Aspects of the life history and population dynamics of the Platte thistle (Cirsium canescens Nutt.), a rangeland weed, were studied in the Nebraska Sandhills from 1977 through 1979. This thistle is a monocarpic perennial, reproducing only by seed production, which typically occurs in small, scattered groups of plants. Seedling mortality at four sites was 75% during the first year, mostly because of dry moisture conditions. In plots with good vegetative cover, thistle rosette plants had a lower mortality rate, but growth rate was slower than that of rosette plants in plots with sparse vegetative cover. Seed heads developed during May and June and gained more than one-half of their dry weight shortly before maturing. Plants averaged 72, 197, and 303 seeds at three sites. Within sites, plants produced seeds in proportion to the number of seed heads per plant. Although naturally occurring biological-control agents have little effect on the vegetative growth phase of the life cycle, they greatly reduce the production of seeds.
