Palmer Amaranth on the Move in the Midwest

Native to the southwestern United States, Palmer amaranth has become a problem weed for many Midwest growers in recent years. In 2018, Palmer amaranth moved into North Dakota and Wyoming, and is now present in 39 of the 48 continental U.S. states.

Palmer amaranth is native in Arizona, California, New Mexico and Texas, but since the early 1900s, it has been on the move. As early as 1915, Palmer amaranth was documented in Virginia, and throughout the 20th century spread to the southeastern United States.ⁱ

Palmer amaranth infestations on the rise in the Midwest

More recently, Palmer amaranth has made its way into the Midwest. Indiana was first affected in 2011; followed by Illinois, Michigan, and Ohio in 2012. In 2013, Wisconsin became affected, followed by South Dakota in 2014. In 2018, Palmer amaranth was discovered in North Dakota and Wyoming.ⁱⁱ Now, only nine of the 48 continental U.S. states remain unaffected by this prolific weed.

The number of Midwestern counties affected by Palmer amaranth also continues to grow. For example, in 2013, Iowa had only three counties with known infestations of Palmer amaranth. By mid-2018, the number increased to 53 counties with known infestations. This trend is not unique to Iowa.ⁱⁱⁱ Most Midwestern states have seen increases in the number of counties affected by Palmer amaranth. The chart below details the increases in Midwestern states over the past five years.

The problem with Palmer amaranth

Growers in the south have been dealing with herbicide-resistant Palmer amaranth for years. Palmer amaranth is dioecious, having separate male and female plants. The result is greater genetic diversity for plants to develop adaptive traits, such as herbicide resistance.^iv If a Palmer amaranth plant is herbicide-resistant, it becomes even more important to make sure it is controlled before producing seeds.  

Palmer amaranth is aggressive competition for field crops. Plants can grow two-to-three inches per day. At full maturity, the plants may grow to be six-to-eight feet tall. If left unmanaged, Palmer amaranth has the potential to reduce corn yields by up to 91 percent.^v

Look for Palmer amaranth during harvest

Many growers focus on weed control during the season, but harvest provides a great vantage point from the combine to identify weed-control issues. Look between rows to see if Palmer amaranth is present in the fields. If the weed is found, it is important to work with agronomists and consultants to develop a plan to prepare for future seasons and lower the number of seeds in the weed seed bank.  

As Palmer amaranth has quickly gained traction in the U.S., it is also important to develop a weed management plan to help prevent it from spreading areas that are currently unaffected. Agronomists and local university extension offices can provide information about the presence of Palmer amaranth.

Take control with a zero tolerance approach

To prevent movement and infestation of Palmer amaranth, it is important to take a zero tolerance approach.

“A zero tolerance approach focuses on preventing weeds from producing seeds,” said Frank Rittemann, selective corn herbicides product manager at Bayer. “It is the key to fighting back against and preventing the spread of tough weeds like Palmer amaranth. A zero tolerance approach is also a key component of an integrated weed management program.”

Well-planned integrated weed management programs start with clean fields and focus on keeping fields clean throughout the season. Growers should also use multiple effective sites of action.

"It's also important to use a pre-emergence, residual herbicide,” Rittemann said. "Cover crops and tillage can also help control weeds.”  

Rittemann’s recommendation for a zero tolerance approach starts with Corvus^®, tankmixed with atrazine for a pre-emergence herbicide. "It kills weeds that are not out of the ground and provides burndown for the ones that have already emerged. Corvus helps prevent weeds from becoming a problem right away from the start, early on in the season," he said.

With a zero tolerance approach, it is also important to perform a second-pass with a postemergence herbicide that is effective on problem weeds like Palmer amaranth. Rittemann recommends DiFlexx^® DUO, tankmixed with glyphosate.

Postemergence herbicides work best when applied within the correct application timeframe. "Correct timing strikes a balance between weed size and crop growth stage," said Rittemann. "It is important to take weeds out when they are three inches or less."

As part of a two-pass herbicide program, Corvus tankmixed with atrazine, followed by DiFlexx DUO tankmixed with Roundup PowerMAX^®, provides multiple effective sites of action and uses six distinct and powerful ingredients.

Throughout the growing season, growers should also scout fields and identify problems. This is a key component of a successful integrated weed management program. If a Palmer amaranth plant or other tough weed is found, it is important to pull it out immediately. This practice is most beneficial if done before the plant goes to seed.

Most importantly, be proactive. Take a zero tolerance approach to fight back against Palmer amaranth and other tough weeds.