Musk thistle rosette.
Spring Pasture Maintenance
Control musk thistle early.
If you've had problems with musk thistle on your land in the last year and intend to apply chemical control, early spring is a good time to check for new growth of this noxious weed, said a University of Missouri Extension agronomy specialist.
"For optimal control, herbicides should be applied when musk thistle is in the rosette stage, typically in the fall or early spring," said Travis Harper. "Landowners should walk fields in the spring or fall looking for these rosettes." The musk thistle rosette is a flat circle of leaves that can reach a diameter of 2 feet or more.
Timing is critical. "Spraying after the plant has bolted and/or produced a flower is too late to achieve good control," Harper said. "Typically, the plant will survive, flower and produce viable seed."
Spray rosettes with 2,4-D or other registered herbicide, he said.
Musk thistle, a European weed introduced to the U.S. in the 1800s, can infest old fields and overgrazed pasture. Missouri statute classifies musk thistle as a "noxious" weed because of its ability to cause economic harm to the state's agriculture and the difficulty associated with controlling or eradicating the species, Harper said. According to the Missouri Department of Conservation, all landowners are required to control the plant if it is growing on their property.
Musk thistle is an erect biennial (two-year life cycle) with spiny stems and leaves that may reach 6 ft. in height. It is primarily a weed of pastures, hayfields, roadsides and non-crop areas. After germination, the plant develops into a rosette. During the second year of growth, the rosettes elongate and flowering stems are produced. Musk thistle produces an average of 10,000 seeds per plant. These seeds are disseminated by wind and will germinate in the fall or following spring.
"Letting just one musk thistle go to seed usually results in multiple musk thistle plants the following year," Harper said.
Mowing has long been a popular method of musk thistle control, but musk thistles must be mowed within two days after blooming to inhibit seed production and prevent rebolting.
"The problem with trying to control musk thistle through mowing is that not all thistles bloom at the same time," he said. "To effectively control musk thistle, you would have to mow a given area every couple of days. Not only can this be costly and time-consuming, it is usually detrimental to desirable plant species."
Field infested with musk thistle.
There are natural enemies that can aid in the control of musk thistle. These include the musk thistle rosette weevil and the musk thistle flower head weevil. The larvae of the rosette weevil feed on the underside of the rosette leaves and on the leaves and stems of bolted plants, while larvae of the flower head weevil feed in the base of the developing flower. Entomologists released flower head weevils in Missouri in 1975 and rosette weevils in 1979. Areas where these weevils have been present the longest have seen a 70%-90% reduction in thistle populations, Harper said.
Even with all of the available control methods, musk thistle continues to thrive, especially along roadways. If you have concerns about musk thistle populations along state-managed roadways, contact the Missouri Department of Transportation at 1-888-275-6636. For concerns about musk thistles on roadways not managed by the state, contact your county commission or local township.
If you would like to know more about controlling musk thistle on your own land, contact your local MU Extension center or see MU Extension Guide IPM1010, "Biological and Integrated Control of Musk Thistle in Missouri," available for purchase or free download at http://extension.missouri.edu/publications/DisplayPub.aspx?P=ipm1010.
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