The
Thomas Mill Ravine is changing.
The section of Philadelphia's Wissahickon Park known as
the Thomas Mill Ravine has had a bit of a make-over in
the past few years. Some mature trees have been
cut down. Some new saplings have been planted.
Wildflowers and woods shrubs have been planted.
New signs and information boxes have appeared. What's
going on?
Like many wooded
areas in the vicinity of Philadelphia, this section of
the park, near the intersection of Seminole
and Chestnut Hill Avenues (see map),
was once in danger of becoming overrun by invasive exotic
plant species. These species were originally brought
to the area mostly as ornamental specimens, but their
seeds got out of the private gardens and into the woods.
The offspring of these non-native species have
been living and reproducing in the wild ever since.
The native plant community in the
park has been disrupted because these plants tend
to take over, and the native species are almost completely
crowded out.
The invasive
species causing the most damage in the Philadelphia area
are mostly from Europe and Asia. The most invasive
non-native trees here are Norway
maple (Acer platanoides), tree-of-heaven (Ailanthus
altissima), and empress-tree, a.k.a. princess-tree
(Paulownia tomentosa). Problematic exotic
woody perennials and understory trees
include winged euonymus, a.k.a. burning-bush (Euonymus
alatus), jetbead (Rhodotypos scandens),
Siebold viburnum (Viburnum sieboldii), oriental
bittersweet (Celastrus orbiculatus), Amur honeysuckle
(Lonicera maackii), Japanese maple (Acer
palmatum), and an Asian species of privet (Ligustrum
sp.). Some of the herbaceous invasives that
have been running rampant through the ravine are garlic-mustard
(Alliaria petiolata), Japanese stilt-grass (Microstegium
vimineum), and goutweed, a.k.a. "Chestnut-Hill-weed"
(Aegopodium podagraria). Pictured below
is a solid mat of uncontrolled goutweed; very few other
herbaceous plants can grow through it because it sprouts
leaves earlier in the spring than most other ground-cover
plants, then shades them out almost completely!
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Pam Morris (no relation to
the Morris family that gave its name to the Arboretum),
the Horticulture Section Leader of Natural Areas at the
Arboretum, has been at the forefront of an effort to combat
the invasive plants which are threatening the integrity
and diversity of the park's plant community. Over
the past few years, in collaboration with the Fairmount
Park Commission and PECO (our local electricity utility),
she has felled dozens of the Norway maples and Ailanthus,
some of them quite large, to make room for displaced native
species such as tuliptree (Liriodendron tulipifera),
American beech (Fagus grandifolia), black and
red oaks (Quercus velutina & Q. rubra),
white ash (Fraxinus americana), black walnut
(Juglans nigra) and wild black cherry (Prunus
serotina). [Actually, PECO did the actual chainsawing
of the large trees-- thanks, PECO!] She has re-planted
the area with native canopy trees such as tupelo/black
gum (Nyssa sylvatica) and white oak (Quercus
alba); native understory trees such as eastern redbud
(Cercis canadensis) and flowering dogwood (Cornus
florida); and native shrubs such as spicebush (Lindera
benzoin), sevenbark (Hydrangea arborescens),
and arrowwood viburnum (Viburnum dentatum).
Native herbaceous perennials that have been used in the
restoration effort have been Virginia water-leaf (Hydrophyllum
virginianum ), three-lobed coneflower (Rudbeckia
triloba), golden ragwort (Senecio aureus),
and fire pink (Silene virginica). For more
information on woodland plants in the Wissahickon Creek
area, see the woods section
of this web site.
Periodically throughout
the year, the Arboretum organizes volunteer project
days to maintain this area. Some of the less glamorous
improvements to the Ravine in the past few years have
included narrowing some of the trails and replanting
along the edges, rebuilding and repairing stone walls,
shoring up the sides of some of the trails, and installing/repairing
water-bars (which keep trails from washing out and becoming
creek-beds during rain storms). Volunteers originally
contributed over 1,400 hours of work to make this project
possible, and they planted more than 700 1-gallon container-grown
plants. Volunteers continue to donate their time
to this project, and they have been a major factor in
making it a success.
Chris Dartley, the Arboretum's
Urban Forestry intern for the 2003-2004 season, worked
extensively on these tasks in the Thomas Mill Ravine.
Most of the images on this page were taken from his final
project presentation, which he completed as a requirement
of his internship.
(Thanks, Chris!)
Pieces
of rock found within a few feet of the trail were used
to build this wall in the
park. This photo was taken in the winter following
construction.
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