Bull thistle is the least serious of the introduced thistles in North Dakota,; however, populations of bull thistle have been increasing these during these wet falls. Native Americans used bull thistle to treat hemorrhoids. Many plant parts from the root to the flower are edible. The flower petals are used as chewing gum.
Bull thistle occurs in all 48 contiguous states and most of Canada, but is designated noxious in only a few states. Bull thistle generally is found growing singularly or on small patches. The large size and showy flowers makes it quite noticeable in pasture and rangeland, but it has little economic or ecological consequence.
Bull thistle is a biennial that grows from a flat rosette of the first year to a flowering stem the second year, often 5 feet or more tall. Plants are multibranched; stems having purple veins and are winged. A distinguishing characteristic are the leaves. Leaf margins ar deeply toothed and toothed again (double dentate) with prominent stiff spines. The leaf surface ha a distinct center vein with slight prickly hairs above and cottony pubescence below.
Bull thistle flowers heads usually are found singularly at the end of each stem branch. The flowers are gumdrop shaped and large (2 to 3 inches tall) with long, stiff, yellow-tipped spines. Bull thistle flowers from July to September.
Control on non-crop lands:
Bull thistle seldom reaches high enough densities to warrant treatment.
Chemical: Fall is the preferred time for applying herbicides. Seedlings that emerge in summer are in the rosette stage and are most susceptible to herbicides. Milestone (aminopyralid), Curtail (clopyralid), Tordon (picloram) or dicamba (various) easily control bull thistle. Spring applications of Escort or Cimarron Max will eliminate seed production when applied in the bolting to bud stage.
Cultural: Hand-digging the rosette prior to bolting will kill the plant prevent seed-set.
Biological: No biological control agents or pathogens are available.