Aggressive Palmer amaranth weed found in northern Indiana

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Populations of the fast-spreading Palmer amaranth weed have been confirmed in five counties in northwestern Indiana, a Purdue Extension weed specialist says.

At least 50 corn and soybean fields of Jasper, Newton, Pulaski, LaPorte and Cass counties have verified infestations.

Palmer amaranth is a green, flowering plant that has caused widespread damage in cotton production in southern states. Most populations are glyphosate-resistant, and the weed thrives in summer heat, can grow upwards of 2 inches per day and reach heights greater than 7 feet.

Travis Legleiter, Purdue weed science program specialist, said the rapid growth and general hardiness of the weed makes it a problem in corn and soybean fields.

"Palmer amaranth's competitiveness is what makes it a concern for us," he said. "We know from our counterparts in the South how devastating this weed can be to crops. And now we know Palmer amaranth can be competitive in northern geographies, too."

Farmers might be familiar with other amaranth species such as Redroot, smooth pigweed or waterhemp. In fact, the confirmed cases were misidentified as waterhemp before farmers noticed that their applications of traditional herbicides such as glyphosate weren't controlling the problem.

Legleiter said farmers across Indiana should be aware of the Palmer species and know it differs from other species.

"Palmer amaranth does have distinguishing characteristics. The first thing is the length of the petiole, the stalk that joins the leaf and stem. It will be significantly longer, especially on older plants. If you take the petiole and leaf blade and fold them over, the petiole is longer on Palmer amaranth."

The plant is hairless and has multiple seed heads, with one terminal, female spiny seed head that's 3-4 feet long and can produce 500,000 seeds.

Farmers should use pre-emerge herbicides on both corn and soybean fields. Some post-emerge herbicides can help farmers with infestations in corn fields, but those with soybeans must start with a clean field the next season and use the pre-emerge herbicides.

For more information, see a publication from Legleiter and Bill Johnson, Purdue weed scientist, at https://ag.purdue.edu/btny/weedscience/Documents/Palmer_Amaranth.pdf

Legleiter advises farmers to contact their local Extension agent if they suspect a Palmer amaranth infestation in their fields.

Farmers should also be aware of what they're bringing onto their farms that the Palmer seed could travel in or on, such as manure, feeds and equipment. The source of these weed populations may be from cattle manure spread over fields, with feed rations that contained cotton seed or cotton seed hulls infested with the palmer seed.

This isn't the first discovery of Palmer amaranth in Indiana, but it's the most significant because of its crossover to upland fields in northern Indiana. During the 2011 growing season, populations seemed to be confined to river bottoms in southwestern Indiana.

Legleiter and Johnson are continuing to research Palmer amaranth in Indiana and plan to release more publications as they gain more insight into the competitive weed in the northern United States.



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