Bull thistle identification and control

Why it’s a problem

Bull thistle can outcompete native plants, damaging habitat for desirable wildlife. It can invade most disturbed areas and grow in dense thickets. It can also outcompete livestock forage plants and damage the value of hay.

Plant description

Bull thistle is a widespread biennial (2-year life cycle) thistle originally from Europe and Asia, but now introduced throughout North America. It can grow 2 to 6 feet tall.

Leaves are deeply lobed and hairy. There are coarse hairs on leaf tops, making leaf feel rough to the touch, and woolly hairs on the underside. Long, sharp spines grow on the leaves at the midrib and the tips of the lobes. Leaf bases extend down onto stems and form spiny wings along the stems.

Each stem is topped by a pink-magenta flower head. These flower heads are "gumdrop" shaped, and spines extend all around their base. Bull thistle flowers from June to September.

Bull thistle has a two-year life cycle, flowering and setting seed in the second year. Seeds are short-lived on the soil surface but can persist for many years when they are buried, such as form cultivation activities. Seed germination generally occurs in the fall and spring. It has a short fleshy taproot with several primary roots.

Basal rosettes (a circular cluster of leaves at ground level) form and continue to grow until winter and can grow quite large, up to 3 feet in diameter. Rosettes that are not large enough by spring may not flower until the following year. Flowering usually starts in mid-June and continues into early fall. Plants can be self-pollinated, or insect pollinated. Bull thistle does not reproduce vegetatively and does not have rhizomes.

Although it is intimidating in appearance and can sometimes form large infestations, this thistle is not as challenging to control as many others and is mainly a problem in hay fields and pastures. Bull thistle is also commonly found along trails, roads, and vacant fields.

Bull thistle prefers sunny, open areas and can tolerate a wide range of conditions, from moist to dry soils, and is typically found in disturbed areas such as roadsides, trails, logged areas, vacant land, pastures, and cultivated land. Overgrazed pastures are susceptible to bull thistle encroachment, and it can sometimes form dense stands that reduce productivity and stocking levels. Bull thistle may also dominate forest clear cuts and reduce growth of tree seedlings. 

Be aware of look-alike plants

Bull thistle might be confused with native thistles as well as more serious noxious weeds such as milk thistle, a Class A noxious weed in Washington, and creeping thistle, a Class C noxious weed that is much more difficult to control.

There are two native thistles common to our area that look like bull thistle: edible thistle (C. edule) and clustered thistle (C. brevistylum).

When in doubt, take photos and report them on iNaturalist.

What to do if you find it

Because bull thistle is so widespread, property owners in King County are not required to control it. King County is not generally tracking infestations. We can provide advice on how to control bull thistle, but there is generally no legal requirement to do so. The King County Noxious Weed Control Board encourages property owners to remove bull thistle where possible, and to avoid introducing it to new landscapes.

Bull thistle only reproduces by seed so prevention of seeding and taking care not to spread seeds are key to preventing new infestations. Contaminated hay is a primary means of spread of this species so be careful to purchase weed free hay or watch closely for new plants in the areas hay is kept or spread. Do not leave cut stems of flowering bull thistle on the ground because they are likely to form viable seed after they are cut.

Control methods 

We recommend using a combination of methods to control noxious weeds. In areas with few weeds, it is important to act quickly before they become harder to control. Make a long-term plan as it often takes several years to get rid of most weeds. Start in the least infested areas first and then move into more heavily infested areas. 

For more in-depth control information, read our Best Management Practices (BMP) for bull thistle

Manual control

Bull thistle can be dug up with a shovel. Usually removing the top couple of inches of root is enough to kill the plant, especially after it has bolted (produced stems). A shovel or other tool can used to chop off leaves from one side of the plant to gain easier access to the roots, which can then be dug up. Flowering stems should be collected and destroyed to keep them from forming viable seed.

Mechanical control

Mowed thistles will produce new branches from basal buds but close cutting or cutting twice per season will usually prevent seed production and reduce the population over time. For more effective control, cut plants with a sharp shovel at 1-2 inches below the soil surface prior to flowering. If only one cutting a year is possible, cut when plants are in bud for best results. Cultivation and tilling can also be effective to control bull thistle. 

Biological control

The bull thistle seed head gall fly (Urophora stylata) lays eggs on closed flower buds in June and July. After hatching, the larvae burrow into the seed-producing tissues to feed, forming galls and reducing seed production. If the bull thistle population is large enough to support a good-sized population of this insect, it can be an effective way to reduce seed production of the bull thistle. This insect will not get rid of the bull thistle, however, just reduce its impact.

For more information or a site-specific control recommendation in King County, contact the noxious weed program. For information in other counties in Washington state, contact your local weed board or your local cooperative extension office.

Chemical control

Stay safe when using herbicide:

• Always read the label before use.
• Wear a long-sleeved shirt, long pants, shoes, and eye protection.
• Follow state and local regulations.

There are several herbicides that are effective on bull thistle, but it is important to carefully match the product with your site, local conditions and regulations, and other weed and land management issues, and always follow the label directions for the product you are using. If your site has animals grazing, please read instructions carefully on grazing restrictions. For grassy areas, it is best to use a selective broadleaf herbicide to keep the competitive grasses intact. After spraying, wait two weeks or more to give the herbicide time to work. 

See the PNW Pest Management Handbook for the most up to date and specific method for chemical control of bull thistle. 

Disposal instructions

If there are any signs of flowering, all parts of the plant should be bagged and destroyed. Do not compost the plant if there are any signs of flowering.  

Noxious Weed Disposal - Washington State Noxious Weed Control Board

Are your weeds piling up? - Noxious Weeds Blog