Yellow Star-Thistle: A "Wanted" Weed

Invasive crew member sprays blue-dyed herbicide on a steep slope

Fall protection-certified NPS crews apply herbicide with a blue indicator dye to yellow star-thistle.

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.
Two workers spray blue-dyed herbide on yellow flowerheads

With yellow star-thistle's blossoms visible here, NPS crew members treat this invasive plant.

What is the best way to limit this plant's growth? Hand-pulling contains small to medium-sized infestations, but at some point, infestations spread faster than they can be pulled. Timed mowing has worked well, but is too dangerous on the slopes that remain. Now, herbicide treatments are the only reasonable option for successfully controlling the largest patches of yellow star-thistle, at least until the density is low enough to be hand-pulled again. Combined methods can effectively reduce the number of seeds produced. Current herbicide treatments (glyphosate and aminopyralid) are safe and efficient options on the steep slopes of the Merced River Canyon, maximizing success of yellow star-thistle control while minimizing negative impacts, such as soil erosion and danger to workers on nearly vertical slopes. When treatments are effective, fewer re-treatments are needed, and larger treatment areas can be handled. Resources are freed up to prevent and to detect the plant early on with a rapid response (Invasive teams look for new infestations and eradicate these small patches by hand before herbicides become necessary.)
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.
Herbicide Treatment Totals
Year NPS (canopy acres)
USFS (canopy acres)
2009 12.2 0.19
2010 27.33 9.22
2011 16.13 19.13

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.

Crew members in a cherry picker lift
Herbicides used by the Yosemite Invasive Plant Crew to control yellow star-thistle are called glyphosate and aminopyralid. Both contain a blue indicator dye used to mark sprayed foliage during treatment application; the blue dye fades in a day or two.
Invasive crew members carefully carry their equipment on steep slopes

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.

Cherry picker machine along a steep slope
With the Yosemite Forestry "cherry picker," staff can safely access the steep and unstable cut-bank slopes of Foresta Road--minimizing erosion and minimizing the exposure of workers to hazardous slopes.
Crew members hand-pull invasive star-thistle on the river shoreline

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.

Botanist stands on roadside pulling invasive plants

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.

large patch of blue dye can be seen on hillside
Blue dye marks areas on El Portal's steep slopes in Yosemite treated for yellow star-thistle. In 2011, 16 acres were treated in Yosemite, and 19 acres on adjacent Sierra and Stanislaus national forest lands.
Worker climbs steep slope with ropes as re-inforcement

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.
  • The Upper Merced River Watershed Council and the Sierra National Forest partnered with American Conservation Experience Crew youth in June 2011 in a project funded by a Sierra Nevada Conservancy Grant. The multi-national ACE youth spent a week hand-pulling yellow star-thistle next to the river where herbicides are not used.
  • The Bureau of Land Management worked with the Upper Merced River Watershed Council and the Mount Bullion California Department of Corrections Crew to control yellow star-thistle and Italian thistle along seven miles downstream of Briceburg on BLM landin a project funded by the Sierra Nevada Conservancy.
  • Sierra National Forest personnel hand-pulled six patches of yellow star-thistle along Highway 140 in June 2011 and hand-pulled 65 garbage bags of Italian thistle along Highway 140 in April 2011.

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