Palmer amaranth confirmed in 17 Indiana counties

July 10, 2013  

Palmer amaranth

Palmer amaranth grows quickly and creates seeds rapidly, making it difficult to control. Weed scientists say it could be the most aggressive weed Indiana farmers have had to face. (Superior Ag Resources photo/Tom Sinnot)
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WEST LAFAYETTE, Ind. - Palmer amaranth now has been confirmed in 17 Indiana counties and is no longer confined to the northwestern part of the state.

Palmer amaranth grows quickly and creates seeds rapidly, making it hard for farmers to control. Populations earlier were found in the northwestern counties of LaPorte, Newton, Jasper, Pulaski and Cass. Since then, more have been confirmed in Benton, Porter, St. Joseph and White counties in that part of the state; Adams, Kosciusko, Huntington and Noble counties in other northern areas; Henry in the east-central area; Clay in the west; and Posey and Vanderburgh in the extreme southwest.

"Palmer amaranth is potentially the most aggressive agronomic weed Indiana producers have ever dealt with and must be managed with an aggressive control program," said Bill Johnson, Purdue Extension weed scientist. "Seed bank populations will increase quickly in fields where Palmer amaranth is not correctly identified or managed, leading to several years of expensive control programs."

Identifying this weed can be tricky because it closely resembles three other common amaranth species: redroot pigweed, smooth pigweed and common waterhemp. They can be differentiated by the presence or lack of hair, leaf shape, petiole length, apical meristem growth pattern, seed head structures and the leaf blade watermark.

Identifying characteristics of Palmer amaranth:

* No hair.

* Leaves are wide and ovate to diamond-shaped.

* Petioles, stemlike structures that connect the leaf blade to the main stem, are as long or longer than the leaf blade itself.

* The apical meristem grows to capture as much light as possible, resulting in a rosette-like appearance when looking directly down at the top of the plant.

* Females have a long main terminal seed head that can reach up to 3 feet long.

If growers identify what they believe to be Palmer amaranth in their fields, they should report it to Johnson at 765-494-4656,, or Travis Legleiter at 765-496-2121,

The best method of control, Johnson said, is a multistage approach of crop rotation, thorough tillage, full rates of pre-emergence residual and postemergence herbicides, hand weeding and monitoring ditches and field borders, and cleaning equipment before moving from infested to non-infested fields.

Many herbicides are labeled for Palmer amaranth control in corn, and growing corn for a few years consecutively might be the best option for growers who don't want to invest in expensive soybean herbicide programs.

This far along in the growing season, Legleiter said the best bet for farmers who find Palmer amaranth in their fields is to see if specific herbicides are allowed in reproductive-stage soybeans and to hand-remove escapes to prevent seed production. Herbicides are available for treating the plant in ditches and fencerows.

More information is available in Johnson and Legleiter's publication, Palmer Amaranth Biology, Identification, and Management, available for free download from Purdue Extension's The Education Store at Search for WS-51.

For Palmer amaranth updates, visit Purdue Extension's Pest and Crop Newsletter at

Writer: Jennifer Stewart, 765-494-6682, 

Sources: Bill Johnson, 765-494-4656,

Travis Legleiter, 765-496-2121,

Ag Communications: (765) 494-2722;
Keith Robinson,
Agriculture News Page

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