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.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on August 30, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.
- Retired Weed Scientist and Professor of Agronomy
- Field Agronomist in Central Iowa