Nature Article
Non-native Plants
01/01/2010
Kyle Spinks, Natural Resources Technician
Urban natural areas are subjected to pressures from surrounding development. Many areas that appear natural have actually been changed by the presence of humans. Many of the changes may appear benign at first glance, but closer examination reveals harmful effects to the ecosystem. One example of a change caused by human influence is the introduction of plants from other parts of the world. Many of these plants have been present for so long that some people do not realize they are not native to Oregon. Non-native plants can run rampant in the wet, mild climate of the Pacific Northwest, without the diseases, insects, parasites, etc., which kept them in check in their native land. Invasive plants are aggressive and damage the ecosystems to which they have been introduced by over-growing and pushing out native vegetation. The result is called a monoculture, whereby a single species dominates the environment. Monocultures of non-native plants can change the nutrient cycles, habitat values, sedimentation rate, soil chemistry, and hydrology of a site.
Non-native plants are usually introduced accidentally, spreading from ornamental landscaping, but they are sometimes intentionally planted without awareness of all the consequences. This article focuses on the most notorious introduced species in the Park District, how to recognize them, why they are a problem, and how these plant pests can be discouraged. If you are interested in attacking these pests in the Park District's natural areas, please contact the Natural Resources Department at 503-629-6305 x 2950 or nature@thprd.org.
Himalayan Blackberry (Rubus armeniacus, formerly R. discolor)
Description:
The evergreen leaves of the Himalayan blackberry are made up of 3-5 leaflets that are oval and toothed. The flowers are white to pink and the fruits are black when ripe, with a solid central core. The stems have distinct angles and large, curved thorns.
Natural History:
Himalayan blackberry was introduced from Asia as a desirable berry producer. It escaped from cultivation and rapidly spread by underground growth and through seed dispersal via animal feces. It is usually found in the sun but can grow in the shade. Under ideal conditions it can grow 20 to 30 feet in one season.
Negative Impacts:
This species climbs and spreads over native plants, blocking their access to sunlight. Although wildlife eat the fruit and use the brambles for shelter, blackberries can form impenetrable hedges that reduce habitat diversity. The soil beneath blackberry bushes is exposed and provides poor filtering and erosion control compared to native vegetation.
Control:
Himalayan blackberry is extremely hardy and persistent. The most environmentally safe way to control it is by removing the canes during the growing season. This can be done by mowing the canes or by using goats to graze on it. It can take up to three years of diligent removal of new shoots to weaken the root system. Success can be increased by digging up the main root nodule during the winter after the canes have been removed. It is a good idea to replant the area to provide shade and competition to discourage the return of blackberries. Red-osier dogwood (Cornus sericea) or elderberry (Sambucus spp.), are good choices. Another method is to spray the bramble with an herbicide (it must be an approved herbicide if used near wetlands or streams). Always follow the label directions...they are the law.
English Ivy (Hedera helix)
Description:
This evergreen vine spreads across the ground or climbs up surfaces. When mature it produces clusters of small greenish flowers that form black berries. The leaves are triangular with 3-5 lobes and are dark green and glossy.
Natural History:
English ivy is native to Europe and has spread from landscaped sites. It grows in upland areas in the sun or shade.
Negative Impacts:
This plant forms dense carpets that exclude all other vegetation and prevents regeneration of native plants. It kills mature trees by blocking the sun or causing damage with its anchoring structures. Infestations of ivy result in a dramatic reduction in habitat diversity. In a study of Forest Park, no reptiles or amphibians were found in ivy-dominated sites.
Control:
If ivy is growing up a tree, cut the vines at ground level and, if possible, pull them off the tree trunk. If ivy is growing as a groundcover, pull or dig it up during the winter to avoid disturbing native plants that may be present. Replant the area with native plants and monitor the site for any surviving pieces of ivy, removing any regrowth as it occurs.
Scot's Broom (Cytisus scoparius)
Description:
Scot's broom is a spindly, deciduous shrub in the pea family. A mature shrub is 8 feet tall with a stem 1-2 inches in diameter at the base. The leaves are small and the branches are green with 5 ridges running down their length. The flowers are yellow and turn into flattened, black seedpods about 1 inch long.
Natural History:
Scot's broom is from the Mediterranean region of Europe. It prefers sunny, open sites at low elevations with dry soils. It was originally planted as an ornamental and to stabilize road banks.
Negative Impacts:
Scot's broom takes over low-elevation uplands in the Pacific Northwest. It is toxic to most livestock. It produces a tremendous number of seeds which may remain viable for over 30 years.
Control:
Pull or dig Scot's broom plants during August, when they are dry and under water stress, or in the spring, when root resources are lowest. Cut the base of the plant as close to the ground as possible (below the 'budding stock') to prevent resprouting.
Purple Loosestrife (Lythrum salicaria)
Description:
Purple loosestrife is a tall shrub with spikes of bright purple/pink flowers that have a crinkly, tissue paper-like appearance. The stems are square in cross-section. It blooms from July to September.
Natural History:
This species grows only in wet areas. It spreads by seeds and underground stems called rhizomes. Purple loosestrife is native to Europe and Asia and was first introduced to the Willamette Valley as an ornamental landscape plant. From yards and gardens it has spread into nearby wetlands.
Negative Impacts:
Purple loosestrife is classified as a noxious weed in all Pacific Northwest states because it invades wetlands, displaces native species, forms impenetrable monocultures, and offers few benefits to wildlife. These stands are long-lived and a mature plant can produce 1,000,000 seeds per season. The seeds remain viable in the soil for many years. In January 1993, the Oregon Department of Agriculture declared a quarrantine against the plant, but purple loosestrife can still be found in wildflower seed mixes. Some nurseries carry a 'sterile' version of the plant, but this version is still capable of spreading by rhizomes.
Control:
The most effective form of control is to prevent infestation. Once an area has been infested, the plant can be physically removed by pulling it up if the infestation is small (fewer than 100 individuals). Mowing or cutting will not kill it. Herbicides approved for use in wetlands is effective, but always follow label directions and avoid use of herbicides where overspray or runoff may get into water.
Current research is investigating successful and safe forms of biological control for purple loosestrife using insects and diseases. Contact the Natural Resources Department at 503-629-6305 x 2950 or nature@thprd.org to report infestations.
Reed Canarygrass (Phalaris arundinacea)
Description:
This coarse, perennial grass can grow up to 7 feet tall. It has sturdy, hollow stems and 8-inch-long flower stalks that stand high above the leaves.
Natural History:
Reed canarygrass is a native of temperate regions of Europe and Asia. It is usually found in wetlands and spreads by underground growth and through seeds. It was planted for erosion control in creek corridors after sewer lines were completed in the 1950s. It has also been planted for livestock forage.
Negative Impacts:
This species is hardy and exhibits rapid growth, forming tall, thick, impenetrable stands that replace native species within a few years of infestation. It provides little wildlife value because it grows too dense for waterfowl nesting, and is unpalatable as forage except when the plant is young. It uproots in clumps during flooding and can clog streams and culverts, increasing the chances of flooding. It produces abundant pollen and chaff, which aggravate allergies.
Control:
Reed canarygrass is notoriously difficult to control and, unfortunately, this species dominates most Park District wetlands. Some research has been conducted to evaluate different control methods under various conditions. Small clumps can be removed by hand, but mowing alone does not eradicate the plants. Controlling water levels can drown the plant, and it is somewhat intolerant of shade. Experience is needed to correctly apply the appropriate herbicide at the right time of year. Always follow label directions...they are the law. No biological controls are known at this time.
Bittersweet Nightshade (Solanum dulcamara)
Description:
Bittersweet nightshade is a perennial, vine-like plant with egg-shaped or heart-shaped leaves, some of which have ear-like lobes at their base. The flowers resemble shooting stars with 5 blue-violet petals that arch back to reveal a yellow center. The berries are bright red and oval.
Natural History:
Bittersweet nightshade is native to Eurasia. It has escaped from gardens and now is frequently found growing in both wetlands and uplands. It grows from underground stems (rhizomes) and creeps along the ground or climbs up other vegetation.
Negative Impacts:
This plant dominates the riparian vegetation along many urban streams. It smothers native plants and climbs up trees, covering the area like a carpet. The leaves and berries are moderately poisonous.
Control:
Remove the above-ground portion of the plant by repeated cutting. Carefully pull and dig to remove the roots. Digging up the underground rhizomes is difficult because they break easily, leaving viable pieces in the ground to resprout. Pieces of stem and roots can propagate easily, so cuttings should be removed from the site and destroyed. Cutting should be carried out before the plant flowers to minimize the distribution of seeds. This procedure should be followed by dense seeding or replanting with native plants to provide competition to resprouting nightshade. Spreading a black covering over the infested area may kill the rhizomes because of the heat generated in the soil underneath.
