Velvet grass

Prevention and early detection of invasive plants are the most important and economically feasible means to control the spread of invasive plants, as noted on our general invasive plants webpage. Yosemite National Park Service work crews, park partners and volunteers have been using techniques such as hand-pulling, lopping, and mowing to manage the park’s most invasive species as part of an Invasive Plant Management Plan. Yosemite, in 2008, began using two herbicides--glyphosate and aminopyralid--as additional tools to manage the most threatening plants that manual methods have not effectively controlled.

Yosemite's removal efforts are focused on the control of nine high-priority species: yellow star-thistle; Himalayan blackberry; spotted knapweed; bull thistle; common velvet grass; cheat grass; French broom; Italian thistle; and perennial pepperweed.

In 2009, Yosemite’s invasive plant biological technicians have treated 156 gross-infested acres (27 acres in wilderness) and mapped 231 acres of invasives so far this season. On the whole, invasive plant ecologists have recorded 805 acres (285 in wilderness) of non-native plant infestations in Yosemite, but there is more to document. Some species, such as cheat grass, are so widespread that it’s a challenge to map.

Velvet grass fills some of Yosemite's meadows(represented by light brown patches). Plant removal crew members, in the bottom center of photo, hand-pull the invasive plant.

The park's management of these invasive plants incorporates the following goals:

• Prevent new invasions through early detection, equipment inspections, use of certified weed-free pack stock feed, and use of weed-free soil and rock for construction projects.  
• Eradicate existing infestations to minimize threats to natural and cultural resources and scenic values.
• Preserve native plants and sites valued by American Indians.
• Promote restoration of native species and habitats in ecosystems degraded by invasive plants.
• Ensure the program is environmentally safe and supported by research.
• Implement the most appropriate control technique for each species and site. 

Cheat grass

So, how exactly is Yosemite's invasive plant management one of the highest scientific concerns at Yosemite? In 2009, approximately 30 seasonal NPS crew members were involved in removing invasive plants—25 crew members, for example, removed star-thistle in June of 2009. In addition, Yosemite benefitted from 619 volunteers and 8,763 volunteer hours specific to invasive-plant removal in 2009.

Invasive species not only displace native plants, but they also can have severe negative impacts on many of the cultural and natural features. Invasive species are able to:

• Disrupt Native Wildlife: Many animals are adapted to use very specific plants for food and habitat. When desirable plants are replaced by invasive species, the animal’s food source or habitat is lost. As native plants shrink in numbers, so will the number of animals that depend upon the plants. Additionally, many invasive species are poisonous or harmful to animals.
• Change Fire Regimes: Invasive species such as cheat grass can increase the frequency, seasonality, and intensity of fires. Many native plants and animals can be displaced by changes to the fire regime.
• Alter the Visitor Experience: Invasive plants can transform spectacular displays of showy wildflowers into large, unattractive monocultures. Thorns and irritants on invasives can transform inviting and accessible areas into impassable and unattractive thickets of thorns and brambles.
• Cause Impacts Beyond Park Borders: Invasive species have no regard for political boundaries. They can rapidly spread from Yosemite onto adjacent lands.

Fire is one method to control invasives.

The following methods or techniques are commonly used for controlling invasive plants.

• Mechanical removal includes hand-pulling, tilling, and mowing. Benefits: Selective if hand-pulling; only target species are affected. Disadvantages: Labor intensive; requires repeated treatments; disturbs ground.
• Fire control includes both prescribed burns and hand-torching. Benefits: Can cover large area; reduces biomass. Disadvantages: Stimulates re-sprouting and germination of particular invasive plants; impacts air quality; can affect native species. (Learn more about fire ecology.)
• Chemical control is the use of herbicides. Benefits: Effective to cover large areas; less labor intensive; potential to irradicate a population of invasive plants. Disadvantages: Can affect native species; training costs; potential toxicity.
• Biological control uses the introduction of an herbivore (such as a beetle that consumes seeds) or pathogen (such as a fungus) that will infest invasive species. Benefits: Continues to work for many years; effective treating large infestations. Disadvantages: Potential threat to non-targe plants; introduces another exotic species.

Learn More about the Invasive Plant Management Plan

• View the 2008 Invasive Plant Management Plan. Key documents include the Invasive Plant Management Plan Workplan - October 2008 [33 kb PDF] and the Finding of No Significant Impact - September 2008 [436 kb PDF].
• Watch a slide show about Yosemite's Invasive Plan Management Plan.
• Read a general guide to Yosemite's invasive plants on a separate webpage. 

• Or, volunteer to help remove invasive plants in Yosemite through the “HaPY” program (Habitat Protectors of Yosemite).