Vegetation/Invasives
Definition and Source
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
birdlife.
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.
How Invasives Spread
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.
 Control Program
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.
Brochure
The 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.
A printed brochure is FREE and
available at the FOW office.
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