Bull thistle

Bull thistle

The issue: Bull thistle

Bull thistle (Cirsium vulgare) is a biennial, native to Eurasia. It invades roadsides, pastures, rangeland, and other disturbed sites.

Bull thistle forms a rosette during the first year of its life cycle, when conditions are right during the second year, it bolts, sending up flowering stems. Stems are typically 1-4 feet tall and are covered in spines. Rosette leaves are oval shaped with spines along the edges. As leaves mature, they become longer, and they develop deep lobes that become very pointed. Leaves have hair on both sides, the hair on the top is coarse, while the hair on the bottom is wooly. Bull thistle produces plumed seeds that are carried easily on the wind, aiding in dispersal. Bull thistle seed can survive in the soil for 10-plus years.

Integrated pest management options:

Prevention: Learn to identify this plant. Clean equipment after leaving an infested area.

Mechanical: Using a shovel to cut the root below the soil surface is a good way to remove Bull thistle when small infestations are present. Mowing can be effective if done before seed is produced. Tillage is also an effective option.

Cultural: Use proper grazing practices in conjunction with other control methods that will favor the growth of beneficial plants.

Biological: In some areas bull thistle seedhead gall fly (Urophora stylata) is used as a biological control agent. While it reduces viable seed production, its effect is minimal. A biological control agent will never eradicate its host, or it will die as well.

Chemical: Refer to Pacific Northwest Weed Management Handbook (https://pnwhandbooks.org) for herbicide recommendations (Search “bull”). Always read and follow herbicide label directions.

Justin Hatch is a University of Idaho Extension Agriculture Educator in Caribou County. He can be reached at 208-547-3205 or JLHatch@uidaho.edu.

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