What's the Problem with Invasive Plants?
Invasive plants displace native plants by outcompeting them for water, light, nutrients, and space. As native plants are lost, native animals who rely on those plants for food and shelter are threatened. Invasive plants can also negatively affect ecosystems by changing fire intensity and frequency, soil moisture regimes, and by producing thickets of plants so dense that the stands are impassible for large animals. Yosemite's invasive plant management focuses on many non-natives, including: Himalayan blackberry, yellow star-thistle, bull thistle, common velvet grass, cheat grass, French broom, Italian thistle, perennial pepperweed, and spotted knapweed.

Let's take these plant terms one at a time: 

Native plants: Plants that have evolved in a given area as part of the ecosystem. Through natural selection, they have adapted to soil, water and climatic conditions as well as to other plants and native wildlife with which they have co-existed for thousands of years. Yosemite hosts approximately 1,340 native plant species.
Exotic plants (also called non-native, alien, or introduced): Plants that were intentionally or accidentally introduced by humans to a location outside their natural range. Many exotic plants brought to Yosemite for cultivation, like Himalayan blackberry, have become naturalized, meaning they now reproduce in the wild. Others, like spotted knapweed, were accidentally introduced. In 2008, a dozen new exotic plant species were identified here. At present, some 190 exotic plants are documented in the park. Fortunately, many do not cause harm to the environment.
Invasive plants: Exotic plants that spread aggressively and displace native plants, causing significant ecological or economical damage. Exotic plants did not evolve as part of the ecological community. As a consequence, many of the forces that keep native plant populations in check—such as diseases, pests and predators, and competition with other plant species—do not exist for exotic plants. Yosemite's efforts focus on the control of nine invasive plants.

What Is the National Park Service Doing to Help?
Employees at each national park work with partner organizations and volunteers in an effort to control invasive species in and around the parks. In addition, 17 Exotic Plant Management Teams have been set up across the country, each tailored to a specific region. These teams assist parks with ongoing efforts to remove invasive plants and provide management guidance for invasive plant control in individual parks.

What Actions Are Being Taken in Yosemite?
Prevention and early detection of invasive plants are the most important and economically feasible ways to control the spread of invasive plants. Additionally, Yosemite National Park Service work crews, park partners and volunteers have contributed 100,000 hours to fight infestations. Since 1987, crews have used techniques, such as hand-pulling, lopping, and mowing, to manage the park’s most invasive species. Under the 2008 Invasive Plant Management Plan, Yosemite employs two herbicides--glyphosate and aminopyralid--as additional tools to manage the most threatening plants that manual methods have not effectively controlled.

What Can You Do to Help?

• Choose non-invasive ornamental plants or native species for your garden; ask your nursery for native, non-invasive alternatives.
• Scout your property for invasive species and remove them before they become a problem.
• Avoid transporting seeds by removing all seeds from your clothing and wipe your shoes before leaving any natural area.
• Prevent pets from walking through areas with invasive species where they could pick up seeds.
• Don’t camp in weed-infested areas.
• Drive on established roads and hike on designated trails.
• Wash mud or dirt off vehicles before going onto public lands.
• Wash boats, trailers and gear before going to a new river.
• Alert people in your neighborhood and place of employment about the problem with invasive species and what to do about them.  

Get Involved

• Volunteer to help remove invasive plants in parks and natural areas. Yosemite’s "HaPY" (Habitat Protectors of Yosemite) meets Friday mornings from Memorial Day through Labor Day. Call 209/379-3286 to sign up.
• Visit the National Park Service's systemwide invasive species page.
• Learn more from The Nature Conservancy.
• Learn more from the California Invasive Plant Council.
• Print out a "Control of Invasive Plants" brochure for Yosemite [1.39 MB PDF].

Follow Yosemite's Invasive Plant Management Plan

• Learn more about the planning process behind Yosemite's 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].
• Since the plan's creation, follow how Yosemite botantists have implemented invasive plant management strategies. 
• Watch a slide show about Yosemite's Invasive Plan Management Plan.