2006 Annual Report 4d.Progress report. This report serves to document research conducted under a Specific Cooperative Agreement between ARS and the University of Idaho. Additional details of this research can be found in the report for the parent CRIS 5325-22000-020-00D, Biology and Management of Invasive Weeds in the Western US. Results of a field experiments in Idaho indicate that yellow starthistle abundance is limited by interspecific plant competition on gentle slopes (<15°) and by soil moisture on steep slopes (>55°). Yellow starthistle continues to spread in canyon grasslands of Idaho. A computer simulation model based on landscape topography predicts how fast and where the weed spreads. It identifies habitats where the weed is most invasive and is helping to guide management decisions to help stop spread of the weed.