Palmer amaranth (Amaranthus palmeri) is a problematic annual broadleaf weed in the amaranth genus. It has several common names including carelessweed, dioecious amaranth, Palmer’s amaranth, Palmer amaranth and Palmer’s pigweed.
Palmer amaranth is native only to the Southwest and is considered a weed throughout the country. Field staff at the U.S. Department of Agriculture, Natural Resources Conservation Service, their partners, as well as farmers and landowners are working to eradicate these infestations before they spread to new areas.
Palmer amaranth is an annual weed that is fast growing (possibly more than 3 inches per day), can grow to more than 6 feet tall, and produces many small viable seeds (100,000–500,000 seeds). Seed can remain dormant in the soils and germinate years later.
Palmer amaranth has the potential to spread to agricultural fields, can be very difficult to control, can significantly increase production costs, and may reduce crop yields due to competition. Additional conservation practices, such as Conservation Crop Rotation, Integrated Pest Management, Residue and Tillage Management, No-Till or Reduced Till, and Cover Crops, may be used if found in crop fields. Fast-growing cover crops and those with allelopathic qualities (the release of chemicals by one plant that inhibits the growth of adjacent plants) should be considered.
Identification of Palmer amaranth should be done with assistance of your State department of agriculture and land grant university extension specialists. Program participants are responsible for contacting these agencies for assistance.
Palmer amaranth can develop resistance to herbicides, such as glyphosate and acetolactate synthase-inhibiting herbicides (ALS).
The landowner, operator, or program participant should seek recommended control measures from State agricultural and extension specialists. NRCS will not develop chemical pest suppression recommendations, consistent with 190-GM, Part 404, “Pest Management.”
A multi-pronged approach to address Palmer amaranth may include a combination of mowing during establishment before seed maturity, hand weeding, burning, and spraying. Mowing alone will not disrupt seed production.
The presence of Palmer amaranth does not necessarily constitute a stand failure. Stand success or failure is determined by plant density. States will use their established stand establishment guides for determining stand success or failure. Successful stands with Palmer amaranth should also have it removed.
After seed maturity, terminating the stand by tillage spreads and incorporates the mature seed, making it harder to achieve control in the future. Inverting the soil to bury the seed will leave the field susceptible to erosion and is not recommended by NRCS.
Broadcast herbicide application on the entire field may not kill Palmer amaranth and, since the herbicide may kill all broadleaf species, the reduced competition from desirable plants allows the weed to further establish a monotypic stand. Desirable plants may need to be reestablished following appropriate program requirements.
Sources of Palmer amaranth introductions may include cattle feed, manure, birds, hay or mulch, clothing, equipment or vehicles that have not been cleaned properly, or as a contaminant of native seed mixes purchased for conservation plantings.
Program participants should purchase native seed from reputable sources. Land owners planning conservation seed mixes should seek relevant important information about what they are purchasing from their commercial source.
Producers and landowners should look closely at the seed label, in particular the “weed seed” analysis. An “all State noxious weed seed” test may be useful for detecting species of concern which may not be on your State’s noxious weed seed list. Seed laboratory test results and information on weed seed content and species that may be present for each species in a mix may be obtained from the seller. A weed seed listing of “Amaranthus sp.” or “pigweed” may indicate a number of species in this genus. Palmer amaranth is difficult to distinguish from other Amaranthus species. A genetic test for Palmer amaranth seed has been recently validated and may be available in the coming months for additional testing when Amaranthus seed is detected in a seed test.
Confidential and voluntary safe reporting is essential.
What You Need to Know
Palmer amaranth is a weed that can significantly reduce crop yields and increase crop production costs.
Palmer amaranth is a weed that can be difficult to control and can develop resistance to several commonly used herbicides.
Palmer amaranth was added to the Iowa noxious weed list in July 2017.
Besides the introduction through seed mixes, other sources of Palmer amaranth introduction may include contaminated cattle feed, manure, birds, hay or mulch, and equipment or vehicles which have not been properly cleaned.
In the upper Midwestern states, Palmer amaranth cannot be readily identified in fields until late June or early July.
Landowners and farmers should be proactive in identifying palmer amaranth on their property to prevent Palmer amaranth from reseeding. Landowners can actively search for it in crop fields, borders, ditches, conservation lands and around dairies.