A class “B” noxious weed in Benton County
Scotch thistle is a biennial plant but it can behave as an annual or a short-lived perennial as it is not bound by daylight or temperature for growing and flowering. In the first year, scotch thistle grows very spiny leaves in a large rosette. The second season is when flowering shoots grow.
Scotch thistle plants grow eight to twelve feet tall and can be five feet wide. The shoots and leaves are covered with thick hairs which give the plant a blueish green look. The flowers are vibrant purple in color.
Scotch thistle can be controlled by mechanical, cultural, and chemical methods. Mechanical control is very effective if you do it before the plant goes to seed. Seeds remain viable for up to seven years and fall close to the plant, but wind, water, wildlife, livestock, and human activities disperse the seed.
Using a combination of methods is best and re-vegetation after treatment is recommended. Planting competitive grasses helps prevent the invasion and establishment of new scotch thistle plants. Re treatment may be necessary for four to six years or until the seed bank in the soil is exhausted. Simply severing the roots of the rosette or plant kills it. Be careful though, because the plant has stout spines.
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