Yosemite National Park has documented more than 130 non-native plant species within park boundaries. These non-native plants were introduced into Yosemite following the migration of early Euro-American settlers in the late 1850s. Natural and human-caused disturbances, such as wildland fires and construction activities, have contributed to a rapid increase in the spread of non-native plants. A number of these species aggressively invade and displace the native plant communities, resulting in impacts on the park's resources. Non-native plants can bring about significant changes in park ecosystems by altering the native plant communities and the processes that support them. Some non-native species may cause an increase in the fire frequency of an area or increase the available nitrogen in the soil that may allow more non-native plants to become established. Many non-native species, such as yellow star thistle ( Centaurea solstitialis ), are able to produce a long tap root that allows them to out-compete the native plants for available water. A volunteer removes bull thistle in Sentinal Meadow.
Bull thistle ( Cirsium vulgare ), common mullein ( Verbascum thapsus ), and Klamath weed ( Hypericum perforatum ) have been identified as noxious pests in Yosemite since the 1940s. Additional species that have been recognized more recently as aggressive and requiring control are yellow star thistle ( Centaurea solstitialis ), sweet clovers ( Melilotus spp.), Himalayan blackberry ( Rubus discolor ), cut-leaved blackberry ( Rubus laciniatus ) and periwinkle ( Vinca major ).