Although Minnesota has been successful in controlling the invasive weed, Palmer amaranth, a 10th county in the state has been infested.

First detected in the state back in 2016, it was found in Douglas County in 2017. A total of 73.11 acres were infested across seven sites, according to the Minnesota Department of Agriculture.

The weed was also confirmed in neighboring Todd County that year and 132.49 acres were infested in six sites.

In late May, the agriculture department positively identified the weed in Polk County. The confirmation came after the MDA inspected a field being used for the disposal of agricultural screenings.

The MDA collected a sample of the screenings material and determined by genetic testing that it was contaminated with Palmer amaranth seed. The field was scouted, and MDA staff found several dead Palmer amaranth plants on the field’s edge remaining from last year, suggesting the plants had grown and matured.

The landowner is working with the department to eradicate any of the weeds moving forward. At this time, the MDA believes the issue is isolated to only one field. The field and the surrounding area will be a priority for MDA field scouting this summer.

Fortunately, most of the sites in Minnesota have been successfully eradicated and the remaining are being closely monitored, according to the MDA.

Agricultural leaders credited their success to keep the weed in check to a collaborative effort among the Minnesota Department of Agriculture, University of Minnesota Extension, farmers, and other partners. They've been able to identify the weed in fields, determine how it got to Minnesota, and implement strategies to eliminate infestations.

"Although our work with Palmer amaranth is far from completed, what we have accomplished has been critical to protecting Minnesota’s ag economy from this serious threat,” said Minnesota Agriculture Commissioner Thom Petersen. “We believe this collaborative effort to fighting invasives can work in many settings.”

About Palmer amaranth

Palmer amaranth is listed as a noxious weed in Minnesota. All above and below ground parts of the plant must be destroyed, and it cannot be moved.

The weed is also listed as a prohibited weed seed in the state. This means no Palmer amaranth is allowed in any seed offered for sale in Minnesota.

Left uncontrolled, a single female Palmer amaranth plant typically produces 100,000 to 500,000 seeds, according to the department of agriculture. It is resistant to multiple herbicides, can cause substantial yield losses, and greatly increase weed management costs in soybeans and corn.

Palmer amaranth is documented in 28 states including Minnesota and neighboring South Dakota, Iowa and Wisconsin.

Palmer amaranth competes aggressively with crops. It has a fast growth rate of 2-3 inches per day and commonly reaches heights of 6 to 8 feet, greatly inhibiting crop growth. Yield losses have been up to 91% in corn and 79% in soybean.

Prevention and management

The MDA offers the following advice:

Be proactive and prevent Palmer amaranth establishment. Familiarize yourself with Palmer amaranth identification and actively look for it in crop fields, borders, ditches and around dairies. Palmer amaranth is difficult to control because it can be resistant to multiple classes of herbicides and their different modes of action. Populations of Palmer amaranth have been documented with resistance to one or more of the following classes: Dinitroanilines, triazines, ALS (acetolactate synthase) inhibitors, glyphosate and HPPD (4-hydroxyphenylpyruvate dioxygenase) inhibitor herbicide.

Mowing alone is not as effective as cultivation, as Palmer amaranth plants are usually not killed by mowing. They can regrow from cut stalks and set seed close to the ground. Mowing must therefore be done in conjunction with other methods of control like herbicide application, prescribed fire or propane weed torching to be effective.

Prevent all Palmer amaranth plants from producing seed if possible. Plants can be hand weeded and removed for disposal. If hand weeding is not feasible, contact your local University of Minnesota Extension agent, co-op, or certified landscape care expert for a specific herbicide recommendation.

Always clean vehicles and equipment after exiting infested areas. If seed was produced, deep tillage will reduce the quantity of seeds that can readily germinate.

A cereal rye cover crop can reduce Palmer amaranth germination and growth.

How to spot Palmer amaranth

Other information from the MDA:

The green leaves are smooth and arranged in an alternate pattern that grows symmetrically around the stem. The leaves are oval to diamond-shaped. There is a small, sharp spine at the leaf tip.

The leaves of some Palmer amaranth plants have a whitish V-shaped mark on them. Not all Palmer amaranth plants display this characteristic.

There are separate male and female plants.

Palmer amaranth looks similar to native pigweeds such as tall waterhemp, redroot and smooth pigweeds.

Seed is the means of spread and female plants are prolific seed producers. A study in Missouri documented more than 250,000 seeds produced per plant. Seed can be spread in water movement, by wildlife and via agricultural practices such as plowing, harvesting and spreading manure.

For more information, go to the website, www.mda.state.mn.us/palmeramaranth.