Ohio Perennial and Biennial Weed Guide

Family:

Sunflower Family (Asteraceae)

Other Names:

Carduus lanceolatus, C. vulgare, Cirsium lanceolatum, bank thistle, bell thistle, common thistle, plume thistle, spear thistle.

Origin and Distribution:

Originally from Eurasia, bull thistle is now established on every continent except Antarctica. The species was introduced to northeastern U.S. during colonial times and is now widespread throughout the U.S. and Canada. Bull thistle is naturalized in over half of the counties in Ohio. It is a problem weed in disturbed areas such as pastures, rangelands, roadsides, waste places, and abandoned fields. The weed prefers relatively rich, moist soils.

Plant Description:

Bull thistle is a typical biennial species in that, during the first year of growth, it produces leaves in the form of a low growing basal rosette. Leaves are hairy on the upper surface and irregularly indented around the edge with groups of 2 or 3 spine-tipped lobes interspersed with unlobed portions. Bull thistle rosettes remain green and continue growing throughout the winter. The rosette is 3 feet in diameter by the second spring when it gives rise to a 5-foot-tall upright flowering stem. Stems have distinctly spiny wings and bear solitary, purple, globe-shaped flower heads on the ends of branches. Reproduction is solely from seeds.

• Root System:

  Plants usually form a large fleshy taproot with many secondary fibrous roots.

• Seedlings and Shoots:

  The two seed leaves (cotyledons) are egg-shaped and spineless and the first true leaves are oval with notched edges that are fringed with prickles. Young leaves are crisp and easily broken. Seedlings grow into a rosette of basal leaves.

• Stems:

  The two seed leaves (cotyledons) are egg-shaped and spineless and the first true leaves are oval with notched edges that are fringed with prickles. Young leaves are crisp and easily broken. Seedlings grow into a rosette of basal leaves.

• Leaves:

  Rosette leaves are between 3 to 6 inches long and irregularly indented around the edge with groups of 2 or 3 deep lobes interspersed with unlobed portions. At the tip of each lobe is a 2/5-inch-long spine while smaller spines are randomly distributed along the rest of the leaf edge. The upper leaf surface is dark green, covered with sharp hairs, and prickly to the touch. The lower surface is light green and covered with wooly hairs that feel soft. Stem leaves are similar in appearance to rosette leaves but smaller and alternate (1 leaf per node). Leaf bases attach to and extend down the stem giving it a winged appearance.

• Flowers:

  Many disk flowers are contained in each flower head, which is rose to reddish-purple, globe-shaped, 1 1/2 to 2 inches wide, surrounded by narrow, overlapping, outward-pointing, green, spiny bracts, and usually solitary at the end of a branch.

• Fruits and Seeds:

  Single-seeded fruits are approximately 1/6 inch long, straw colored, streaked with black, and similar in shape to a chile pepper. Attached to one end are many brown, feathery, 3/4-inch-long hairs (pappus).

Similar Species:

Lobes of Canada thistle (Cirsium arvense) leaves end in small spines and the upper leaf surface is smooth while lobes of bull thistle leaves end in large spines and the upper leaf surface is rough. Also, Canada thistle has creeping roots and rhizomes that spread great distances while bull thistle forms a taproot. Bull thistle grows as a rosette during its first year while Canada thistle rarely forms only basal leaves.

Biology:

Bull thistle blooms from June to October. A single plant growing in a pasture can produce approximately 7000 seeds. The pappus aids in wind dispersal of seeds. Seeds dispersed from a height of 2 1/2 feet have been observed to travel as far as 40 feet in the wind. However, the pappus is easily separated from the seed prior to dissemination, which explains why so many seeds tend to be deposited directly under the mother plant. To control mechanically, plants should be mowed just before flowering or rosettes can be cut below the crown with a hoe. Bull thistle does not persist under cultivation. Several herbicides are available that selectively control the weed.

Toxicity:

None known. However, bull thistle spines can irritate the mouths of grazing livestock and significantly reduce weight gain.

Facts and Folklore:

• The Latin name 'Cirsium' is a corruption of the Greek 'kirsos', which roughly translates to 'swollen vein' and describes the result of being pricked by this spiny plant.

• Bull thistle roots are sold commercially in Australia for rabbit bait.

• Some people find roots and young leaves tasty, providing the spines are removed.