Identifying early growth stages can be aided by locating seedlings emerging under the dried skeletons of the previous year's growth. The rosette stage extends through winter and spring. Large plants can eventually contain up to 26 deeply lobed leaves, but the size and lobing pattern may vary. The best diagnostic characteristic is the large triangular lobe at the leaf tip. Rosettes are usually prostrate when growing in open sites, but the basal leaves grow upright when plant densities are high or where shaded by competition.
Plants generally mature in late summer, long after most other annuals have senesced.
The stems of the mature plant are ridged, and the entire plant is covered with soft, appressed hairs. The rigid spines that surround the flower heads shatter in late fall and winter. Under some conditions, yellow starthistle behaves as a biennial, surviving a second winter and regrowing the following spring. Seed maturation coincides with the progressive loss of the bright yellow pigment of the developing flowers. As flower color fades, the seed quickly develop.