For years growers have been hearing about this super weed, nicknamed Satin, that has been infesting southern and midwestern states for the past 12 years. They’ve heard horror stories about how it can rob yield, become resistant to any herbicide that is thrown at it, or plug up a combine in the blink of an eye. Since 2004, this invasive weed species known as Palmer amaranth, has migrated from its indigenous desert habitat of the southwestern United States all the way to the southeast and is approaching the doorstep of the upper Midwest, including southern Minnesota and the Red River Valley.
Palmer amaranth (Amaranthus palmeri) is a member of the pigweed family. Depending on where a grower is located, they may encounter other pigweeds such as redroot pigweed, waterhemp, powell pigweed and smooth pigweed. Palmer amaranth is most closely related to waterhemp because of the fact they share many key characteristics. Perhaps most notably is the dioecious nature of the two weeds, meaning the species has both male and female plants.
“What that ultimately means is there is a lot more diversity in waterhemp and palmer amaranth than in pigweed,” said North Dakota State University and University of Minnesota Sugarbeet and Weed Science Extension Agronomist, Tom Peters. “Diversity means two things. If you are out walking and see these plants, they will look slightly different. The plants can be taller or shorter, leaves can be wider or narrower, the morphology of the plant can be different. The other concern with diversity is weed resistance. Many of the different families of herbicides that we are using have already developed resistance to waterhemp, and especially Palmer amaranth.”
The reason for that is the biotypes that have been discovered in southern Minnesota have come from other regions where they developed herbicide tolerance or resistance. “There is a characteristic that weed scientists talk a lot out about, it’s called multiple resistance,” continued Peters. “It describes how a plant can not only have resistance to glyphosate, but it can also have resistance to other herbicides such as PPO inhibitors, Triazines, or maybe ALS inhibitors. A single plant can be resistant to sites of action two, five, nine, and 14. That’s what we’re scared about with Palmer amaranth.”
Currently, Palmer amaranth has been identified in four counties in Minnesota; Yellow Medicine, Big Stone, Lyon and Swift. Those four counties are in sugarbeet producing areas. “This will be an extremely important topic for farmers within the Southern Minnesota Beet Sugar Cooperative as well as farmers that are within southern regions of Minn-Dak Farmers Cooperative.”
Recent infestations of Palmer amaranth have been related to newly-seeded conservation plantings where the seed was contaminated. Once the weed is introduced to an area, it’s aggressive nature can take over quickly. The Palmer amaranth may produce enough seed to build up a seed bank in the conservation habitat and in surrounding cropland before perennial plants have a chance to become established and compete with the Palmer amaranth. “We want to make sure we fight it when it’s in CRP. It’s important that we identify it and eliminating it when it is in the CRP plantings and not allow it to get established in our sugarbeet fields,” said Peters.
Once Palmer amaranth escapes and moves into cropland, the effects can be devastating if it is left unidentified or untreated. It’s emergence pattern mirrors that of waterhemp where it requires more growing degree days to germinate. That means it won’t surface until mid-May on average in the Red River Valley or southern Minnesota. However, once emergence has begun, Palmer amaranth will continue to emerge into early August, in response to a rainfall event, and more often than not the extremely late emerging weeds will still produce seed. “Mother nature is programed to reproduce. It wants to make seed. Even if it’s a few seeds on a plant that is two or three inches tall.”
While it has its similarities to waterhemp, once developed, Palmer amaranth grows much quicker and becomes a much longer plant overall. Given pristine growing conditions, waterhemp can grow up to one inch per day, while Palmer amaranth has been noted to grow between two and three inches per day. Furthermore, waterhemp’s average seed production at 150,000 seeds per plant is miniscule compared to the 500,000-one million seeds that Palmer amaranth is capable to producing. In general, Palmer amaranth is just a bigger, stronger weed. Its height can reach up six to eight feet, while branching out to four to six feet wide, with extremely dense stems that can become up to three inches thick.
Once in the seedbed, Palmer amaranth seeds can be viable for four to six years. “Generally they are most plentiful in the first four years. Then they diminish and become less viable in sequential years and are essentially gone after six years,” continued Peters.
All of those factors make it essential to control Palmer amaranth in it’s earliest stages. “The key is to get it when it’s small. In the greenhouse we sprayed at three different stages. At zero to two inches, we have reasonably good control. Many of our treatments are getting near perfect control. When it grows to two to four inches, the effectiveness drops down to 60-70%. When it’s four to eight inches, it declines to 20-40% control. The control really declines rapidly as the plant gets bigger.” Peters’ greenhouse trial included a postemergence program that consisted of glyphosate, ethofumesate, Betamix and UpBeet, which is modeled after the method used in Michigan where Palmer amaranth is present. The effectiveness of pre-emerge herbicides is still unknown, but studies show it has good control if the herbicide becomes activated.
As it currently stands, there have been no documented cases of Palmer amaranth in the state of North Dakota. Many fear, however, that it is only a matter of time before the superweed migrates into the Peace Garden state. “It’s inevitable that [North Dakota] will get Palmer amaranth, but we can prevent it from entering for as long as possible. There are a few ways to do that, starting with scouting. If you see things that don’t look right get help. Talk to your retailer, crop consultant, agriculturist, or the extension service.”
Additionally, soil-applied chloroacetamide herbicides that sugarbeet growers use to protect against waterhemp, are effective on small-seeded broadleaves and can also be used in corn and soybeans. Another key method of prevention is the maintaining of areas that border the fields. “I believe that a lot of the weeds that we have moved from non-crop areas, such as ditches or areas of farmsteads where equipment is parked. Maintain those areas, specifically, mow them off and don’t let those weeds make seed.”
When preventative measures fail, eradication should be the next goal. “I would not combine an area where there is a heavy density of a pigweed species like waterhemp and Palmer amaranth. First of all, I would prefer that we mow those areas off before they start producing seed. Just make a commitment to yourself early in the season that you are not doing to harvest that patch and cut your losses. But if they do produce seed, don’t harvest them. You would just be using your combine as a spreader.
“Thresholds are also an important factor when dealing with this weed. When I was a college student in 1980, what I learned in the classroom is that we should target 85-90% control. What my instructors said at the time was that the cost to get the extra 10% control did not warrant the expense. So there was this idea that a 90% threshold was enough. That’s not the case anymore with resistant weeds. The reason for that is every one of those plants you leave behind is a factory making more seeds. Part of the weed management strategy is to not allow new seed to enter the seed bank. We need to strive for complete, 100% control, even if that means going out and cultivating or pulling weeds, that’s what he have to do. I’m not saying that my professors in 1980 have changed their minds, it’s just a whole different dynamic now than it was years ago,” concluded Peters.
Editor & General Manager of The Sugarbeet Grower