Invasive plants are plants that grow aggressively and spread rapidly displacing native plant species. Most invasive plants (and animals) have little or no predators to control their populations. The rapid growth and spread of invasive plants make them difficult to control.
When invasive plants take hold of a plant community, the effect is wide spread. When native plants are displaced, so are all of the organisms that rely on those plants for food and cover. In addition, the spread of invasive plants tends to lead to a simplification of the community, including a loss of biodiversity and structural diversity. Most of the invasive plants in the Wissahickon Valley Park were introduced by people one way or another. Also, the Wissahickon Valley Park has a history of anthropogenic disturbance (human-caused disturbance). Many of the invasive plants that we deal with thrive in disturbed conditions. Therefore, the changes in the forested community caused by invasive plants are at least in part our fault and for that reason, it is our responsibility to restore the natural lands to more natural and more native conditions. We may never have the forest that existed before Europeans arrived on the scene, but we can restore it to conditions that protect our existing native organisms and encourage the return of displaced native organisms.
There are several different methods for controlling invasive plants. Because we are a volunteer-based organization, we focus primarily on mechanical controls that can be administered by hand. Herbicides are often used to control invasive plants, but we limit our use of herbicide to worse-case situations. In which case, a contractor is usually called in.
The Wissahickon Valley Park, like so many parks and natural areas, is plagued with a plethora of invasive plants. The following list is made up of species that we have made priority targets. We are always learning more about effective control of these plants. New plants are occasionally added to our list of targeted species.
Devil's walking stick
Japanese knotweed, Polygonum cuspidatum: This herbaceous plant originated in Eastern Asia. It was introduced to the U.S. in the late 1800's as an ornamental plant, erosion control, and landscape screening. It spreads by seed and vegetative means, including rhizomes. It can be carried by wind, water, and contaminated fill-dirt or on the soles of shoes. It can also escape from neglected gardens. There are no easy ways to control this plant. We focus our efforts on weakening the rhizome to prevent future growth by cutting repeatedly during the growing season and finally cutting after seeding when the rhizomes are pulling energy down from the leaves.
Mile-a-minute, Polygonum perfoliatum: This vine-like annual originated in Asia. It was introduced in York County, Pennsylavania in the 1930's as a nursery site. It is spread by seed and carried by birds primary long distance dispersal agents, including ants. It can also be transported by water. The fruit remain buoyant for 7-9 days. The vine has countless small thorns that make it possible for the plant to stick easily to anything, especially other plants. The focus for controlling this plant is two-fold: suppressing seeds and preventing growth. As its name suggests, mile-a-minute grows very fast and it can successfully shade out other plants during a growing season. Long-term control requires focusing efforts on seed suppression. Once it begins to grow around June, it immediately has to be controlled. We remove it by hand with leather gloves. Once separated from the soil, the plant dies quickly, but immature seeds are known to germinate even after the plant has been killed.
Multiflora rose, Rosa multiflora: This shrub originated from Asia. It was introduced in 1866 as an ornamental rose. In the 1930's, the plant was used as erosion control. It was later used to promote wildlife habitat. Seed is spread by birds. This plant has hooked thorns and can make controlling them uncomfortable. Like so many invasive plants, controlling this plant requires diligence and patience.
Japanese honeysuckle, Lonicera japonica: This vine was introduced from Eastern Asia in the 1800's as an ornamental plant. It was also used for erosion control and to promote wildlife. It grows vigorously and can smoother other plants.
Oriental bittersweet, Celastrus orbiculatus: This vine originated in Asia. It was introduced in the 1860's as an ornamental plant. Its seed is spread by birds and people. It is a popular ornamental plant for wreaths and other decorations. It can also grow by rhizomes and root suckering. There is a similar native plant, Celastrus scandens, American bittersweet. Oriental bittersweet, Celastrus orbiculatus, has very round leaves, whereas the native bittersweet has oval leaves that tend to be shinier.
Devil's walking stick or Japanese angelica tree, Aralia elata: This perennial shrub or tree originated in Asia and has become naturalized in forested areas. It is easily recognized by its thorny stem and leaves. This plant sends root shoots and can produce stump sprouts after cutting. We also have a similar native plant, Aralia spinosa. It is our understanding that this native plant is being displaced by Aralia elata, which is very difficult to distinguish.
Common reed, Phragmites australis (communis): This tall perennial grass is found in wet conditions. It can spread by rhizomes. Although this plant is not found throughout the park, it is a serious concern in areas where it is found, because it can dominate a wetland community.
Wineberry, Rubus phoenicolasius: This member of the raspberry family originated from Asia. It was introduced in the 1890's for its berries. It is spread by seed by birds, mammals, and people. New plants can grow from arching canes that touch the ground and form root buds. It is easily identified by its red, hairy/thorny stems.
Norway maple, Acer platanoides: This canopy tree originated in Europe and West Asia. It was introduced as an ornamental landscaping plant. It spreads by vegetative reproduction and by seed. Norway maple produces a phytotoxin that prevents other seedlings to thrive. We actively control the seedlings, but full-grown Norway maples require a little more care, therefore, we target them less frequently.
Tree of heaven, Ailanthus altissima: This tree originated from central China. It was introduced to the U.S. in 1748 by a Pennsylvania gardener and by California immigrants during the gold rush. It was available commercially in 1840. It is a prolific seeder. It also produces a phytotoxin that prevents other plants from growing in the area. It can also spread effectively by resprouting after cutting or breaking. It can grow in extremely poor soils and is often the only tree growing in dense urban settings. Unfortunately, it also grows well in the forests of the Wissahickon Valley Park.
Japanese stiltgrass, Microstegium vimineum: This grass originated in Asia. It was introduced to the US in 1919 when it was used as packing material for porcelain sent to Tennessee. It spreads by rooting at the joints along the stem and by seed. It produces 100 to 1000 seeds per plant per year. The seeds can remain viable for 3 to 7 years. It can be carried by water, animals, and people. It is unusual for a grass in that it can grow and germinate in shady, forested conditions. It can completely dominate the herb layer of a forested community.
Plume-poppy, Macleaya cordata: This herbaceous perennial plant is new to the Wissahickon Valley Park, but has already proven its right to be on this list. It originated in Eastern Asia. It can grow 10 feet tall. It has large white clusters of flowers and chalky-white stems. It spreads by seeds and rhizomes. The sap of this plant is toxic and stains clothes easily.
Wild grape, Vitis sp: This vine is common throughout the Wissahickon. It is most likely a native grape or a hybrid cultivar that has escaped. This vine works in concert with non-native invasive vines. Due to various disturbances, this vine has been very successful in the Wissahickon. We control it in order to allow canopy trees to grow.
Goutweed, Aegopodium podagraria: This herbaceous plant originated in Eurasia. This herb has naturalized in the Wissahickon Valley Park. It is still sold in nurseries, but is generally considered a nuisance weed by most gardeners as well as land managers.
More information on invasive plants is available from the following sources:
Plants of Pennsylvania: An Illustrated Manual, by Ann Fowler Rhoades and Timothy Block (University of Pennsylvania Press, 2000).
Plant Invaders of the Mid-Atlantic Natural Areas, by National Park Service and U.S. Fish and Wildlife Service.
Weeds of the Northeast, by Richard Uva, Joseph Neal, and Joseph DiTomaso (Cornell University Press, 1997).
Alien Plant Working Group: http://www.nps.gov/plants/alien
Invasive Plants of the United States: http://www.invasive.org/weedcd/index.htm
National Invasive Species Council: http://www.invasivespecies.gov