Palmer Amaranth is in the Area

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Last year I saw a few fields in the area (Delaware and Maryland) with infestations of Palmer amaranth. Palmer amaranth is a pigweed, which looks similar to the smooth pigweed that is so common (and often called redroot). However, Palmer amaranth is a very aggressive species that grows very rapidly. It is native to the southwest region of the US, and does better than most plants under dry conditions. Palmer amaranth has been described as pigweeds on steroids because of its ability to grow very rapidly, get very tall, and be very competitive with crops. Palmer amaranth is found throughout the southern US and is moving northward. Palmer amaranth is not as sensitive to Group 2 herbicides as smooth or redroot pigweed (this includes Pursuit, Sandea, Accent, Matrix, etc.). It is sensitive to PPO herbicides (Reflex, Valor, etc); atrazine, and HPPD (Callisto, Impact, and Laudis. Furthermore, glyphosate-resistant Palmer amaranth is present in Georgia, North and South Carolina and other southern states. I am not aware of any herbicide-resistant Palmer amaranth in our area.

It is critical that you control plants early; and that you do not allow the plants to produce flowers. Plants will produce a very high number of seeds that will quickly infest fields. In the southern cotton growing regions where they have herbicide-resistant Palmer amaranth, they have had to resort to hand weeding.

To help identify Palmer amaranth, see the chart, websites, and photos below.

Characteristics Redroot Pigweed Smooth Pigweed Palmer Amaranth
Stem hairs Hairy Hairy No hairs
Stems Often ridges running length of stem Often ridges running length of stem Mostly smooth
Leaf petioles Petioles no longer than length of the leaf Petioles no longer than length of the leaf Long drooping petioles
Seed head Short, stout, prickly Long, slender, slightly prickly Very long, thick, very prickly

A couple of good publications include:

http://mulch.cropsoil.uga.edu/weedsci/HomepageFiles/PalmerBiologyEcology.pdf
http://www.extension.iastate.edu/Publications/PM1786.pdf
http://www.ksre.ksu.edu/library/crpsl2/s80.pdf

 

Smooth pigweed

 

Palmer amaranth

 

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