Invasive Watch: Bull Thistle

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Bull thistle flower

Recognized as a noxious weed in many states, bull thistle is an invasive biennial that can wreak havoc along roadsides and other disturbed areas. Learn how to identify and control it effectively. 

Bull thistle (Cirsium vulgare) is a troublesome weed that is self-pollinated or insect-pollinated. It doesn’t take long for this dangerous plant to take over roadsides and inhibit roadway safety as well as environmental sustainability. Understanding the threats it poses and how to prevent it from invading roadside rights-of-way can make a world of difference for vegetation managers, drivers and wildlife alike. 

Why Bull Thistle Is a Nuisance Along Roadsides

Bull thistle tolerates a wide range of conditions, including sunny, open areas and various soil types. The plant’s bur-like seeds cling to animals, pant legs and machinery and remain viable in the ground for years. At full maturity, this tall-growing pest is capable of outcompeting desirable low-growing plants and grasses that support roadway safety and environmental sustainability.   

What Does Bull Thistle Look Like?

For a wildflower, bull thistle is fairly large. Its erect structure can grow up to 6 feet tall and features a number of distinguishing features: 


The long, robust leaves of bull thistle plants are covered densely with sharp spines and extend from spiny-winged stems. Each leaf is rough to the touch; the top surface is deeply lobed and hairy while the underside commonly features wool-like hairs.  


Basal rosettes of bull thistle form in the first year of development and commonly grow up to 3 feet in diameter. A cluster of leaves extend from the taproot either on or near the soil surface. Each taproot can grow up to 28 inches tall. 

Flowers and Fruit

During the second year of development, bull thistle will develop flowering stems that produce spine-covered fruit and bright-pink or white flowers atop the terminal root. Flowering in the second year most commonly starts in mid-June and continues into early fall.  

How To Control Bull Thistle Effectively

As bull thistle only reproduces by seed, avoiding mowing practices and other activities that facilitate seed spreading is critical. Digging and other manual control methods can be used but they can be time-consuming and particularly costly. Comparatively, selective herbicide applications are a preferred alternative as they offer sustainable results for optimum efficacy.   

When bull thistle is less than 2 inches tall in early or mid-spring, apply HighNoon® herbicide from Corteva Agriscience at a rate of 16 fluid ounces per acre. For longer residual control of susceptible late-spring or early summer weed emergence, apply up to 20 fluid ounces of HighNoon herbicide per acre. For optimum results, apply when weeds are actively growing and conditions are favorable for plant growth. Use a higher rate when growing conditions are less favorable and weeds are mature, foliage is tall and dense, or residual control is important.  

Preemergence control of germinating seeds or seedlings also is possible after weeds have emerged post-application. Simply increase the application rate to the high end of the rate range specified to extend residual control.  

For more information on invasive plants and vegetation control strategies or chemistries that can maintain effective control, explore our digital archive

Under normal field conditions HighNoon® is nonvolatile. HighNoon has no grazing or haying restrictions for any class of livestock, including lactating dairy cows, horses (including lactating mares) and meat animals prior to slaughter.  Label precautions apply to forage treated with HighNoon and to manure and urine from animals that have consumed treated forage. Consult the label for full details. 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. HighNoon, 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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