New Population of Palmer Amaranth Detected in Montana, September 2024
Palmer amaranth (Amaranthus palmeri) was confirmed for the first time in Carter County, Montana in September 2024. This is the third confirmed occurrence of this plant in Montana. Be on the lookout for this species and its close relative waterhemp (Amaranthus tuberculatus) this fall.
Palmer Amaranth and Waterhemp in Montana
Palmer amaranth (Amaranthus palmeri) and waterhemp (Amaranthus tuberculatus) are both pigweed species that have very limited known distribution in Montana. Both species have developed genotypes resistant to multiple herbicide modes of action. If they become established, these two species will have a large economic impact on Montana agriculture. Both weeds have the potential to establish in warm season crops such as corn and millet, and also irrigated sugarbeets, cereal, and pulse crops, but have also been associated with birdfeed.
In September 2024 one male, flowering Palmer amaranth plant was found growing on a roadside in Carter County. The plant was removed from the site and no seeds were produced. This detection follows two in 2023 in Daniels and Toole Counties. To date, fewer than ten Palmer amaranth plants have been found in Montana, and all were removed from their sites prior to seed production.
Palmer amaranth plant growing on the roadside in Carter County, September 2024 |
Two waterhemp populations have also been found in eastern Montana in recent years. One was detected in an irrigated durum field in Roosevelt County in 2020, and a few plants were found in a sugarbeet field in Prairie County in 2021.
Now is a good time of year to scout fields for Palmer amaranth and waterhemp because these warm season pigweeds are best identified when flowering. Special attention should be given to scouting warm season forages and irrigated fields. Look for large pigweeds with long, thin inflorescences compared with more common relatives like redroot pigweed.
 Photo of long, thin inflorescence of Palmer amaranth. Note the numerous yellow anthers suggesting it is a male plant. |
Early detection and rapid response are critical to contain these weeds and prevent populations from establishing, so learn to identify them, take action to manage and control spread as needed, and learn to recognize potential vectors. Identification can be difficult, but the MontGuides and resources linked below can help identify the different pigweeds.
Contact your local Extension office or agronomist if you suspect you have found either of these species or if you have questions about pigweed identification.
Resources for Identification and Management
- Montana Department of AgricultureEarly Detection Rapid Response Palmer Amaranth Page
- Palmer Amaranth MontGuide
- Waterhemp MontGuide
- NDSU Palmer Amaranth Webpage
Contact
Noelle Orloff, Schutter Diagnostic Lab ([email protected])
Tim Seipel, Extension Cropland Weed Specialist ([email protected])