WEST LAFAYETTE, Ind. — Palmer amaranth now has been
confirmed in 17 Indiana counties and no longer is 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
“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
n No hair;
n Leaves are wide and ovate to diamond-shaped;
n Petioles, stem-like structures that connect the leaf blade to the main
stem, are as long or longer than the leaf blade itself;
n 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;
n Females have a long main terminal seed head that can reach up to 3 feet
If growers identify what they believe to be Palmer amaranth
in their fields, they should report it to Johnson at (765) 494-4656 or
firstname.lastname@example.org or Travis Legleiter at (765) 496-2121 or
The best method of control, Johnson said, is a multistage
approach of crop rotation, thorough tillage, full rates of pre-emergence
residual and post-emergence herbicides, hand weeding and monitoring ditches and
field borders and cleaning equipment before moving from infested to non-infested
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
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
www.the-education-store.com. Search for WS-51.