Giant ragweed in soybean field (Purdue University photo)
I read a story recently about cogongrass, which is being compared to kudzu for its invasiveness and is called one of the ten worst weeds in the world. Alabama has created a Cogongrass Control Center to try to contain the weed in the state, but it will be a challenge to keep it from moving north, according to the project coordinator. "Left unchecked, 'it could spread all the way to Michigan,' " Ernest Lovett told The New York Times.
The piece reminded me of a 2001 Agriculture.com story in which Indiana farmers were surveyed to name their ten worst weeds. At that time, they were:
1. Giant ragweed
2. Canada thistle
4. Common lambsquarters
6. Hemp dogbane
9. Common ragweed
10. Common cocklebur
Glen Nice, a Purdue University weed scientist, says that although the farmer survey has not been repeated since then, many of the weeds on that list would remain there today. "A few weeds come and go, but the big ones are always there," he says. An Extension joke in the state is that Indiana is the 'giant ragweed national forest,'" he added.
In recent years, while many of the grasses have disappeared from the "most-wanted" list, others, like the herbicide-resistant marestail, have cropped up due to take their place, Nice says.
Farmers discussing the issue of the toughest weeds in Agriculture.com Crop Talk agree. Said one: "Marestail or horseweed is getting to be a real problem in KS."
Other bad-boy weeds, according to farmers joining the discussion, are field bindweed, kochia, cheatgrass, fall panicum, tall waterhemp, and dandelion.
"The list of noxious and herbicide-resistant weeds gets longer every year," said one farmer.
Using all our tools
Controlling the world's worst weeds require the grower to take a long view, says Nice.
"These weeds can be controlled but it takes persistence and, unfortunately, inputs in the way of rotations and herbicides," he says. "Growers have wrangled giant ragweed and velvetleaf seed beds down to acceptable levels by gearing their weed control programs to controlling these pests over the long haul. The use of effective residuals and timely post applications can reduce populations over time."
Perennials like Canada thistle require a "more localized approach," Nice says. "In many cases spot treatments in the spring or fall are required. Applications in the heat of summer can inhibit flowering, but generally are not effective at controlling the underground roots," he says.
"Weeds have always been a problem from the days of conventional tillage, to the adoption of no-till and herbicide tolerant crops," Nice says. "We have to use all of the tools available to us and be able to adapt to the problem as it adapts to our strategies."