Bull thistle is an introduced biennial herb which produces a first-year rosette. Several primary roots develop from the root crown, and each bear a number of smaller lateral roots.
All flowers are hermaphroditic.
Bull thistle is a monocarpic herb which must regenerate from seed. In west-central Idaho, bull thistle seeds were found in the seedbank of three forest habitat types, although bull thistle plants were rare or absent. In Yosemite National Park, California, clipping in plots where bull thistle was previously absent promoted bull thistle seedling establishment, and digging further promoted it. It also occurs in woodland clearings, rock outcrops, on the banks of rivers and streams, and in hedgerows. It occurs on soils with a wide range of pH values, having a peak on almost neutral soils, and being very rare on soils of pH less than 4.8 to 5.