Preventing Palmer amaranth in Minnesota
In September 2016, Palmer amaranth (Amaranthus palmeri) was initially discovered and confirmed in Minnesota. Efforts to eradicate this weed are critical to Minnesota’s commodity crop producers.
Palmer amaranth is on the Minnesota Noxious Weed List as an “Eradicate” weed. This legal status means the plant must be destroyed and that no transportation, propagation or sale of this plant species is allowed.
Report possible infestations to the Minnesota Department of Agriculture (MDA) at 1-888-545-6684 or email@example.com.
Handling Palmer amaranth
Palmer amaranth isn’t native to the northern United States, but has spread northward from southern states. It’s been confirmed in Wisconsin, Iowa, Illinois and Nebraska from 2011 to 2013, South Dakota in 2014 and other northern states.
In August 2016, it was discovered in newly seeded Conservation Reserve Program (CRP) land in Iowa, and that September, it was confirmed in Yellow Medicine county in Minnesota.
Palmer amaranth is the most competitive and aggressive pigweed species. It’s related to waterhemp and, like waterhemp, it emerges throughout the growing season, from May to August. However, Palmer amaranth is much more aggressive than waterhemp, growing 2 to 3 inches a day.
Palmer amaranth can quickly adapt to herbicide management tactics that don’t include diverse effective sites of action (SOAs), ultimately limiting control options. In the northern states, Palmer amaranth is expected to be resistant to multiple herbicides, including glyphosate (SOA group 9) and acetolactate synthase (ALS) inhibitors (SOA group 2).
Palmer is dioecious, with male and female plants. Outcrossing results in rapid spread of herbicide resistance.
It’s a prolific seed producer, with a single female plant typically producing 100,000 to 500,000 seeds.
Timing of herbicide application is critical. Effective preemergence herbicides need to be followed by timely (plants less than 3 inches tall) and effective postemergence herbicides.
Threat to crop production
Palmer amaranth infestations have caused substantial yield losses and greatly increased weed management costs in cotton, soybeans and corn in the southern states. Once established in the northern corn and soybean states, it’ll likely do the same and significantly increase costs and decrease yields.
First, scout your fields with a particular focus on native seed planting areas.
Look for pigweed plants that can reach heights of 6 to 7 feet in unmown areas, as well as the following key characteristics:
Leaves: Ovate- to diamond-shaped leaves, which give it a poinsettia-like appearance (Photo 3). No hairs.
Inflorescences: Prolific seed producer with a long main terminal seed head (inflorescence) that’s up to 3 feet long (Photos 1 and 4). Female inflorescence have sharp, spiny bracts (Photo 5).
Plant characteristics: No hairs on stems and leaves, unlike redroot pigweed. Petiole is as long as or longer than the leaf (Photo 6).
Palmer amaranth’s legal status as a state-prohibited noxious weed on the “Eradicate” list means the plant must be destroyed and that no transportation, propagation or sale of this plant species is allowed.
This law gives MDA, county, city and township officials the right to inspect land areas believed to be affected by this weed and ask owners to destroy the plants. It also allows MDA officials to investigate where potential seed lot contamination sources are occurring.
How to report
If you suspect a weed is a Palmer amaranth, email the following to firstname.lastname@example.org:
Photos similar to Photos 4 to 6.
The pests’ location. Be as specific as possible.
Your contact information.
Seed source (if known).
A description of the area where the plant was found.
Why to report
This information helps the MDA understand the weed’s current distribution and its potential to spread to your fields and adjacent lands. Reporting also allows local officials to help monitor the site in subsequent years to make sure there were no escapes. This is particularly important if land ownership changes in the near future.
Destroying the plants
After photographing and reporting, destroy the plants in question.
Note there’s a very short timeframe to accomplish eradication. However, due to the economic significance of this invasive weed, it’s worth our collective effort to try.
If the Palmer amaranth population is small in number:
Weed plants by hand.
Place them in a large paper bag.
Remove it from the field to a site suitable for burning the plant.
Burn the plant.
Again, weed it, bag it, drag it and burn it. Please don’t literally drag it; it just makes the process easier to remember.
For a larger population of plants at the mature stage of the weed’s life cycle, particularly when the seed is being set:
Mow the area.
Clean off the mower on-site. This prevents the spread of any weed seed shed onto the mower.
Mowing doesn’t kill the entire plant, but it’ll keep the seed on the ground in the affected area where insects and rodents can feed on it. Mowing will reduce movement away from the affected area. Plus, any seed that germinates the following year will have to compete with the more established plants that surround it.
It’s important to closely monitor the affected and surrounding area the following year to make sure there are no weeds that escaped detection or seeds that have moved into adjacent lands. By reporting your affected site to the MDA, they’ll be able to assist you with this monitoring procedure.
Palmer amaranth has spread from the south through:
Contaminated feed, including cottonseed and hay.
Contaminated grain and seed.
Farm equipment and manure.
In Iowa and Indiana, the most recent infestations were in newly seeded conservation plantings (i.e., Conservation Reserve Program (CRP), wildlife, pollinator and cover crop plantings) where the seed mix was contaminated. Ohio and Illinois also reported contaminated conservation seed mixes as a source of Palmer amaranth introduction.
Palmer amaranth may not persist in areas being established for conservation habitat. This is because Palmer amaranth should be crowded out once native, perennial vegetation establishes. However, until perennial plants become established, Palmer amaranth may produce enough seed to establish a seed bank and move into neighboring corn and soybean fields.
More about Palmer amaranth in other states:
Early identification is key to prevent the spread of Palmer amaranth. If plants are found, remove and destroy them. Above all, they shouldn’t be allowed to go to seed.
According to Minnesota statutes, landowners must attempt to eradicate any Palmer amaranth found by destroying all the above and below ground parts of the plants.
Full labeled rates of residual herbicides with multiple Site of Actions (SOAs) are a must in corn and soybeans.
If a field becomes infested, rotate it to a crop with more diverse management options, such as alfalfa, small grains or even corn.
Early postemergence herbicide applications (when Palmer is less than 3 inches) are critical in managing this weed. Remember, Palmer can grow 2 to 3 inches in a day, so it can quickly exceed heights that herbicides can acceptably control.
In addition to contacting the MDA if you suspect Palmer amaranth, please contact University of Minnesota weed scientists:
Reviewed in 2018