Invasive Watch: musk thistle

Musk Thistle

Musk thistle (Carduus nutans) can develop as an invasive biennial or annual forb. This invasive weed robs desirable plants of the water, light and nutrients needed to survive.

What to Look For

In the right conditions, musk thistle (Carduus nutans) can fully develop as a summer or winter biennial. However, in most cases, this invasive weed develops as an invasive biennial.

In its first year of development, which is known as the rosette stage, the plant will usually feature pale green leaves that appear waxy and slightly hairy.

In the following year, known as the bolt stage, the weed will become a branched plant and grow over 6 feet high. Long narrow and deeply lobed leaves commonly reach 6 inches in length and will feel smooth to the touch on both sides. With lobes featuring white or yellow points, the leaves appear alternately on the stem.

Musk thistle’s most distinguishing factor is its flowers, which grow up to 1 to 2 inches in diameter and are usually puffy with purplish hues. Each plant can hold anywhere from 3,500 to 10,000 seeds.

Where Is Musk Thistle Found?

Known to establish in a variety of natural areas across the United States, musk thistle is most commonly found in grassy environments like prairies, meadows and other grazing areas.

After flowering for approximately two months, the plant disperses seeds roughly two weeks after blooming. Seeds are then spread through various carriers, including wind, livestock, machinery and other vehicles.

How to Treat Musk Thistle

When musk thistle has yet to bloom, hand-weeding strategies are an applicable option for plant extraction. However, it is important to note that after removing a blooming brush thistle by hand, the plant must be burned immediately thereafter. Failure to do so can encourage seed production and subsequent spreading, which leads to unwanted plant development in adjacent areas. Considering this, the most effective method of control for musk thistle is the use of selective herbicides when the plant is developing in the rosette stage.

By using TerraVue herbicide at an application rate of 2.85 ounces per acre, vegetation managers can effectively treat areas in which musk thistle and other broadleaf weeds or brush species have developed. As a selective herbicide, TerraVue is safe on grass and most beneficial forbs.

For optimum control, apply TerraVue during the spring when the flower stalk has yet to develop. If inclement weather is in the forecast, applying TerraVue at least 12 hours prior to expected rainfall will allow the plant to absorb the herbicide more thoroughly.

Applications in the fall can also yield effective results, but be sure to apply herbicides before low temperatures cause the soil to freeze; such conditions can hinder efficacy. As herbicides work to reduce the number of seeds per plant, and thereby eliminate future populations of invasive plants, applications are recommended before flowering begins in either season.

 

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Trademarks of Dow AgroSciences, DuPont or Pioneer, and their affiliated companies or their respective owners. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. TerraVue is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.

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