Bull thistle is a biennial plant that can grow up to six feet tall. Bull thistle grows in disturbed areas such as pastures, roadsides and ditches. Bull thistle is distasteful to most grazing animals, giving the thistle a competitive edge.
Caution: Plants have spines along leaves and stems.
Bull thistle is a biennial herbaceous plant. In its first year, it grows as a small round clump of leaves close to the ground (a rosette). Plants overwinter in the rosette stage, then the next spring the plants send up a flowering stalk, set seed, and then die. In its second year, bull thistle can grow up to three to six feet high with a single erect stem with multiple branches.
Leaves and stem
Leaves are alternate (come off the stem one at time at each leaf node). Leaves are coarsely lobed with each lobe having a spine at its tip. Leaves are hairy and the upper leaf surface is rough. Bull thistle stems have hairs and green ridges with spines.
Disk-shaped flower heads can be two inches wide and contain hundreds of tiny individual purple flowers. There are long, thin green bracts below the purple flowers. Flowers bloom from July through August.
Numerous straw-colored seeds with white, feathery tufts are dispersed by wind. They remain viable in the soil for over 10 years.
Each plant has a fleshy taproot.
Bull thistle is a biannual, meaning it has two distinct growing phases. The first year it grows a clump of leaves close to the ground called a rosette. The second year it completes its life cycle by growing a stalk and flowering. Bull thistle prefers to grow in disturbed areas like pastures, roadsides and ditches. Bull thistle is distasteful to most grazing animals, giving the thistle a competitive edge.
Origin and spread
Bull thistle is native to Europe, Asia, and Africa and was introduced to the United States in the early 1800s. Bull thistle was likely accidentally introduced from seeds in ship ballast or as a seed contaminant with other seeds that were brought over purposefully. Today, bull thistle is found in every state and throughout Minnesota.
Refer to EDDMapS distribution maps for current distribution.
Don't be fooled by these look-alikes
- Musk thistle, Carduus nutans (invasive) – flowers nod to the side at a 90 degree angle, leaves do not have hairs.
- Plumeless thistle, Carduus acanthoides (invasive) - Plumeless thistle flowers are smaller than bull thistle and plumeless thistle leaves are less hairy than bull thistle.
- Canada thistle, Cirsium arvense (invasive) – Canada thistle has small flowers (3/4 inches long) and its stems generally do not have spines on them.
- Swamp thistle, Cirsium muticum (native) – swamp thistle does not have the long, spreading bracts underneath the flower head that musk thistle has. Minnesota has several native thistle species. View the Minnesota thistle comparison webpage by the Friends of the Eloise Butler Wildflower Garden for descriptions and photographs of the species.
- Regulatory classification
The Minnesota Department of Agriculture does not regulate this species; however, it is regulated by some Minnesota counties. Check the list of county noxious weeds to see if it is regulated in your area.
- Threat to Minnesota
- Once bull thistle has been established, it spreads quickly replacing native plants and decreasing diversity. It is generally found in disturbed areas.
- It is distasteful to most grazing animals, reducing forage quality.
- What you should do
One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.
- REMOVE plants, animals and mud from boots, gear, pets and vehicles.
- CLEAN your gear before entering and leaving the recreation site.
- STAY on designated roads and trails.
- PLANT non-invasive species.
- Native substitutes
- Control methods
Mechanical control can be done by pulling the plant by hand or with equipment such as a shovel. If pulling plants, leave them to decompose on-site or follow Minnesota Department of Agriculture guidance for plant disposal. Mow second year stalks in May or June during the flower bud stage. Do not mow after plants have gone to seed as this will spread seeds. Contact University of Minnesota Extension for advice related to pasture management.
Herbicide control can be done using systemic herbicides which are taken up by plants and move within the plant, which can kill leaves, stems, and roots. Spot-spray with glyphosate, triclopyr, or 2,4-D. The most effective timing is to apply herbicide to the rosettes in spring (April, May or June) or fall (October and November).