At Pinnacles National Park, out of approximately 625 plant species, about 100 are nonnative. Several of these species are invasive, with the potential for creating serious ecological damage and detracting from the uniqueness of the Park’s native plant community. Pinnacles National Park Weed Control Program is focused primarily on horehound (Marrubium vulgare), mustard (Hirschfeldia incana), and yellow star thistle (Centaurea solstitialis). Weed control efforts focus on these three species because of their potential for native habitat destruction. Yellow star thistle and mustard are controlled by working through a sequence of large areas on a monthly basis. Horehound is much closer to being eradicated within the Park and is controlled by monthly visits to 140 small plots. Eradication methods include hand pulling and herbicide application.
Yellow star thistle (Centaurea solstitialis)
Yellow star thistleproduces a deep taproot, which extends below the zone of root competition of associated annual species. This allows yellow star thistle to grow well into the summer after most other annuals have dried up. Each seed head produces stiff spines, 1-3 cm long that make the plant unpalatable to wildlife and painful for park visitors. Yellow star thistle is less abundant and somewhat less widely distributed within the Park than mustard. Nonetheless, it imposes a serious long-term threat because of its ability to produce large numbers of seeds and its growth during the hot summer months.
In January 1999, an integrated pest management (IPM) action plan was drafted for managing yellow star thistle at Pinnacles. The control objective of the IPM action plan was to reduce the abundance of yellow star thistle to 5% of its abundance at that time by the year 2002.
Horehound (Marrubium vulgare)
Summer Mustard (Hirschfeldia incana)
Did You Know?
Pinnacles National Park began as a volcanic field that originated about 195 miles south of its present location. It has traveled northward along the San Andreas Fault, and currently moves at a rate of about 3 - 6 centimeters per year.