URBANA, Ill. – Illinois farmers are all too familiar with invasive plants like Palmer amaranth, buckthorn species, bush honeysuckles, giant hogweed, garlic mustard, chaff flower and stiltgrass.

Now, add one more weed to that list: Asian copperleaf.

The first confirmed populations of this species were documented in New York in 1990. The next time Asian copperleaf was positively identified in the U.S. was in an Iowa seed corn field in 2016. Since then, it has been confirmed in 10 counties across central and western Iowa.

Like the invasive plants mentioned above, this weed can reduce crop yield, degrade pasture quality and alternative plant communities.

Illinois is the third state with a confirmed population of Asian copperleaf. Asian copperleaf was discovered in a farm field in Stephenson County in northern Illinois following corn harvest last fall. The Asian copperleaf plants observed in Stephenson County ranged from one to two feet tall, though many were smaller.

It’s the first confirmed Asian copperleaf population in Illinois but not the only sighting. The weed has since been spotted in fields in the Illinois counties of Calhoun, Humboldt, Boone, Wright, Franklin, Hardin, Grundy and Black Hawk.

Its presence is often noted under crop canopies, particularly during harvest time, where it can form dense mats that may go unnoticed until the crops are removed.

“It is more advantageous to remain in front of a new weed challenge rather than allow it to get in front of researchers and producers,” said Aaron Hager, weed science professor and University of Illinois Extension faculty specialist in the Department of Crop Sciences.

Asian copperleaf is non-native to the United States. It is an invasive plant species native to regions such as Russia, China, Japan and Philippines. It belongs to the spurge family but is notable for lacking the milky sap that many other spurges produce. The plant can grow to heights of two-three feet and features lanceolate to egg-shaped leaves that are two-four inches long, with fine, blunt teeth and deep veins. The bracts beneath its flowers are circular to heart-shaped, which helps distinguish it from other similar species.

Reports of herbicide resistance in Asian copperleaf populations in its native range raise concerns that it could evolve herbicide resistance in U.S. populations as well. Hagar said its rapid spread and persistence could quickly become problematic.

“Our goal is to give Illinois farmers and weed management practitioners the information necessary for them to scout and identify populations of Asian copperleaf,” Hagar said. “We hope this will allow any additional populations to be identified so that management can be implemented to reduce the movement of seed to additional fields.”

Asian copperleaf is a summer-annual species that reproduces exclusively by seed, but its emergence timing and duration are not known. Weed scientists at Iowa State University have received reports indicating that populations are frequently found at corn harvest, but there have also been reports of emerged seedlings at soybean planting.

According to David Zaya, associate research scientist and botanist with the Illinois Natural History Survey at the University of Illinois, identifying Asian copperleaf before reproductive structures are present can be difficult.

“Its leaves are alternately arranged on the stem and have serrated margins, pointed tips and slightly to very rounded bases,” he said. “Stems are erect and flowers are not showy. Leafy tissues, called bracts, grow beneath the female flowers. These bracts are important for distinguishing Asian copperleaf from native species such as Virginia copperleaf, rhomboid mercury and the rarer Deam’s mercury.

“Asian copperleaf’s relatives are native to Illinois, growing in grassy areas, woods, and sometimes as yard weeds. The best way to tell Asian copperleaf apart is the leafy tissue under the flowers. If it looks like a small hand or lopsided star with long ‘fingers’ it is one of the native species. If it looks like a heart or egg with a pointed tip, it is probably Asian copperleaf. The other clue would be if there are hundreds of thousands of them in an agricultural field, then it is probably Asian copperleaf.”

Zaya said farmers are encouraged to keep an eye out for this species while scouting during the upcoming growing season. While initial reports of this weed in Iowa were made during harvest, it is advised to scout for it prior to planting and throughout the growing season. Preventing the spread of seed is crucial to limiting future infestations. Fields containing Asian copperleaf should be harvested last and combines should be cleaned before moving to fields to prevent seed movement.

“Asian copperleaf is a growing concern in Illinois agriculture, and its identification and management are essential to mitigate its impact on crop production,” Zaya said. “The plant can potentially resist certain herbicides, making it a real challenge for farmers. Continued monitoring and research will be vital in developing effective strategies to control this invasive species.”

Growers who suspect Asian copperleaf is in a corn or soybean field should consider consulting local agricultural extension services for guidance on management practices. The Extension staff can also assist in verifying its identification.