Production Blog

Meet the Worst Weeds in the Patch

Pamela Smith
By  Pamela Smith , Crops Technology Editor
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The fight against waterhemp continues in both corn and soybeans, such as in this field in central Illinois. (DTN photo by Pamela Smith)

When woolly cupgrass was mentioned as a weed to watch during a recent University of Minnesota webinar, I did a double take. I can't remember the last decade I heard this weed cited as problematic.

Lately I've been hearing other familiar names pop up as worries again. Long-ago weeds that seem to be making a comeback include cocklebur, Eastern black nightshade, velvetleaf, giant ragweed and fall panicum -- the latter ironically appropriate for this time of year.

That's the thing about weeds, we may cuss their stubborn ways, but their ability to adapt and overcome is often remarkable. That's confirmed in John Cardina's recently published book Lives of Weeds/Opportunism, Resistance, Folly (Cornell University Press).

Cardina, a professor in the Department of Horticulture and Crop Science at The Ohio State University, outlined how and why plants become botanical bullies and the human role in the corruption process. Most of the species were at one time considered desirable, or at least innocuous, he wrote.

"Some were coveted, protected, and enjoyed for ornamental, medicinal, culinary, or other practical purposes," Carina wrote in the book. "Conversely, the thorniest enemies of farmers of old include some species that are insignificant today; some can hardly be found." Indeed -- Cardina noted examples from old botany books that itemize the most serious weeds to be things like oxeye daisy and red sorrel.

Today's row crop farmers will not be surprised to learn that the latest survey conducted by the Weed Science Society of America (WSSA) found Palmer amaranth to be the hardest to control weed in grass crops, while foxtail species are the most prevalent.

More than 315 weed scientists across the United States and Canada participated in the survey, which was conducted during 2020. Seven categories of grass crops were included, covering weeds found in corn; rice; sorghum; turf; spring cereal grains; winter cereal grains; and pastureland, rangeland and other hay.

In addition to leading the overall survey results as the most troublesome weed, Palmer amaranth was identified as the most troublesome weed in corn, followed closely by its cousin waterhemp. Palmer amaranth also ranked as both the most troublesome and the most common weed in sorghum.

In a recent WSSA release about this survey, University of Georgia weed scientist Stanley Culpepper, pointed to the reasons Palmer amaranth is such a tough adversary.

"Palmer amaranth grows rapidly, has an extensive root structure and produces massive amounts of seeds that are easily transported and spread," he said. "Even more impressive are its genetic capabilities. Palmer amaranth can quickly evolve resistance to many important herbicides and has the potential to transfer that resistance to new plants through pollen movement."

Culpepper advised growers to have a healthy respect for Palmer amaranth's ability to dominate the landscape. "Removing it before it produces seed is key," he said. "You won't win with herbicides alone. You need timely intervention using a holistic approach."

Read more about how to identify Palmer amaranth and separate it from waterhemp here:….


The survey also confirmed the rising importance of bluegrass weed species, mainly annual bluegrass. Bluegrass species ranked second only to Palmer amaranth as the "most troublesome" weeds overall. They also were ranked as the most troublesome weeds in turf.

Jim Brosnan, head of turfgrass weed science research and extension at the University of Tennessee, said bluegrass has much in common with Palmer amaranth. For example, it is a prolific seed producer and is highly adaptable.

Less than a decade ago, Brosnan confirmed the first instance of herbicide resistance in annual bluegrass when the weed escaped treatment by glyphosate on a Tennessee golf course. Today, annual bluegrass has adapted and evolved resistance to multiple herbicides globally, targeting 10 different sites of action.

"Annual bluegrass is especially problematic in urban ecosystems, including golf courses, lawns, sports fields, public parks and greenways," Brosnan said in the WSSA release. "Long-term, sustainable control will require a diversified approach that doesn't rely solely on herbicides."


Below are results for weeds in each of the grass crops from the WSSA 2020 survey.

Most Troublesome, Hard to Control Weeds

-- Corn: Palmer amaranth and waterhemp

-- Pasture, rangeland and other hay: Canada thistle

-- Rice: Sedge several species (spp.) (yellow nutsedge and rice flatsedge) and Echinochloa (barnyardgrass and coast cockspur)

-- Spring cereal grains: Foxtail spp. (green and yellow foxtail)

-- Sorghum: Palmer amaranth and johnsongrass

-- Turf: Bluegrass spp. (annual and roughstalk bluegrass)

-- Winter cereal grains: Italian ryegrass and Bromus (downy and Japanese brome, cheat, rescuegrass)

Most Commonly Found Weeds

-- Corn: Common lambsquarters and foxtail spp. (green, yellow and giant foxtail)

-- Pasture, rangeland and other hay: Bromus (downy, red and smooth brome, cheat) and Canada thistle

-- Rice: Sedge spp. (yellow nutsedge and rice flatsedge) and barnyardgrass

-- Spring cereal grains: Common lambsquarters and foxtail spp. (green, yellow and giant foxtail)

-- Sorghum: Palmer amaranth

-- Turf: Crabgrass spp. (large, smooth and southern crabgrass)

-- Winter cereal grains: Lamium (henbit and purple deadnettle)

WSSA conducted a survey regarding weeds in soybeans and some other important row crops in 2019 and will be resurveyed in 2022. Here's some of those findings from the U.S. and Canada (166 weed scientists responding).

Most Troublesome, Hard to Control Weeds

-- Alfalfa: pigweed (including redroot, spiny, waterhemp and Palmer amaranth)

-- Canola: wild oat

-- Cotton: Palmer amaranth

-- Peanut: Palmer amaranth

-- Pulse Crops: kochia

-- Soybean: waterhemp and horseweed (marestail)

-- Sugarbeets: common lambsquarters

Most Commonly Found Weeds

-- Alfalfa: pigweed spp.

-- Canola: wild oat

-- Cotton: Palmer amaranth

-- Peanut: nutsedge spp. (yellow and purple)

-- Pulse Crops: kochia

-- Soybean: waterhemp, horseweed (marestail), common lambsquarters, and foxtails (giant, green, yellow)

-- Sugarbeets: common lambsquarters

For more information on WSSA's weed surveys go to….

Naming the worst weed in my own personal world is an easy choice. It's spiny amaranth and it's my own fault. I couldn't say no to the offer of some free compost from a local cattle producer and have been paying the price ever since. The more I battle prickly pigweeds the more tenacious they become. Every punctured finger or festering sticker leaves a deeper understanding of how the weeds insert their way into our lives.

These days, waterhemp might as well be the state plant in my home state of Illinois. Like corn and sorghum, pigweeds are C4 plants, making them very efficient at fixing carbon and well-adapted to high temperatures and intense sunlight. Should we be surprised that they routinely outrun soybeans and cotton, and can even be found towering above cornstalks?

Hopes to overcome these adversaries quickly evaporate reading Cardina's chapter devoted to pigweed. He wrote extensively on the biology of Palmer and waterhemp and how they were of no particular concern until modern practices put them on the path to "extraordinary weediness."

Cardina covered the history behind eight specific weeds and how we got where we are today. Woolly cupgrass isn't among them, but yet, it is. The narratives are all about evolutionary biology and how plants respond to the environments humans put before them.

Pamela Smith can be reached at

Follow her on Twitter @PamSmithDTN


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