Invasive Plants

Three people in orange tops wearing backpack sprayers walk along spraying plants.
Yellowstone works to prevent the spread of invasive plant species, which can displace native species, change vegetation communities, affect fire frequency, and impact food for wildlife.

NPS / Pat Perrotti


Invasive nonnative plants can displace native plant species, including some endemic to the park’s geothermal habitats, change the nature of vegetation communities and affect fire frequency and the distribution, foraging activity, and abundance of wildlife. These changes can profoundly affect the entire ecosystem. For example, nonnatives that are unpalatable to wildlife may replace preferred native plants, leading to changes in grazing activity. In turn, this stresses plants not adapted to grazing.

Invasive plants have altered views of the park’s cultural landscapes and historic districts. Seeds may be spread by people and their vehicles, wild and domestic animals, and sand and gravel used for construction and maintenance work. The most vulnerable areas have been disturbed by human use: along the roads, trails, and rivers; though they are spreading from developed areas to the backcountry. Restoring native plants in an area that has become infested is extremely difficult.

In addition to about 1,386 native plant species, 225 nonnative species have been documented in the park through ongoing survey efforts. Not all of these nonnative species are still present in the park, but most of them are.

A park map showing park roads, rivers, lakes, and elevation with invasive plant treatments, mostly along roadways
A total of 120 acres (green circles) of invasive plants were treated in 2015. Most treatment areas are along roadways and in developed areas, though trails and some backcountry areas were also treated.


Managing Invasive Species

Controlling all the invasive plants—some well-established—is unrealistic. Staff prioritize treatments based on the threat they pose to native plant communities and the likelihood for successful control. Some infestations can be eradicated if the species is treated when the outbreak is still small; other species, such as spotted knapweed (Centaurea maculosa), are so common that stopping them from spreading is the primary goal. This strategy has helped prevent high priority invasive species from moving into wilderness areas where control is more difficult.

Nonnative vegetation was found on 7,189 acres of the 7,914 acres inventoried in 2015. Sometimes exotic plants are found in pure populations with few native plants, but are most often mixed within native plant communities. Based on program priorities for species, the equivalent of 120 acres of pure non-native populations were treated during 2015. Physical removal is the preferred method of control when feasible, but pulling or cutting of some of the perennial species serves to stimulate new growth, and the use of herbicides becomes necessary to control aggressive species over large areas. Plants were physically pulled or clipped on 20 acres while the remaining 100 acres were treated with herbicides. Most of the 36 species targeted for treatment are listed by the states of Idaho, Montana, or Wyoming as “noxious weeds,” which means they are detrimental to agriculture, fish and wildlife, aquatic navigation, or public health.

Preventing the Spread of Invasive Plants

Prevention efforts include control of construction materials entering the park, equipment inspections at park entrances, allowing only certified weed-free hay to be transported through the park, restrictions on the use of hay in the backcountry, and planting native species where ground disturbance has occurred.

To improve nonnative plant management throughout the region, park staff work with land managers from other government agencies, the Greater Yellowstone Coordinating Committee’s Weed Subcommittee, and the National Park Service’s Rocky Mountain Exotic Plant Management Team. The park uses Integrated Pest Management—chemical, biological, sociological, and mechanical methods—to control some of the nonnative plants. The park also cooperates with adjacent state and county Weed Control Boards to share knowledge and technology related to nonnative plant detection and control.


Quick Facts

Number in Yellowstone

225 species

Canada Thistle (Cirsium arvense)

  • Throughout the park and adjacent national forests.
  • Airborne seeds enable it to spread widely throughout the park, invading wetlands. Forms dense monocultures, thus radically changing an area by forcing out native vegetation.

Dalmatian Toadflax (Linaria dalmatica)

  • Northern portions of the park, especially around Mammoth.
  • Highly invasive, replaces native plants.

Houndstongue (Cynoglossum officinale)

  • Primarily found in Mammoth and East Entrance.
  • May have been introduced by contaminated hay used by both the National Park Service and concessioners in their horse operations.
  • Seeds easily attach to the coats of animals, and thus spread along animal corridors.
  • Highly invasive.

Leafy Spurge (Euphorbia esula)

  • Small patches in Bechler and along roadsides, so far being successfully controlled but spreading actively in Paradise Valley north of the park and outside Bechler on the Caribou-Targhee National Forest.
  • Becomes a monoculture.
  • Extremely hard to control because of deep underground stems (up to 30 feet) and dense vegetation.

Ox-eye Daisy (Leucanthemum vulgare)

  • Mammoth and Madison areas.
  • Can become dominant in meadows, is unpalatable to elk and other wildlife.
  • Control efforts have substantially curtailed infestation; monitoring and evaluation continue.

Spotted Knapweed (Centaurea maculosa)

  • Along roadsides and in the vicinity of Mammoth.
  • Aggressive species that, once established, forms a monoculture.
  • Aggressive control efforts underway to prevent a catastrophic change in park vegetation.

Management Issues

Resource managers target the most invasive species for control or removal.


Words to Know

Native species have occurred, now occur, or may occur in a given area as a result of natural processes. Native plants evolve within their own ecological niche in concert with other native plants.

Nonnative species (also known as exotic, foreign, non-indigenous, alien, or weeds) live outside their natural range and arrived there by human activity, either deliberate or accidental. Nonnative species typically have few biological controls to keep their populations in check.

Invasive species have the potential to thrive and spread aggressively outside their natural range, and can include native species. A naturally aggressive plant may be especially invasive when it is introduced into a new habitat. An invasive species that colonizes a new area often has an ecological advantage because the insects, diseases, and foraging animals that naturally keep its growth in check in its native range are not present in its new habitat. An invasive plant that sustains itself outside cultivation is considered naturalized and has not “become” native.


More Information


Sheley, Roger L. and Janet K. Petroff. 1999. Biology and Management of Noxious Rangeland Weeds. Corvallis: OSU Press.

Whitson, Tom L. et al. 2002. Weeds of the West. Western Society of Weed Science. Jackson, WY/Grand Teton Lithography.

Last updated: April 11, 2017

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