Invasives in our Gardens

what to watch for

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-Atlantic region.

• 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 neighborhoods.

• Watch for the following plants, all of them known invasives, in your own garden, and look for places they may be growing in your neighborhood.

English Ivy (Hedera helix)

• Although widely recognized as a serious pest of natural areas, it continues to be sold as an ornamental.

• 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 plants.

• 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 centers.

• It is a serious threat to natural and disturbed wetlands, where seeds may be carried from a single

garden planting.

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 Commonwealth.

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 charge.

• 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 Pennsylvania.

• Distinguish Multiflora from native roses by the fringed bracts at the base of each leaf stalk.

Oriental Bittersweet (Celastrus orbiculatus)

• 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 native bittersweet.

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 vegetation.

• Photo by Jeff McMillian @ USDA-NRCS PLANTS Database

Other Invasives in your Neighborhood

• Learn to recognize the following problem plants.

• Eradicate them immediately if they appear in your garden.

• Consider speaking to your municipal officials about removing them from public areas.

• Japanese Stilt Grass(Microstegium vimineum) 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 Europe

Remove by hand. Spray rosettes with Round-up in early spring or late fall, when other plants are dormant.

• Japanese Honeysuckle(Lonicera japonica) native to eastern Asia

Spray with Round-up as soon as flowers appear.

• Japanese Knotweed(Polygonum cuspidatum) 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 China

A major problem; try cutting the trunk low to the ground, and painting with Vine-X (buy online).

• Canada Thistle(Cirsium arvense) native to EuropeThe 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 plants.

• Invasive plants and other exotic aliens disrupt the intricate web of life for plants, animals and microorganisms and compete for limited natural resources.

• 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 small mammals.

• 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.