Yellow Star-Thistle: A "Wanted" Weed
Yellow star-thistle (Centaurea solstitialis) has grown into Yosemite National Park's most "wanted" weed. Despite its rather attractive yellow blossoms, this Eurasian plant quickly takes over Sierra Nevada native habitat. One individual plant, in fact, can disperse up to 150,000 seeds in a single season. In 2011, there were 18 net acres of star-thistle in Yosemite scattered over some 250 gross acres of steeply sloped terrain, and even more on the adjacent Stanislaus and Sierra national forests. To control this plant, an accelerated 2011 interagency management effort went into action all over the hillsides of the Merced River Canyon-spanning approximately 18 river miles. More than seven agencies and groups have worked to control star-thistle in the canyon since 1998, but the problem demands a stepped-up approach to protect the canyon from being overtaken by this spiny, aggressive weed.
In 2011, two five-person NPS crews--led by Heather Smith and Josh Higgins--completed treatment of yellow star-thistle in and to the east of El Portal by early July. Since 2010, these areas have been treated with herbicides by NPS crews with USFS technical support and funding. Both NPS and USFS have used hand-pulling (manual control) near waterways where herbicides are not desired.
Based on the above chart, fewer NPS acres were treated in 2011 for good reason. A 41% decline in treated acres from 2010 to 2011 proves the success of past treatments. Yellow star-thistle is an annual plant that produces seed with a mean viability of about two to six years. Because all known yellow-star thistle received treatment in 2009, each year fewer NPS acres will need to be treated until the seed is no longer viable. On U.S. Forest Service lands, the 207% increase between 2010 and 2011 was due to the ability to extend the treatments to previously untreated areas. Plans exist to continue the USFS-NPS cooperation through 2014-which is the time needed to deplete the seed bank. As evidence of success, botanists have seen an increase in native plants in yellow star-thistle-treated areas where native seeds in the soil have had the opportunity to grow.
Interagency efforts in the Merced River Canyon will continue to treat yellow star-thistle--an annual plant that produces seeds with a mean viability of two to six years.
In some areas, federal land managers choose not to spray herbicides because barrier zones exist near waterways. Here, American Conservation Experience crew members hand-pull yellow star-thistle along the Merced River on U.S Forest Service land.
Sierra National Forest botanist Christina McAdams hand-pulling star-thistle growing along Highway 140. Star-thistle, and other invasives, often take root near roads and disturbed areas.
NPS crews use ropes to secure themselves on steep slopes as they apply herbicide to yellow star-thistle infestations. Annual treatment will continue until efforts deplete the plant's seed bank.
Who Else Has Contributed to Improving the Merced River Canyon
Partners were busy in the canyon with complementary work keeping invasive weeds under control.
Did You Know?
In Yosemite Valley, dropping over 594-foot Nevada Fall and then 317-foot Vernal Fall, the Merced River creates what is known as the “Giant Staircase.” Such exemplary stair-step river morphology is characterized by a large variability in river movement and flow, from quiet pools to the dramatic drops of the waterfalls themselves.