At Pinnacles National Monument, 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 monument’s native plant community. Pinnacles National Monument 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 monument 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 thistle is considered one of the most invasive weeds in California by the California Exotic Pest Plant Council. It has already degraded over 25% of the land in California and is called “the plant that ate California”. Yellow star thistle is a summer-blooming annual in the Sunflower family and native to Eurasia. The plant is found primarily in open, disturbed areas such as road edges and stream channels, but through time moves increasingly into undisturbed locations, including meadows and riparian corridors.

Yellow star thistle produces 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 monument 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)
Horehound is an herbaceous perennial plant native to Europe that arrived in North America as a cultivated herb. It reproduces readily by both seed and vegetative means. The seed is readily distributed by wildlife and visitors due to recurved barbs on the seed which attach to fur and clothing. It is likely that animal fur, possibly the fetlocks of horses and the fur of small animals, has transported horehound seeds at Pinnacles because horehound infestations are often located at corral sites and animal burrows. The monument staff has been successfully controlling horehound since the late 1980s.

Summer Mustard (Hirschfeldia incana)
Summer mustard is a biennial native to the Mediterranean. The plant was first established in southern coastal California and now can be found in Oregon, Idaho, Nevada, and Washington. Summer mustard grows extremely well in disturbed, open and sunny areas. In the first year of growth, the mustard plant produces a rosette; in the second year, the plant bolts, flowers, sets seed and dies. The plant blooms May through October and in late fall forms dense stands of brittle woody seed stalks. At Pinnacles, summer mustard is out-competing native plant species, encroaching on trails and the dried seed stalks are creating hazardous fuel buildup.