Palmer Amaranth: The Most Obnoxious Noxious Weed in Maryland

The Concern

Palmer amaranth was first discovered in Maryland during the mid-twentieth century, but wasn’t added to the Maryland Noxious Weed Law until 2020. This 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. Palmer amaranth is problematic for landowners and farm tenants because of its dominance once established and the difficulty to fully eradicate it. For this reason, Palmer amaranth has been chosen as MISC’s Invader of the Month for October 2023.

Physical Description

Palmer amaranth has three growing stages. The first stage is the cotyledon ‘seedling’ stage when the plant will start to emerge in late April to early May with a similar look to other pigweeds. Palmer amaranth leaves are diamond-shaped, lack hairs on surface, and sometimes contain a white or purple watermark. Leaf petioles of older leaves are as long or longer than the leaf blades and stems are smooth and hairless. Other pigweed species leaves have a similar appearance, but have hair on the stems and the petiole length to leaf blade length ratio is not as dramatic. The second stage is the intermediate growth stage where a female and male plant are present. Palmer amaranth is dioecious, having separate male and female plants. The female Palmer amaranth plant will start growing stiff bracts at the leaf axil that have a prickly feel to it, while the male Palmer amaranth plant will have a softer feel with no prickly bracts. The third and final stage is the reproductive adult stage where the female Palmer amaranth will produce 250,000—600,000 seeds per plant, and flower heads will harden, becoming pricklier. Male Palmer amaranth plants will produce pollen.

Habitat & Distribution

Native to the Sonora Desert of Arizona and Northern Mexico, Palmer amaranth is well adapted to the hot Maryland summers. It is mostly found in agricultural crops such as corn, soybeans, lima beans, and other vegetable fields or anywhere the land has been disturbed and seed can make bare ground contact. It’s most problematic in organic systems where the farmer is limited in the methods used to control it. It has recently been found along roadway routes where commodity crops travel to grain elevators and mills, in guardrails and right of ways. Although it’s been identified in each region of Maryland, it’s most prevalent on Maryland’s Eastern Shore.


Palmer amaranth is a summer annual weed in the pigweed family that has become a major challenge for the farmers across the U.S. for much of the decade. It is one of the most troublesome weeds in the United States due to the economic and ecologic harm it causes to our private and public lands, specifically in the agronomic crop production systems. Studies have shown 79% – 91% yield loss in corn and soybean crops once Palmer amaranth is established. Female plants can produce upwards of 600,000 seeds per plant and their rapid growth of 2-4 inches per day make them difficult to control in a timely manner. Once plants get to be larger than 4” in height, control methods are more difficult and expensive. This adds an additional cost to farmers and land managers and limits landowners who may not have herbicide knowledge to properly control the weed.


Control practices include treatment with an approved herbicide, cultivation or hand pulling. However, Palmer amaranth has shown resistance to multiple herbicides which limits its use. In organic systems, weed electrocution (Weed Zapper) or flaming (the use of fire) in between the rows may be used to control Palmer amaranth. The best way to prevent the spread of Palmer amaranth is to scout the land early in the growing season, identify new infestations quickly and initiate steps to prevent its establishment and spread. Additionally, prevent seeds from entering the farm by cleaning equipment thoroughly.

The Maryland Department of Agriculture has entered into an agreement with 16 Counties throughout the State to provide technical assistance to landowners for initiating noxious weed control programs. Many of the programs provide herbicide applicate to landowners on a fee for service basis. To learn more, please contact your area’s weed control program. For assistance with weed identification, contact your local Extension office.