A new, difficult to control weed is showing up in some Iowa corn and soybean fields called Palmer amaranth. It's a close cousin to waterhemp, but is even more difficult to control. Palmer amaranth has caused big problems in the southern U.S. and now it has marched northward and is starting to show up in Iowa.
"Now is the easiest time to scout your fields to try to find new infestations of Palmer amaranth and initiate programs to either eradicate or limit its spread," says Bob Hartzler, an Iowa State University Extension weed management specialist. "We at ISU Extension weed science appreciate being informed of new Palmer amaranth infestations in the state and we are willing to help farmers, crop consultants and others in properly identifying suspect plants." The lack of reliable traits to distinguish Palmer amaranth and waterhemp during vegetative stages complicates efforts at stopping the spread of Palmer amaranth across the state, he notes. "However, both plants should be in full reproductive mode at this time, greatly simplifying the identification of the two amaranths," says Hartzler.
How to tell the difference between Palmer and waterhemp
While most agronomists and weed scientists prefer to identify weeds using vegetative traits, the small bracts (modified leaves) associated with the flowers of Palmer amaranth and waterhemp are the most, if not only, reliable way to differentiate the two species, says Hartzler. Palmer amaranth has relatively large, green bracts that extend well beyond the other flower parts, whereas on waterhemp the bracts are similar in length to the tepals surrounding the seed capsule.
On close examination, Palmer amaranth's bracts on mature female plants are easily seen protruding from the plant's seedheads without the use of a hand lens, he explains. Redroot and smooth pigweed also have large bracts; however, these species have hairy stems in contrast to the smooth stems of Palmer amaranth and waterhemp.
Palmer has now been found in five counties in Iowa
Several people from across the state have submitted photos or samples of plants suspected of being Palmer amaranth, to ISU Extension weed specialists for proper identification. In all but one case they were simply 'healthy' waterhemp, says Hartzler. The exception was from Lee County in the southeast corner of Iowa. This brings the number of counties with confirmed infestations of Palmer amaranth to five: Fremont, Harrison, Lee, Muscatine and Page.
Palmer amaranth work is making a difference
A Muscatine County farmer's determination to eradicate a relatively new yield-robbing weed in Iowa is inspiring others to do the same.
Roger Hargrafen discovered Palmer amaranth — a tough herbicide resistant weed native to the southwest U.S. capable of decimating row crops — in a 5-acre section of a soybean field last September. Since then, he's diligently worked to get rid of the weed and helped spread the word why that's important and how to do it.
Hargrafen, with assistance from ISU Extension, has the Palmer outbreak under control. ISU Extension and the Iowa Soybean Association are working together to provide information to keep the weed at bay. ISU weed and agronomy experts believe Hargrafen's actions are making a difference. Steps taken include not harvesting the affected area, planting cover crops, intensive management such as scouting and hand weeding, participating in a herbicide trial and using herbicides with effective, multiple modes of action, thoroughly cleaning equipment and more.
Hargrafen also became a weed control advocate, spreading the word about the need to scout fields for Palmer amaranth and take action to control it. "This is having a positive impact," says Virgil Schmitt, ISU field agronomist in southeast Iowa. "I said before he was the 'poster boy' for doing things right, and that's still true."
In Iowa, five counties now have documented cases of Palmer
The only new documented case of Palmer in Iowa this year is in Lee County. Other counties include Muscatine, Harrison, Page and Fremont. Earlier this year, ISU officials were concerned Palmer in Harrison County could get out of control. But after Hargrafen's story broke, with assistance from ISA, fears subsided.
Bob Hartzler, ISU weed specialist, says the farmer dealing with Palmer in Harrison County followed Hargrafen's lead and is actively working to control and eradicate the weed. "He got things on track, which is good news," Hartzler adds.
Producers statewide are more vigilant as well. More and more weed samples are being sent to ISU to be identified, Hartzler says. Farmers can take pictures of suspected Palmer plants and email them to ISU weed experts Hartzler, [email protected] and Mike Owen, [email protected] or any ISU Extension field agronomist.
Mid-July through mid-August is best time to identify Palmer
Mid-July through mid-August is the best time to identify Palmer, Hartzler says. Palmer looks almost identical to waterhemp, the most prevalent herbicide-resistant weed in Iowa, but Palmer is more hardy and tougher to control.
There are differences between these two weeds, which are easier to distinguish when the plants are flowering. For example, Palmer generally has a longer and thicker flower stalk and its bracts (modified leaves at the base of flowers) are 3 to 7 millimeters long, while waterhemp bracts are shorter.
Control strategies that work for waterhemp will work for Palmer. But Palmer is hardier and weaker doses of herbicides won't work. That's why identification is important. "That's what we're trying to stress now. Anything that looks like Palmer needs to be checked out," Hartzler says.
Farmers need to prevent Palmer from going to seed
Hargrafen, the Muscatine County farmer, isn't letting Palmer go to seed. By preventing it from going to seed, the seedbed becomes less and less abundant, and eventually disappears. That could take five years. "Roger made a commitment to stick with a plan until nothing comes back," Schmitt says.
Taking action against Palmer early, even though there's a significant investment in time and money, is the best alternative. Hargrafen sacrificed thousands in crop revenue by idling the ground that has the Palmer until it's totally gone.
Palmer produces twice the amount of biomass as waterhemp, which means it steals more water and nutrients from crops. A mild to moderate infestation of Palmer can result in soybean yield losses of up to 30%. That could mean a potential revenue hit of more than $200 per acre. Some severe cases have left fields in the Southern U.S. unharvestable. "I think the strategy Roger, the Muscatine County farmer, is using will save him significant money down the line in weed control costs," Hartzler says.