DETECTION OF YELLOW STARTHISTLE (CENTAUREA-SOLSTITIALIS) AND COMMON ST-JOHNSWORT (HYPERICUM-PERFORATUM) WITH MULTISPECTRAL DIGITAL IMAGERY

Ground-based surveys to regularly document the size, shape, and locati on of weed populations usually are not economically feasible. Digital images of vegetation in semiarid rangeland were obtained from four cha rge-coupled devices with spectral filters mounted in an airplane. The ability to distinguish yellow starthistle and common St. Johnswort fro m other rangeland vegetation in images with 0.5, 1, 2, and 4 m spatial resolution was assessed. Detection, positioning, and measurements of size and density of yellow starthistle and common St. Johnswort coloni es with densities as low as 30% ground cover were possible at all thos e resolutions, Images indicated yellow starthistle occupied about one- third of a 180-ha study area, Images taken in June indicated 2 to 10 h a less yellow starthistle ranging from 30 to 100% cover than images ta ken in July for all resolutions. Images indicated common St. Johnswort occupied less than 4 ha of the 180-ha study area. This procedure prov ides a method to establish baseline plant community composition and a way to monitor species population changes and dispersal over time.