Several new infestations of Palmer amaranth, an aggressive and hard to control weed that’s relatively new to Iowa, have been found in the state this summer. Palmer amaranth is an invasive weed that is native to the southwestern United States. Over the last three years, however, this weed has made its way into Iowa and has now been identified in at least nine counties, most recently in Madison, Clayton, Washington and Crawford counties.

LEAVES: Comparing Palmer amaranth vs. waterhemp, note that the petioles on Palmer leaves are often longer than the leaf blade.

Palmer amaranth is related to waterhemp, a common weed found across Iowa. In addition to traditional weedy traits, the characteristic that makes those two weeds unique, and hard to manage, is their ability to evolve resistance to herbicides, says Bob Hartzler, an Iowa State University Extension weed specialist.

 Stopping the spread of Palmer—an aggressive, competitive weed
“Both of these weeds are adapted to a production system that relies on herbicides,” says Hartzler. “Waterhemp is a relatively non-competitive weed so many farmers have learned to accept lower levels of weed control. If Palmer amaranth spreads across the state and farmers try to manage it like they do waterhemp, it will have a huge negative economic impact.”

 The known cases of Palmer amaranth in Iowa are isolated right now, and keeping this weed from spreading is the immediate need.

BRACTS: Female Palmer amaranth flowers have large, sharp bracts (shown here). Waterhemp bracts remain soft.

“Palmer amaranth built its reputation on how it devastated the cotton industry in the south after the near complete reliance on glyphosate in Roundup Ready cotton,” notes Hartzler. “The best way for us to manage it here in Iowa is to slow its spread into and within the state. This is a highly competitive weed that has adapted to our current management system for corn and soybean crops.”

Potential crop impact from conservation/habitat restoration areas
The initial Palmer amaranth infestations in Iowa were found in row crop fields associated with equipment and inputs such as seed grown from outside of the state. More recently, Palmer amaranth has been found growing on ground that’s been set aside for conservation practices, such as CRP acres.

The weed most likely will not persist in these new locations being established for conservation habitat. That’s because the Palmer amaranth should be crowded out once native, perennial vegetation is established. The concern, however, is that until the perennial plants become established, Palmer amaranth may produce enough seed to begin moving into neighboring corn and soybean fields.

“With Palmer amaranth being so much more competitive than waterhemp, Palmer infestations could have a significant economic impact due to higher weed management costs and greater yield losses,” says Hartzler.

 Identifying Palmer amaranth—how to tell the difference
A video of Hartzler describing Palmer amaranth and how to identify the weed can be viewed at vimeo.com/178360206. Hartzler says Palmer amaranth needs to be identified early so it can be eradicated before it has an opportunity to establish itself in a field. The difficulty arises because of how closely Palmer amaranth resembles waterhemp; the two weeds are virtually identical in their early growth stages.

Compare the petiole and leaf blade, consistent vegetative trait that can be used to differentiate Palmer amaranth from waterhemp is the relative length of the leaf petiole and leaf blade. After removing a leaf from the stem, you should bend the leaf backward so the petiole lays over the leaf blade. If the petiole is longer than the blade, the weed in question is most likely Palmer amaranth, says Hartzler. Not all leaves on a Palmer amaranth plant have this trait, but most plants will have some leaves with the long petiole.

IDENTIFY CORRECTLY: This is a typical Palmer amaranth seedhead or inflorescence. From a distance Palmer can often be differentiated from waterhemp due to the presence of long terminal branches on the seedheads.

Female Palmer amaranth plants have long bracts (modified leaves on the flower stem) that extend well beyond the other floral parts of the weed. As the plants mature, these bracts become sharp and painful to the touch while waterhemp bracts remain soft. The flowering branches on Palmer amaranth are longer than those of waterhemp, and have a spiky appearance because of the large bracts.

What are best treatment options to control Palmer?
“The one thing we have going for us is that every corn and soybean field has waterhemp infestations, so farmers have developed weed management programs targeting waterhemp,” says Hartzler.

“Programs that are effective on waterhemp should provide effective control of Palmer amaranth. That alone will make it hard for the weed to spread rapidly. However, Palmer amaranth is more aggressive and grows more rapidly than waterhemp so that reduces the window of opportunity to implement control tactics,” he says.

Much of Iowa’s waterhemp is already resistant to glyphosate (Roundup), and it is likely Palmer amaranth will carry that same resistance. Farmers need to develop diversified weed management programs that use multiple herbicide sites of action and include alternative management strategies to delay further selection of herbicide resistant weeds, Hartzler observes.

Conservation habitat raises question about Palmer spreading
While new detections of Palmer amaranth have been found in ground reserved for non-crop habitat restoration, such as Conservation Reserve Program land, there are ways to manage the weed while also continuing to grow the vegetation needed to support water quality and game birds, pollinators and other wildlife populations.

“The standard management practices for establishing a native planting habitat, is to mow that area three or four times during the first year because the weeds will take off and grow much quicker than native plants. Additional mowing may also be needed in the second year after planting,” Hartzler says.

He adds, “There is no reason to fear establishing this type of habitat. You should go ahead and seed a conservation habitat. But anyone who does this should become familiar with Palmer amaranth so they can distinguish it from waterhemp or other weeds growing in the area. In many of the infested fields today, the number of Palmer amaranth plants was low enough to allow for the hand removal of the plants.”