Pigweed Problems

10 Aug 2016

By Pam Smith
DTN Progressive Farmer Crops Technology Editor

DECATUR, Ill. (DTN) -- Pollinator plots may be designed to attract beneficial insects, but there's a potential side effect to the good will. Weed scientists in Iowa and Illinois have discovered Palmer amaranth lurking in some plots and are warning growers to scout for the invasive weed.

Known as Palmer pigweed or just "pigweed" to Southern growers, the plant is a legitimate threat in the Midwest too, said Aaron Hager, University of Illinois weed scientist. Part of the weedy Amaranthus species, Palmer amaranth grows fast and is extremely competitive within Midwest cropping systems. Soybean yield losses approaching 80% and corn yield losses exceeding 90% have been reported in the peer-reviewed scientific literature.

"While most concern focuses on Palmer amaranth in agronomic cropping systems, keep in mind that Palmer amaranth also can become established in non-crop areas," Hager cautioned in a recent news release. Pigweed populations that set seed in non-crop environments can easily find their way into production fields.

Hager said that crop scientists recently verified the identification of a Palmer amaranth population growing in an area enrolled in the Pollinator Habitat Initiative of the Conservation Reserve Program (CRP). The origin of this population remains unknown, but it is speculated that a forb seed mixture purchased to sow the pollinator area might have been contaminated with Palmer amaranth seed.

IOWA FINDS

Palmer amaranth infestations have also been identified in Iowa in two new sites (Muscatine and Madison counties) this summer. Both infestations were found in CRP acres that were newly seeded this spring with a diverse mix of forbs and grasses, according to an Iowa State University bulletin prepared by agronomists Bob Hartzler and Meaghan Anderson.

The location in Muscatine County was seeded to a wildlife habitat mix, and the Madison County site was seeded to pollinator habitat. Individuals made both discoveries with significant experience working with and identifying weeds.

"We are confident that the Palmer amaranth was not in these fields prior to this year due to the low plant numbers and the random distribution of plants, rather than occurring in patches," the release said. "In addition, due to the expertise of the landowners, we believe they would have observed the Palmer amaranth in 2015 if it had been present."

CONTAMINATED SEED SOURCES

Earlier Midwest Palmer amaranth infestations are believed to have been introduced from cottonseed used for animal feed, commercial grain transportation or machinery. Iowa State agronomists said they've found no evidence that cover crop seed has been the source of Palmer amaranth infestations.

"Regardless of how and where a Palmer amaranth population becomes established, it remains critically important to take all appropriate steps to prevent established Palmer amaranth plants from producing seed," Hager said. "We strongly encourage all who have established pollinator habitats with a purchased forb seed mixture to scout these areas as soon as possible."

"If Palmer amaranth is identified, please take steps to remove these plants before viable seeds are produced on the female plants. Plants should be severed at or below the soil surface and carried out of the field. Severed plants can root at the stem if left on the soil surface, and plants can regenerate from stems severed above the soil surface," he said.

The goal is to identify new infestations before Palmer amaranth establishes a permanent seed bank. With the widespread distribution of waterhemp across the Midwest, it can be easy to overlook a new infestation. Palmer amaranth usually will have some leaves with a petiole longer than the leaf blade. Another clue is the large, sharp bracts on female Palmer amaranth plants that extend beyond the other flower parts, giving the inflorescence a spiky appearance. Palmer is usually larger and more aggressive than waterhemp too.

The Iowa State University agronomists said the finding of Palmer amaranth in native plantings is a concern, but not a reason to avoid establishing these type of plots. Knowing your seed dealer is a first step to procuring uncontaminated seed, Hartzler told DTN. "However, because Palmer amaranth is not considered a noxious weed here, there's nothing stopping anyone from selling seed that has it in there," he said.

"It's a tough situation, but by keeping an eye on these plantings, the risks can be minimized," he added. Scouting new plantings should be a standard practice, especially in mid-late summer when it will be easier to identify the obnoxious intruder.

For more guidelines on how to identify Palmer amaranth go to http://bit.ly/…

Find an Iowa State University bulletin on the issue here: http://bit.ly/…

Pamela Smith can be reached at Pamela.smith@dtn.com

Follow Pamela Smith on Twitter @PamSmithDTN

(AG/BAS)