Non-native plants

What is one of the largest threats to the integrity of the North Cascades ecosystem? Surprisingly, it's an invasion of non-native plants. Non-native plants and animals threaten natural ecosystems around the world. National Parks are no exception. Nonnative plant introductions, often for horticultural or agricultural use, sometimes have unintended consequences.

Over thousands of years, more than 1600 different native plant species have adapted to specific North Cascade habitats. Though most of these plants are resilient after natural disturbances such as fire, they do not fare as well in areas of human disturbance. Most nonnative plants colonize well in disturbed soils. Diffuse knapweed (Centaurea diffusa) poses a huge threat to native plants east of the Cascade Crest . In their homeland, knapweed and other non-native species must compete with each other and with predators (usually insects) which keep them from spreading. When they arrive here, those competitors and predators are absent. For over a decade the National Park Service has toiled to reduce the spread of knapweed and other alien plants. The Stehekin community in the Lake Chelan National Recreation Area assists the effort by hand-pulling plants in the valley. Knapweed has the potential to spread to much higher elevations in the North Cascades ecosystem. The Stehekin valley is surrounded by wilderness, but knapweed has already invaded some wilderness areas 30 miles to the east. Efforts like those initiated in the Stehekin valley should help prevent the further spread of knapweed into the local "high-country" and protect natural plant communities. Annually, in June a volunteer knapweed pull is held along State Route 20.

Reed canary grass is another aggressive nonnative plant that invades disturbed areas and is pervasive throughout the western United States. During vegetation surveys around Ross Lake many infestations of this invasive grass were identified, ranging from individual plants to as much as 5 acre patches. The varied lake level has resulted in a different hydrologic regime from what existed before the reservoir was constructed. Construction of Ross Dam by Seattle City Light inundated a 26 mile long section of river and over 11,000 acres of riparian habitat. This habitat had served as a resting stop for over 150 species of migratory birds and winter range for many mammal species. It was breeding range for approximately 75 species of neotropical migratory birds that winter south of the United States, many of which are declining in population.

Reed canary grass thrives in wetland stream outlets where water levels fluctuate. Thick shoots and root systems begin growing early in the season, excluding native wetland plants and natural establishment of riparian forests. Native fish and other animals are not well adapted to spawn or reproduce in reed canary grass thickets. Reed canary grass threatens the habitats of both animal and plant species that are listed on Threatened and Endangered Species lists including bull trout, cascades frog, Harlequin duck, and the bald eagle. Reed canary grass also directly affects habitats that support 27 Washington State listed plant species.

Nonnative invasive species like reed canary grass need to be removed in order to protect the undisturbed rivers and streams of the North Cascades. A creative multi-faceted approach is being implemented by Park Service plant ecologists, exotic plant management crew and plant propagation employees to restore wetlands around Ross Lake. Nutrient rich wetlands will be created with the woody debris which washes into Ross Lake annually. In the past, Seattle City Light (SCL) crews collected this wood and disposed of it by burning. In 2004, new riparian habitats were created using some of this wood (1,320 cubic yards) to mimic naturally occurring wetlands and riparian areas which are found in adjacent drainages. These new riparian areas allow the planting of a diversity of plant species and the elimination, by shading, of reed canary grass. Native plant species either salvaged from on site or propagated in the Park nursery, will be planted to create native plant communities which mimic those lost to the creation of the reservoir.

Employees and volunteers gather native grass seed, which is used to fill in the "voids" created by the removal of the offending nonnative plants. Hopefully, the grasses will become established in the human-disturbed areas, and stem the tide of knapweed, reed canary grass and other nonnative invasive plants. Treatments, monitoring, and native reestablishment plantings will continue with the help of agency cooperation and community volunteers. If you are interested in volunteering to help with native plant restoration please contact us.

Over 258 exotic plant species have been documented, in or near, the North Cascades National Park Complex. This list comprises a few common culprits.

  • Diffuse knapweed (Centaurea diffusa)
  • Spotted knapweed (Centaurea maculosa)
  • Rush skeltonweed (Chondrilla juncea)
  • St. John's-wort (Hypericum perforatum)
  • Scotch-broom (Cytisus scoparius)
  • Japanese knotweed (Polygonum cuspidatum)
  • Foxglove (Digitalis purpurea)
  • Common mullein (Verbascum thapsus)

Last updated: August 11, 2017

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