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Problematic Weed Species Pushes into MN

May 6, 2019

Palmer amaranth is not native to the northern U.S. Yet, reports of the weed have popped up in states such as WI, IA, IL, NE, SD and, now, MN.

The weed was first discovered there in 2016, but just 3 years later it has been reported in 6 more counties. The Minnesota Department of Agriculture lists the weed as “Prohibited-Eradicate,” meaning all above- and below-ground portions of the plant must be destroyed under Minnesota Noxious Weed Law. It cannot be transported, propagated or sold.

It’s safe to say MN is taking this weed seriously. Here’s why.

If left unchecked, Palmer amaranth can be devastating to soybean and corn fields. In its report on the spread of Palmer amaranth, the Minnesota Department of Agriculture states that this weed has caused yield losses of up to 91% in corn and 79% in soybeans.

The University of Minnesota Extension calls it the most competitive and aggressive of the pigweed species, growing between 2 and 3” per day. Taking that a step further, Palmer amaranth can grow up to 8’ tall, with stalks several inches thick at the base. This could make harvest nearly impossible. Additionally, one female plant can produce up to half a million seeds in one season. If not dealt with early, it could continue to be a problem for years in a field.

Identifying Palmer Amaranth

With Palmer amaranth, early identification is important. Because they are in the same species, Palmer amaranth can look similar to another weed that is more common in parts of the northern U.S.: waterhemp.

However, there are some clear differences to watch for:

  • Palmer amaranth is broader and taller than waterhemp.
  • Its leaves are broader and end in a sharp point.
  • The petiole – the spine that attaches to the stem and the leaf – is longer than the leaf.

Additionally, these images show some more ways to identify Palmer amaranth:

This agronomic image shows identifying characteristics for Palmer amaranth

This agronomic image shows identifying characteristics for Palmer amaranth

For more information on identifying Palmer amaranth check out our video below:

Managing Palmer Amaranth

With weed control in general, it’s crucial to use a 2-pass herbicide system with overlapping residuals. This means applying a pre-emergence residual herbicide and a post-emergence residual herbicide. Such a program should prevent the weed from breaking through applications.

It’s also critical to use full labeled herbicide rates. Herbicide resistance continues to be a primary concern, and using half or cut rates may not kill the weed. If a damaged weed is able to regain strength, it may do so with a new resistance to the herbicide application.

For corn growers, we recommend using Acuron® or Acuron Flexi corn herbicides followed by Halex® GT herbicide for Palmer control. This program uses 4 effective sites of action for both contact and residual control of tough weeds.

For soybean growers, consider Boundary® 6.5 EC or BroadAxe® XC herbicides followed by Tavium® Plus VaporGrip® Technology or Flexstar® GT 3.5 herbicides. Tavium is a new offering that combines the effective contact control of dicamba with the residual control of ­S-metolachlor, providing 3 weeks longer residual than dicamba alone.

While herbicides are a good tool to fight weeds, they aren’t the only option. In addition to a strong 2-pass herbicide program, growers should utilize diversified management plans that include the use of cover crops and/or crop rotation, and sound agronomic practices that include narrow rows, increased plant populations and other practices to promote crop growth.

Visit ResistanceFighter.com to learn more about this and other techniques for managing tough and resistant weeds.

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