Bolts, flowers, and dies in the second year. The seeds of this weed are short-lived on the soils surface but can lay dormant in the soil for many years. The middle and upper stems of large plants with narrow, very spiny leaf-like wings running lengthwise. The hairy, prickly leaves overwinter to develop stems and branches of up to 2 feet in spring. It tends to grow best in soils with intermediate moisture although it will grow in dry and wet soils. Life Cycle. Life Cycle. It forms occasional side stems, but remains erect in stature, reaching 3-6' in height. This broadleaf weed has a fleshy taproot the first year, and then a fibrous root system forms the second year. The flowering stems elongate and flower in the second year. Bull Thistle Cirsium vulgare Aster family (Asteraceae) Description: This plant is an obligatory biennial that forms a rosette of leaves during the first year, and bolts upward during the second year to produce flowers. Reproduction: By seed . The plants die after flowering or after the first frost. Musk thistle (Carduus nutans) is a 3- to 8-foot biennial thistle. Musk Thistle. Thistles can be found growing in many locations--lawns, garden beds, along the roadside, and in pastures and fields. About Bull Thistle: The Bull Thistle is a biennial weed that reproduces by the wind blowing their seeds. The heads of purple Reproducing only by seed. Bull Thistle leaves grow alternate from each other and form a rosette. Other Names: Life Cycle: Biennial Description: The bull thistle is primarily a pasture weed having spiny-winged stems and leaves. As a biennial, bull thistle has a two‐year life cycle. Flowering stems reach 2-5 ft in height. The tip of each of the lobes of the bull thistle has a long, yellow spine. It has a deep taproot, which makes manual pulling a challenge. Plants grow vegetatively their first year as rosettes of green, sparsely hairy leaves. Bull Thistle has a two year life span. Bull thistle starts life as a spiny leaved rosette. Biennial. Stems are erect, 30-150 cm high but rarely much taller, usually widely branched and short-hairy. It likes pastures, edges of woods, waste places, fence lines and roadsides. Bull thistle has a biennial (two-year) life cycle. Bull thistle (Cirsium vulgare):Common in Clackamas County, moderately invasive, see our weed profile Biennial; Stems are leafy, stout, branched, and somewhat woolly; Flower heads are large, rose to purple with many sharp spine-tipped bracts Stems. Bull thistle is a weed that can grow in a wide range of environments but prefers disturbed areas. Bull thistle reaches a height of from 2 to 6 feet. Propagation. Bull Thistle spreads by seeds. Weedy characteristics: Bull thistle is highly competitive and aggressive, and forms dense colonies that choke out native vegetation important to wildlife and pollinators. In summer the plant grows a scented flower that resembles a spiny globe topped with fringed pink petals. top. Life cycle: Biennial. Flowers are dark pink to purple with spined bracts. It is often confused with musk thistle, which usually lacks the leaf hairs. The leaves have rough hairs on the upper surface and soft hairs on the lower. Basal rosette forms the first year. Leaf margins are toothed with sharp spines. The flowering stems elongate and flower in the second year. They are in rosette stage in the first year and flower and set seeds in the second year. top. Plants grow vegetatively their first year as rosettes of green, prickly, sparsely hairy leaves. Seeds germinate rapidly and are spread by wind. The plants die after flowering or after the first frost of that second year.