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Bull Thistle

Bull Thistle flowerPlant 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.

Bull Thistle young rosettePlants usually form a large fleshy taproot with many secondary fibrous roots. Stems are 1 to 5 feet tall, stout, leafy, wooly, and widely branched. On the middle and upper portions of stems, there are spiny leaf-like wings. 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.

Bull Thistle rosetteMany disk flowers are contained in each flower head, which is rose to reddish-purple, globe-shaped, 11/2 to 2 inches wide, surrounded by narrow, overlapping, outward-pointing, green, spiny bracts, and usually solitary at the end of a branch.   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 21/2 feet have been observed to travel as far as 40 feet in the wind.   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.