Iowa State University Extension Weed Science

It's Palmer Time!!
by Bob Hartzler

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August 6, 2014 -  The lack of reliable traits to distinguish Palmer amaranth and waterhemp during vegetative stages complicates efforts at stopping the spread of Palmer amaranth across the state.  However, both plants should be in full reproductive mode at this time, greatly simplifying the identification of the two amaranths. 

While most agronomists and weed scientists prefer to identify weeds using vegetative traits, the small bracts (modified leaves) associated with flowers of Palmer amaranth and waterhemp are the most, if not only, reliable way to differentiate the two species.  Palmer amaranth has relatively large, green bracts that extend well beyond the other flower parts, whereas on waterhemp the bracts are similar in length to the tepals surrounding the seed capsule.  On close examination, Palmer amaranth’s bracts on mature female plants are easily seen protruding from the plant’s seedheads without the use of a hand lens. Redroot and smooth pigweed also produce large bracts, however these species have hairy stems which easily differentiates them from Palmer amaranth.

Several people from across the state have submitted photos or samples of plants suspected of being Palmer amaranth.  In most cases the Palmer suspects were simply ‘healthy’ waterhemp.  There have also been cases of people mistaking redroot pigweed for Palmer amaranth. Redroot and smooth pigweed both have bracts similar in size to those of Palmer amaranth. The easiest way to differentiate redroot and smooth pigweed from Palmer is that the leaves and stems are hairy.

Figure 2. Seedhead of redroot pigweed displaying large bracts. The presence of
stem hairs distinguishes redroot from Palmer amaranth.

The one confirmed new finding of Palmer amaranth was from Lee County in the SE corner of Iowa.  This brings the number of counties with confirmed infestations of Palmer amaranth to five:  Fremont, Harrison, Lee, Muscatine and Page.

Now is the easiest time to find new infestations of Palmer amaranth and initiate programs to either eradicate or limit its spread.  We appreciate being informed of new Palmer amaranth infestations and are willing to aid in identifying suspect plants.

 Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
1126 C Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1164

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