Scientists rank Palmer amaranth as most difficult weed, foxtail most widespread

Three of the most troublesome plants for row-crop and pasture producers are, from left, foxtail, Palmer amaranth and bluegrass, as listed by the Weed Science Society of America.Weed control may feel like a continuous—and often frustrating—battle, but there are ways to develop more effective management strategies. It starts with identifying which types are the most problematic.

A recent survey from the Weed Science Society of America (WSSA) categorizes the most common and troublesome weeds found in grass crops, pasture and turf. Conducted in 2020, the survey includes input from more than 315 weed scientists in the U.S. and Canada. Seven categories of grass crops were included in addition to pastureland, rangeland and other hay.

Foxtail was identified as the most prevalent species. These grasses, characterized by their fuzzy seed head, are typically found during the spring and summer months. In Missouri, the most common types are yellow, green and giant foxtails.

The most worrisome weed is likely no surprise to most growers: Palmer amaranth. In addition to being identified as the weed causing the most trouble overall, it is the top concern in corn as well as sorghum.

Palmer amaranth is a summer annual that resembles other varieties of pigweed. If left uncontrolled, it can have potentially devastating impacts on row-crop yields. The nitrates found in its leaves can also be toxic to livestock.

“Palmer amaranth grows rapidly, has an extensive root struc­ture and produces massive amounts of seeds that are easily transported and spread,” said Stanley Culpepper, professor and extension weed scientist at the University of Georgia. “Even more impressive are its genetic capabilities. Palmer amaranth can quickly evolve resistance to herbicides and has the potential to transfer that resistance to new plants through pollen movement.”

To reduce its spread, Culpepper recommends a timely and holistic approach. He says removing plants before they produce seeds is key, in addition to treating with herbicides.

Other rising contenders are bluegrass species, primarily annual bluegrass, which ranks second overall as the most troublesome weed. It’s particularly problematic in turf and urban ecosystems, such as golf courses, sports fields and public parks. Bluegrass is very similar to Palmer amaranth in that it’s highly productive and easily adaptable. Plant scientists say a diversified approach is the most sustainable—and effective—long-term control strategy for this weed.

A full list of survey results can be found on WSSA’s website at wssa.net/wssa/weed/surveys/.