maryland during the mid-twentieth century, but the presence of herbicide resistant Palmer amaranth was not confirmed until 2014. Since then it has spread to most regions of the state including Eastern, Southern, and Western Maryland. 

Key Impacts

• Palmer amaranth competes aggressively with crops. 
• Plants can grow over 1 inch per day during summer months and commonly reaches heights of 6- 8'. 
• Yield losses have been up to 91% in corn and 79% in soybean. 
• Other traits: prolific seed production, season-long emergence, & resistance to herbicides.

Appearance

Palmer amaranth leaves are diamond-shaped, lack hairs on the surface, and sometimes contain a white or purple watermark. A single, stubby hair is often found in the notch at the leaf tip. Leaf petioles of older leaves are as long or longer than leaf blades. 

Palmer amaranth stems are smooth (hairless) and can be green, red, or red green striped in color. Male and female flowers are produced on separate plants. These terminal flower spikes can be 2-3 ft. or more in length. Male flowers are soft to the touch while female flowers contain sharp bracts.

For assistance with weed identification, contact your local Extension office.

Key Characteristics

• Flowers — Upright spikes can be 1-3 feet or more with male flowers that are soft to the touch, female flowers contain sharp bracts.
• Leaves — Diamond-shaped leaves lacking hairs, leaf petioles are as long or longer than leaf blades.
• Stems — Smooth stems (green, red, or red green striped in color).
• Roots — Strong taproot sometimes light pink, but not dark pink like redroot pigweed.

Prevention

• Scout your farm frequently in the early growing season. Controlling plants while they are less than 4" will preserve yield potential and save costs. Plants greater than 8 to 10 inches tall often need sequential herbicides to achieve complete control and prevent seed production.  
• Preventing seeds from entering the farm is also important. Equipment is the most common method for moving seeds. Palmer amaranth seeds move readily in and on equipment, particularly combines. When harvesting crops, do not move equipment between infested and non-infested fields. 
• Clean equipment thoroughly between fields using a leaf blower or compressed air. Sending one or two bales of clean straw through the combine before entering non-infested fields can also help to dislodge seeds. 
• Cotton meal or hulls used for livestock feed as well as straw, hay or mulch from infested fields can carry Palmer seeds. Palmer amaranth seed can survive the digestive system of most animals and the seeds remain viable in manure.

Management

• Chemical — Glyphosate-resistant weeds, including palmer amaranth require intensive management and careful herbicide selection to ensure good control.  
  • Pre-emergence Control: Start clean using a good burndown herbicide or tillage. Apply full rates of residual herbicides with multiple effective modes-of-action at planting. 
  • Post-emergence control:   Apply effective POST herbicides before weeds are 3-4” tall.  Include an additional residual herbicide in the tank to extend control later into the growing season.  
     
• Mechanical — Main mechanical control includes tillage, cultivation/inter-row cultivation, mowing multiple times during early flowering stage, and manually hand pulling.  Additional options  include flame weeding and weed electrocution.  Moldboard plow once every four years is a highly effective, research-backed "strategic tillage" method to suppress severe Palmer amaranth infestations.  Use harvest weed seed control tactics to avoid spreading seed at harvest.  
   
• Cultural — During harvesting, harvest weed free areas first and infested areas last. Clean the combine after harvesting infested areas.

Combining Weed Control Strategies Effectively

Palmer amaranth and waterhemp are herbicide-resistant weeds that can easily adapt to conventional spray programs. By combining different weed control strategies, these weeds are less likely to become more problematic as they cannot adapt to a constantly changing system.

Each practice listed has been assigned points based on the relative level of control obtained in university studies. The greater the number of points accumulated, the greater the likelihood of success of the management program. 

• 1 = Fair practice
• 2 = Good practice
• 3 = Excellent practice

It is recommended that conventional farming operations accumulate at least 8 points to control Palmer amaranth and waterhemp successfully. Organic operations should accumulate at least 6 points, including 2 different harvest weed seed control options. 

Adapted from Efficacy of Weed Management Tactics SPES-268N9 

Note: Land under the Conservation Reserve Program should consult with USDA Farm Service Agency on weed control and management.

Resources

• Palmer Amaranth Factsheet - PDF - 2.06 MB​
• Control of Large Palmer Amaranth and Common Ragweed in Soybean or Corn
• Thoughts on the Control of Palmer Amaranth in Organic Systems
• Evaluating Late Season Burndown Options for Palmer Amaranth
• Additional Palmer Information (Grow IWM)

For specific herbicide recommendations as well as organic options, contact University of Maryland Extension Weed Management Specialist, Kurt Vollmer: [email protected] or Ben Beale: [email protected]

MDA has entered into an agreement with many counties in the State to provide technical assistance to landowners. Many of the programs provide herbicide application to landowners on a fee-for-service basis. To learn whether your County has a Weed Control Program, visit plants and pests. 

For weed identification contact your local Extension office or your regions MDA Agricultural Inspector.