In addition, some isolated problems exist in north central and western counties. A severe infestation of musk thistle. The 1994 law has been amended twice. Roger Mills County was added to the law in 1995, and in 1998, musk thistle was designated a noxious weed in all counties of Oklahoma. Spread and proliferation of musk thistle are caused by movement of seed-contaminated hay to uninfested areas and dissemination of airborne seed from mature plants along waterways.
Seedlings can emerge anytime during the growing season when moisture conditions are favorable.
In most years, primary emergence of seedlings occurs in September and October. In the spring of the second season, plants usually start bolting by mid-April, produce flower heads from May through June, then die after seed is produced. Musk thistle solitary flower heads are borne on long bending stems, and the colorful flowers are usually deep rose to violet or purple. Like other thistles, musk thistle is covered with sharp spines but has no hairs on its leaves, while all other thistles in Oklahoma have hairs on their leaves. In the rosette stage of growth the leaves are in a basal- cluster with no visible stem.