Bull Thistle
Cirsium vulgare
Washington State Classification
Class C — Widespread, County-Selected Control
Bull thistle may outcompete native plants and desirable wildlife and livestock forage plants.
Quick Reference
Identification
Growth Habit
Bull thistle is a biennial herbaceous plant growing between 3 to 7 feet tall with one upright branched stem. It grows a rosette (cluster of radiating leaves at plant base) in its first year and blooms in its second year.
Leaves
Leaves alternate and coarsely lobed. Each lobe has a spined tip. Leaf bases extend downward from the leaves along prominent ridges of the stem. Upper leaf surface is rough with bristle-like spines while the undersides are covered with white woolly hairs.
Flowers
Flowerheads many, 1.5 to 2 inches in diameter. Bracts at base of flowerheads are spine-tipped. Flowers are purple or rarely white, blooming July through September.
Fruit & Seeds
Seeds are less than 0.16 inches (4 mm) long.
Impact
Bull thistle may outcompete native plants and desirable wildlife and livestock forage plants. It can invade most any disturbed habitat and grow in dense thickets. Hay price may decline with the presence of bull thistle.
Ecology & Spread
Habitat
Bull thistle colonizes primarily in disturbed areas such as pastures, roadsides, and ditch banks as well as in hayfields, disturbed prairies and logged mountain areas. Please click here to see a county level distribution map of bull thistle in Washington.
Spread Mechanisms
Reproduction
Bull thistle reproduces by seed and not by vegetative means.
"
Control Methods
Mechanical
- Hand-pull and dispose of flowering plants in trash to prevent seed spread. Mowing can be effective, but make sure the plants do not flower. If cut too early before flowering, plants may re-sprout and flower again that season. Remove stems from site if plants are cut or pulled with flowers.
Cultural / Prevention
- Cultivation (rototilling or hoeing) will effectively eliminate plants. Careful digging is useful to manage weed populations. However, digging can carry undesirable weed seed to the surface and foster further germination.
Biological Control
- Bull thistle seed production is impacted by the seedhead gall fly, Urophora stylata. This gall fly’s larvae induce and feed on gall tissue in the developing bull thistle seedhead, reducing seed production up to 60%. Urophora stylata may significantly reduce seed production if bull thistle populations are sustained for many years. For more information about the biological control of bull thistle, please visit WSU Extension Integrated Weed Control Project