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This week waterhemp (Amaranthus tuberculatus) was documented in Montana in Roosevelt County. This is the first report of waterhemp in Eastern Montana. Waterhemp is a very problematic weed that causes extreme yield losses in crop fields in other parts of the United States and Canada. Waterhemp is widespread in cropping systems in the Midwest. Populations of waterhemp have evolved resistance to many herbicide modes of action used in crop production. Amaranthus species are often called pigweeds, and this group of plants includes common weeds like redroot pigweed, prostrate pigweed, and Palmer amaranth.

Now is the time to scout your field for waterhemp because plants are most easily identified now. The flowering inflorescence of waterhemp is the most distinguishing identifying feature. Waterhemp is a dioecious plant meaning there are separate male and female plants. The inflorescence is long and thin and the stems are smooth hairless compared to the more common pigweed species in Montana like redroot pigweed. See this fact sheet from North Dakota State University for a guide to identifying pigweeds.

Most populations of waterhemp are likely resistant to Group 2 (ALS herbicides) and glyphosate (Roundup). If a population is confirmed care should be taken to use effective herbicides and to continue to scout your field in the coming years. The next window for control will be in the spring of 2021.

 

If you have questions or suspect you have waterhemp contact your local Extension agent or agricultural specialist. Together we can prevent the establishment and spread of waterhemp in Montana.