ENID, Okla. —
An invasive species as a species that is non-native (or alien) to the ecosystem under consideration and where introduction causes or is likely to cause economic or environmental harm to human health.
Invasive species can be plants, animals or other organisms. Invasives may have one or more of the following characteristics:
• Tolerance to a variety of habitat conditions.
• Rapid growth and reproduction.
• Compete aggressively for resources.
• Lack of natural enemies or pests.
Bull thistle (Cirsium vulgare), musk thistle (Carduus nutans), Scotch thistle (Onopordium acanthium), distaff thistle (Carthamus lanatus) and Canada thistle (Cirsium arvense) are the five thistles species introduced from Europe and Eurasia, where diseases and insects not present in the United States kept their populations at tolerable levels. The current Oklahoma Thistle Law declares musk, Scotch and Canada thistle to be noxious weeds and public nuisances in all counties of the state and requires a “Plan of Action” for control by all landowners whose land is infested.
Oklahoma Department of Agriculture, Food and Forestry is charged with enforcement. Since other, native thistle species exist in Oklahoma, it is important to properly identify and control the five introduced, invasive thistle species. The one species causing the most headache for landowners and operators at this time of year is musk thistle.
Musk thistle was first reported in Payne County in 1944. By 1994, musk thistle was declared as a noxious weed in Oklahoma. Today, musk thistle infests are present in every county of Oklahoma. Musk thistle is primarily a biennial or winter annual, reproducing only by seed. A single plant can produce in excess of 10,000 seeds. Scattered plants can be removed mechanically by digging below the crown. Other options for control include biological and herbicide application.
The musk thistle head weevil, Rhinocyllus conicus, and the rosette weevil, Trichosirocalus horridus, have been released in Oklahoma to help reduce seed production. Now is the time for weevil releases. Musk thistle is easily controlled by herbicides if treated during the rosette stage. The best chemical control of musk thistles in Oklahoma has been with spring applications to rosette plants in March. If 2,4-D or Transline (1/3 to 2/3 pt./acre) is used in the spring, spray in March or early April before plants start bolting.
Once plants begin bolting, they are more difficult to control; however, 90 to 100 percent control is possible with Grazon P+D (2 pt./acre), or Weedmaster (2 pt./acre), or Cimarron Max applied in late April or early May. Cimarron (3/10 oz./acre) applied to bolted plants in late-April or early May can reduce musk thistle seed production 98 to 100 percent, but some plants may re-sprout to produce new flowers and seed. Therefore, tank mix 2,4-D (1 pt./acre) with the Cimarron to control the plants and prevent seed production.
Grazon P+D, Weedmaster, Cimarron Max, or 2,4-D applied to pastures in late-April or early May also will control most summer weeds like ragweeds, bitter sneezeweed or common broomweed. All herbicide applications should be made to actively growing weeds, with adequate soil moisture, and when daytime air temperatures are above 60 degrees. Always read and follow the herbicide label.
Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Oklahoma Cooperative Extension Service is implied.
Nelson is Oklahoma Cooperative Extension Service ag educator for Garfield County.