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Yosemite Working To Contain Star Thistle Weed

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Yosemite National Park is taking strides to contain the invasive weed yellow star thistle. Yellow star thistle (Centaurea solstitialis) is a highly invasive non-native plant species introduced from Europe and Asia that now covers 15 million acres in California. Yellow star thistle quickly establishes a monoculture and pushes out native vegetation.

There are approximately 2,000 acres impacted by yellow star thistle in Yosemite National Park, primarily in the El Portal Administrative Site. It has also been found in Yosemite Valley, Tuolumne Meadows, Foresta, and along the Tioga Pass Road, the El Portal Road, and the Aspen Valley Road.

Photo documentation in the El Portal Administrative Site of Yosemite National Park reveals the population explosion in the 1990s. It spreads easily through road maintenance, construction, vehicles, and people. It is a constant risk to the ecological health of Yosemite National Park and the Merced River Canyon.

The Division of Resource Management and Science for Yosemite National Park is attempting to control and reduce the infestation within the park. In 1998, ecological restoration crews began an intensive, comprehensive attack designed to protect sensitive rare plant locations, remove small isolated outlying patches of yellow star thistle, mow heavily infested road corridors in the El Portal Administrative Site, and initially contain some of the larger patches.

Over the last five years, crews have continued to build on to this program, working more extensively on containment of some of the larger patches. Two notable areas are the Rancheria Hill slope and the El Portal roadsides. There has been a dramatic reduction of yellow star thistle in these areas of consistent treatment. The successful removal of yellow star thistle along the road corridor has removed a large potential seed source for further infestation within the park.

The method used to remove yellow star thistle used is timed mowing with weed trimmers when the plant is at a two to five percent bloom. This is typically in mid June. 1,823 hours of timed mowing and hand pulling were logged by staff in 2002 and 2003. Volunteer groups, including the dedicated “Weed Warrior” week coordinated through National Park Service and Yosemite Association have been a particular success. In 2003, staff worked controlling 33 acres that had been infested by yellow star thistle. Resource Management has had considerable success in the control of yellow star thistle using mechanical methods. The work is extremely physical, in very hot conditions and allows a brief window of opportunity for the staff to mechanically remove the plants. These factors combine to make this a challenging project to manage.