Palmer Amaranth: Iowa's New Invasive Weed

Palmer amaranth, a new invasive species related to waterhemp, has become a weed of concern for corn and soybean producers across the state in the last three years.  It is extremely invasive and difficult to manage due to its rate of growth and its ability to develop herbicide resistance.  Palmer amaranth has spread across the state and has been identified by agronomists in 28 counties. 
Most new findings of Palmer amaranth are in newly established conservation plantings.  See ICN News, August 19, 2016: Palmer Amaranth in Iowa – What We Know and ICM News, September 9, 2016: Managing Palmer Amaranth in Conservation Plantings.  Discovery in conservation plantings was not unexpected and follows similar trends in other states.  Palmer amaranth seed is likely entering these plantings during establishment with seed mixes that are at least partially sourced outside of Iowa.  The effect of Palmer amaranth in conservation planting should be minimal as plants should not survive once the habitat is established and good establishment practices such as regular mowing are followed.  The concern is that Palmer amaranth seed from conservation plantings may move into production fields (corn, soybeans, and specialty crops). 
Palmer amaranth seed has been found in some bird seed mixes.  This contamination has been documented as a significant problem in other states.  One suspected incidence of Palmer amaranth seed in bird seed was reported in IA but not confirmed.
Home owners with lawns and gardens should not fear Palmer amaranth.  Agronomist and farmers are working diligently to identify and eradicate infestations before seeds form.  For seeds that do survive and get transported elsewhere, seedlings will not compete well with established turf and ornamental plantings.  Standard weed control practices such as mulching, mowing, and hoeing will prevent Palmer amaranth seedlings from becoming established in annual flower and vegetable beds as well as perennial fruits.  Acreage owners, especially those with newly established CRP or pollinator habitat, are encouraged to learn how to identify and scout for Palmer amaranth.  Often infestations are just a few plants that can be removed by hand. 
Palmer amaranth can be identified by two features 1) the leaf petiole is longer than the leaf blade and 2) sharp bracts on the female plants are very long and as they mature, become very sharp.  For more on identification see ICM News, August 5, 2016: Palmer Amaranth Now Identified in at Least Nine Iowa Counties.
See the ISU College of Agriculture & Life Sciences video library to watch Dr. Bob Hartzler describe how to identify Palmer amaranth.

Close-up of Palmer Amaranth in a field
A distinctive characteristic of Palmer amaranth is that leaf petioles are longer than the leaf blade.


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