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A single Scotch Thistle (Onopordum acanthium) is imposing enough as a single plant, but an entire colony can ruin a pasture or destroy a park or campsite. Besides creating an impenetrable barrier, Scotch Thistle nearly eliminates forage use by livestock and big game species such as deer and elk.
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Major infestations include most of the Pacific Northwest along with Colorado, Wyoming, Nebraska, South Dakota, and Utah. Fortunately, it is found here in Weber County mostly in waste areas and vacant ground. We have treated fields of O. acanthium well over 6 feet in height where the operator had to crawl out the truck window every 50 feet, stand on top of the tank and cover what he could with the hand line.

Scotch Thistle is a biennial, producing a large rosette of spiny leaves the first year. The rosette pictured above is representative of this spring's crop. You can see just how large a single plant is even in this early stage. This species can be distinguished by, among other traits, the blue-green color and the thick growth of leaf hairs as shown in the photo below.

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The second year, the weed transforms into a coarse branching plant up to eight feet in height and five feet in diameter. Under poor growing conditions, the plants may stand less than a foot tall, but can produce nearly as many seeds as the larger plants. Scotch Thistle leaves are deeply lobed with long, stiff spines along the margins.

The leaves have a winged appearance that continues down the stems of the plant. Fine hairs give the plant a grayish appearance. Purple flowers more than an inch in diameter are produced in the summer. Flower heads remain upright, rather than nodding as Musk Thistle flowers do. Stocks supporting the flowers are leafy.

Infestations of Scotch Thistle often start in disturbed areas such as roadways, campsites, burned areas, and ditch banks. The weed adapts best to areas along rivers and streams, but can be a serious problem in pastures, grain fields, and range areas. Here in Weber County, this weed is a widespread problem in the cities of West Haven and Hooper, as well as unincorporated western county areas such as Warren, Taylor and West Warren. The inserted photo below shows the mature flower next to a bud. From an artist's viewpoint, the flower is striking in both color and form - but anyone who has had to make their way through a barrier of Scotch Thistle can tell you, it is NOT fun!

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Control of Scotch Thistle starts with good grazing management and attention to disturbed areas where the plants can become established. Small infestations should be eradicated before they spread. Besides encouraging competing vegetation where possible, every effort should be made to prevent established plants from going to seed. Scotch thistle can be controlled by mechanical removal of existing plants and prevention of seed set or by the use of herbicides. Eradication of existing infestations requires at least six years of controlling all germinating seedlings. Residual herbicides offer the best control results.

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