Alien plant invasion

Alien plants may be exotic, but they are unwelcome in North Cascades National Park. Exotic plants threaten the ecosystem integrity of the area. Alien invaders such as knapweed, skeletonweed and scotch broom thrive in areas impacted by humans.

Exotic plants can be brought in as seeds attached to vehicles, pets, cargo or spread by rhizomes such as Japanese knotweed. Often plants like English ivy, holly and herb robert are brought in to beautify an area, but are detrimental to native species.

Because of their shade tolerance and adaptability, alien plants are more successful than natives at colonizing human-disturbed areas. Without controls, such as competing plants and predatory insects, alien species flourish and out-compete native plants.

Over thousands of years, more than 1,500 native plant species have adapted to successfully fit habitats of the North Cascades. Though hardy enough to survive natural disturbances such as fire, most native plants do not fare as well in areas changed by humans. To preserve the native landscape, the National Park Service and USDA Forest Service use a range of techniques to control alien plant invasions.

This summer a team of National Park Service exotic plant managers will partner with the Forest Service and the Nature Conservancy to eliminate weeds throughout the area. To limit environmental damage most weeds are removed by hand. This is extremely work intensive because seeds of some exotic plants can lie dormant in the soil for up to seven years.

Exposing alien plants to predators and diseases from their homeland can also help control their spread. These natural controls have not been found to attack plants native to the North Cascades.

In 1994, two species of gallfly, which feast only on knapweed seeds, were used to help limit an infestation in the Lake Chelan National Recreation Area. Though biological control usually does not eliminate alien species, it can slow down the rate of invasion.

When mechanical and biological attempts to control alien plants such as skeletonweed fail, the NPS may employ chemical controls. Because herbicides can cause unwanted damage to native plants and animals, their use is carefully evaluated and administered. Continued efforts to control alien species will help protect natural plant communities in the North Cascades.

Herb Robert
(Geranium robertianum)

This alien invader is a shade tolerant, low growing geranium that's described as a winter or spring annual. Spread by seeds this alien plant can reach heights up to 10 inches. The stems are often dark red and covered with white hairs. A somewhat sticky or oily feel and a distinct odor from the crushed leaves are attributed to these hairs.

The stems fork at the nodes where a pair of long-stemmed, deeply dissected leaves branch out, giving the overall plant a fern like appearance. Flowers are found from early spring to fall and range in color from magenta to pink to white.

Spotted Knapweed
(Centaurea biebersteinii)

An eight to 48-inch-tall perennial with a stout tap root, knapweed appears hairy and roughalmost woolly at times.

The leaves are blue-gray and divided once or twice into lobes on each side of the center vein. Overwintering rosettes grow in early summer producing 1-15 stems. The pink to purple flowers occur in egg-shaped heads which are at the ends of clustered branches.

Knapweed flowers from June to October producing approximately 400 seeds per plant. A very aggressive species, knapweed can infest large areas very quickly.

Japanese Knotweed
(Polygonum cuspidatum)

This plant can grow to eight feet high, often appears shrubby and has reddish, bamboo-like stems, which are smooth and hollow.

The stems die back each year at first frost leaving tall, brown skeletons. The leaves are alternate, between four and six inches long and usually egg shaped. Small individual flowers bloom in late summer and are whitish to greenish, forming drooping clusters. Knotweed prefers moist, open, sunny locations but can grow on dry sites, as well.

Spread is attributed to rhizome fragmentation rather than seeding. Once established the knotweed forms deep, thick mats obstructing root removal.