About Invasive Plants
Invasive plants are a problem that the Wissahickon Valley shares with most other natural areas throughout the country. Plants that are native in other parts of the world have become established here and have become weed pests. These exotic/invasives are plants that reproduce rapidly and spread over large areas, displacing plants that are native to our region. Invasives often become a monoculture that has little ecological value to our native mammal, insect and bird life.
How Invasive Spread
The plants that we now consider invasives were brought to this country for many reasons: beauty, erosion control, horticultural interest and fast growing habits. In our area there are several dozen trees, shrubs, vines and herbaceous plants that now thrive where native plants once grew.
Invasives can spread throughout a natural area by several means. Many, such as Japanese Barberry, produce vast amounts of seed. Birds eat the seed and spread it around in their droppings. Because most of the invasive plants that grow in our forests grow readily in shade, they can sprout anywhere. Other seeds are spread by water. In the fall of 2004 two huge rainstorms battered the Wissahickon area. In the wake of these storms, Japanese Knotweed was found all along the banks of the lower part of the stream. The high water had washed the seed downstream and deposited it in areas that were favorable for germination.
The other reason for the spread of invasives in the Wissahickon is the overabundance of deer. Deer are usually not attracted to plants that are not native to this area. Therefore they will eat the native plants to the ground but leave the non-native invasives to survive.
A recent study examining the reason some non-native invasive plant species were more successful than others revealed that the chemistry of the plant was the determining factor. The study published in Biology Letters (Vol.2 No. 2, pp. 189-193, 6-22-06) compared 21 species, widely agreed to be invasive by botanists, with 18 non-invasive aliens.
The hypothesis of the researchers was that highly invasive species would have chemical weapons not found in native plants. Native pests, parasites, and native plants would therefore not have evolved resistance to these chemicals.
The comparison revealed that 40% of the invasive species examined had a chemical unknown to native plants, while just over 10% of non-invasive aliens had such a chemical. As further proof of their hypothesis, the researchers found non-invasive aliens shared chemical makeup with at least some native species.
How You Can Help
Fairmount Park has initiated an invasive control program that is supported by the Friends of the Wissahickon. Control of some of the worst invasives is imperative if the forest is to remain a healthy ecosystem.
Volunteers are needed throughout the year to help with this program.
FOW publishes a brochure, A List of Invasive Exotic Plants and Indigenous Plants in the Wissahickon Valley, which can be used by homeowners and gardeners to make responsible decisions when selecting ground covers, shrubs or trees for planting. This brochure is FREE and available at the FOW office.