What is an invasive?
• According to the PA DCNR, an invasive plant is
a name for a species that has become a weed
pest; a plant that grows aggressively, spreads,
and displaces other plants.
• Many invasive plants share some important
characteristics that allow them to grow out of
control. These include: (1) spreading
aggressively by runners or rhizomes; (2)
producing large numbers of seeds that survive
to germinate; and (3) dispersing seeds away
from the parent plant through various means
such as wind, water, wildlife and people.
• Invasive plants are usually exotic aliens, which
were brought into the United States for the
horticultural trade. Once here, they can grow
unchecked, since the competitors, predators,
pathogens and parasites that formerly kept them
in check are no longer present.
• Invasive plants are opportunistic and take over a
site by spreading much faster than local native
plants, especially when soil, drainage, and light
patterns are disturbed by development.
• Ecologists now rank invasion by exotic plants,
animals and pathogens second only to habitat
loss as a major threat to local biodiversity.
• More than 200 exotic plant species have been
identified by natural resource managers as
problematic invaders of natural areas in the mid-
• We are so accustomed to some of these aliens,
that we do not think of them as potential
problems. Nonetheless, they can create havoc,
if not in our own backyards, then in our
• Watch for the following plants, all of them
known invasives, in your own garden, and
look for places they may be growing in
English Ivy (Hedera helix)
• Although widely recognized as a serious pest of
natural areas, it continues to be sold as an
• It threatens all vegetation levels, growing along
the ground as well as into the tree canopy.
• Vines that climb up trees slowly kill the tree from
the base upwards by blocking sunlight, causing
branch and eventual tree death.
• The added weight of ivy vines also makes trees
more susceptible to blowing over during storms.
• The dense growth and abundant leaves of
English Ivy form a thick canopy just above the
ground, that prevents sunlight from reaching
seedlings of native plants.
Norway Maple (Acer platanoides)
• A single tree creates shade so dense that
nothing can grow underneath.
• Seedlings are carried considerable distances by
the wind, and develop more quickly than native
• Distinguish Norway Maple from Sugar Maple by
the milky sap in veins and twig axils.
Burning Bush (Euonymus alata)
• Gardeners everywhere love it for its fall color,
and birds like to eat the berries. Unfortunately,
seeds are carried far and wide as they pass
through a bird’s gut.
• The extensive woodland at Mt. Cuba Center in
Delaware has an understory of nothing but
Burning Bush, simply because the original owner
planted two bushes at her front door.
• Bill Sweeney says Burning Bush presents the
biggest obstacle to native plants in the woods at
Jacobsburg State Park.
Japanese Barberry (Berberis thunbergii)
• The photo of the seeds in autumn makes it clear
that this is another plant which is widely
dispersed by birds.
• The plant also reproduces by stolons, spreading
widely on its own.
Butterfly Bush (Buddleia spp.)
• Although this is a plant frequented by many
butterflies, it is not used by them for egg-laying,
nor do their larvae eat the leaves.
• It can easily become invasive in a variety of
natural habitats such as roadsides, abandoned
railroads, stream and river banks.
• To control it in your own garden, cut off the
flower heads before they develop seed.
(Never add them to your compost.)
Purple Loosestrife (Lythrum salicaria)
• Although listed as a Noxious Weed by the PA
Department of Agriculture, Purple Loosestrife can
still be found for sale in some nurseries and garden
• It is a serious threat to natural and disturbed
wetlands, where seeds may be carried from a single
3 P. S. § 255.3(b)
• Noxious weed: A plant that is determined to be
injurious, to public health, crops, livestock,
agricultural land or other property.
• When a weed is declared noxious it shall be a
violation of this act to sell, transport, plant, or
otherwise propagate that weed within the
Multiflora Rose (Rosa multiflora)
• Multiflora Rose was introduced from Japan in 1866
as rootstock for ornamental roses.
• Beginning in the 1930s, the U.S. Soil Conservation
Service promoted it for use in erosion control and as
"living fences" to confine livestock.
• State conservation departments soon discovered
value in Multiflora as wildlife cover for small game
and as food for songbirds, and encouraged its use
by distributing rooted cuttings to landowners free of
• The tenacious and unstoppable growth habit of
Multiflora Rose was eventually recognized as a
problem on pastures and unplowed lands, where it
disrupted cattle grazing.
• For these reasons, Multiflora is now classified as a
noxious weed in several states, including
• Distinguish Multiflora from native roses by the
fringed bracts at the base of each leaf stalk.
• Who among us hasn’t admired wreaths made
from this workable vine and its beautiful fall
berries? Yet Oriental Bittersweet is an
aggressive invader that threatens vegetation at
all heights in forested and open areas.
• Similar to Kudzu, it has the ability to quickly
smother the vegetation it climbs upon.
• Its flowers spring from the leaf axils, unlike
Periwinkle (Vinca minor)
• This innocuous-seeming plant, used by many
gardeners, can spread unchecked into a dense
monotypic evergreen ground cover.
• A single clone can spread vegetatively and
cover large areas of woodland understory,
crowding out all the native herbaceous
• Photo by Jeff McMillian @ USDA-NRCS PLANTS Database
Other Invasives in your
• Learn to recognize the following problem
• Eradicate them immediately if they appear
in your garden.
• Consider speaking to your municipal
officials about removing them from public
• Japanese Stilt Grass
native to south and east Asia
Remove by hand. For large
areas, mow in July/August while
plant is in flower.
• Garlic Mustard
(Alliaria petiolata) native to
Remove by hand. Spray rosettes
with Round-up in early spring or
late fall, when other plants are
(Lonicera japonica) native to
Spray with Round-up as soon as
• Japanese Knotweed
native to eastern Asia
Try digging, or cut stems low, use
undiluted Round-up. May take
several years to eradicate.
• Tree of Heaven
(Ailanthus altissima) native to
A major problem; try cutting the
trunk low to the ground, and
painting with Vine-X (buy online).
• Canada Thistle
(Cirsium arvense) native to
The seeds remain viable for 20
years, so repeated spraying with
Round-up before the flowers go
to seed may be necessary.
Why worry about invasive plants?
• Invasive plants and other exotic aliens alter our
environment by introducing pests and diseases
which can damage or even decimate native
• Invasive plants and other exotic aliens disrupt
the intricate web of life for plants, animals and
microorganisms and compete for limited natural
• Most plant-eating insects can only eat plants
with which they share an evolutionary history.
• That means that most plant-eating insects can
only eat native plants – not exotic aliens.
• This is why exotic aliens have been so popular
in the horticultural trade: nothing eats them!
• The gardening community has only recently
begun to realize just how much the wildlife
around us depends on native plants.
• Insect populations in areas with many native
plants are larger than insect populations in areas
with many alien (exotic) plants.
• The insects that eat native plants are
themselves eaten by other insects, birds, and
• The fewer the insects available, the fewer the
birds and small mammals to observe and enjoy
in our backyards and woodlands.
• For a thorough explanation of plant-insect
interactions, the dangers of exotic aliens, and
the value of native plants, read Bringing Nature
Home, by Douglas Tallamy.
• The next time you want to plant something
new in your garden, try a native plant
instead of an exotic.
• You’ll be taking one step towards
improving the world’s environment.
• IT MATTERS!
• For more complete information on invasive
plants, contact the Mid-Atlantic Exotic Pest Plant
Council at http://www.ma-eppc.org or the Plant
Conservation Alliance, Alien Plant Working
Group at www.nps.gov/plants/alien/.
• For more information on native plants, check
with the Penn State Cooperative Extension
Office at Gracedale (in Nazareth.)
• Other good sources include Wildlands
Conservancy, the Lehigh Gap Nature Preserve,
Edge of the Woods Native Plant Nursery, and
Bowman’s Hill Wildflower Preserve.