Hill's thistle is a plant species native to Michigan and grows only in the Midwestern United States and Ontario, Canada. Populations are often small, scattered and occupy sites of frequent disturbance. These are factors associated with an increased risk of extinction. Currently, Hill's thistle is listed as a species of special concern. This classification offers no legal protection, but encourages further assessment of the life history of the species. In this study, plants in three populations from northern Lower Michigan were tagged and measured for growth over two field seasons. Plant features measured included: leaf shape, number of leaves, longest leaf length, and plant diameter. Number of leaves and the diameter of the plant were significant predictors of flowering. An experiment was performed on flowering plants in which 24 plants were covered with mesh bags to prevent insect access and therefore were self-pollinated. Thirty-six plants were left to openly pollinate. Seed set in the open-pollinated group was 36.5 ± 19.2% and 7.17 ± 8.4% in the self-pollinated group. Seeds from each group were collected and germinated. There was a significant difference in seed germination between groups. Seeds from the open-pollinated plants had a mean percent germination of 62.4 ± 27.3 compared to the mean percent germination of 9.39 ± 4.03 of seeds from self-pollinated plants. In addition, this species is perennial in that it can flower in more than one season and will produce asexual sprouts from the rootstock.

Key words: Cirsium hillii, pollination, reproduction, seed