Palmer amaranth - A highly variable weed

August 30, 2016 8:35 AM
Blog Post

The first step in managing any pest is correct identification. For Palmer amaranth, we have stressed the need to examine the inflorescences of female plants for the presence of the large, sharp bracts. Farmers/agronomists are not accustomed to examining flowers since almost all weeds can be identified using simple, vegetative traits. Flowers are needed to confirm the identity of Palmer amaranth because of tremendous variability within both Palmer amaranth and its close relative waterhemp.

The variability within Palmer amaranth can appear in plant color, canopy architecture and leaf density, and both leaf and inflorescence size and shape. The photos illustrate a small portion of the variability we've encountered - the majority of the plants pictured grew in the absence of  direct competition with other plants. When facing intense competition, most plants will develop a taller, less branched canopy in order to reach full sunlight before neighboring plants.

Cluster of tightly packed leaves at stem tips (rosette), 

youngest leaves are often chlorotic.
Highly branched plant with multiple long terminal inflorescences.
Watermarked leaves.  This is the only plant

I've seen this year with this trait. Other

pigweeds will also display this trait.
An abnormally long terminal inflorescence.
Palmer with a very dense canopy.
A tall, open canopy even though the plant was

facing no competition with neighbors.
The tremendous variability in growth habit is

why we stress the need to look for female

Palmer amaranth and look for the sharp bracts.



Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...