A Guide for Gardeners and Homeowners
in the Mid-Atlantic Region
In the next century, the greatest threat to our native plants and the wildlife species that depend upon them may well come from other plants. Thousands of plant species have been brought to North America in the past three centuries. Most are well-behaved, rarely penetrating natural areas. Several hundred, however, have no natural controls here, and are able to out-compete and gradually displace our native plants, even deep in forests and undisturbed ecosystems. Variously called alien, introduced, or exotic, these non-natives are highly invasive.
Some of these plants were brought here intentionally, for their medicinal, ornamental, or food value. Others hid in soil, crop seed, or ballast. Most came from other continents, but a few have spread from other parts of the US. In each region, different species are better adapted and therefore pose a greater threat. This guide is for the piedmont and inner coastal plain regions of Maryland, northern Virginia, the District of Columbia, Delaware, and southeastern Pennsylvania.
Many of the plants in this guide are popular, even beloved, landscape plants, but it is now clear that they pose a threat to our environment. If you cannot effectively contain these plants within your property, by clipping seeds, fruits, or runners, PLEASE CONSIDER REMOVING THEM. It is a difficult decision, but each of us has a responsibility not to damage the local ecosystem that cleans our air and water, stabilizes the soil, buffers floods, and provides food and shelter for innumerable species besides our own. EACH OF THE NON-NATIVE PLANTS IN THIS GUIDE SIGNIFICANTLY REDUCES THE NUMBER OF PLANT AND ANIMAL SPECIES ON ANY SITE IT INVADES.
When evaluating exotic plants for your garden, ask these questions:
This guide lists garden plants and weeds which are already causing significant changes to natural areas in the Mid-Atlantic. Measures for controlling each species are indicated by number, e.g., (3), in the text with a full explanation at the end of this article. Click on the word Control: to jump to that section. Then click your "back" button to return to the text. Following each section suggested alternative plants are given. These alternatives are native plants, well adapted and needing little care, attractive to birds and butterflies, and an important part of the food web for our indigenous species.
NORWAY MAPLE (Acer platanoides) has large leaves similar to sugar maple. Break a leaf or stalk - a drop
of white sap will show if it is Norway maple. Fall foliage is yellow. (Exception: cultivars such as 'Crimson
King,' which have red leaves in spring or summer, may have red autumn leaves.) The leaves turn color
late, usually in November. This tree suppresses growth of grass, garden plants, and forest understory
beneath it, at least as far as the drip-line. Its wind-borne seeds can germinate and grow in deep shade.
The presence of young Norway maples in our woodlands is increasing. Our mixed deciduous forests will
give way to pure stands of Norway maple in the next century unless we control its spread now.
Control: (1); (7), (8), (9), or (10); (11) in mid-October to early November, before the leaves turn color.
TREE OF HEAVEN (Ailanthus altissima), known from A Tree Grows in Brooklyn, is incredibly tough and can
grow in the poorest conditions. It produces huge quantities of wind-borne seeds, grows rapidly, and
secretes a toxin that kills other plants. Its long compound leaves, with 11-25 lance-shaped leaflets, smell
like peanut butter or burnt coffee when crushed. Once established, this tree cannot be removed by
mechanical means alone.
Control: (1) - seedlings only. Herbicide - use Garlon 3a (9) with no more
than a 1" gap between cuts, or (10); plus (11) on re-growth. Or paint bottom 12" of bark with Garlon 4
(in February or March to protect surrounding plants). USE MAXIMUM STRENGTH SPECIFIED ON LABEL
for all herbicide applications on Ailanthus. Glyphosate is not effective against Ailanthus.
SAWTOOTH OAK (Quercus acutissima) has oval leaves with sawtooth edges and huge acorns. Often recommended
for wildlife, this Asian tree has spread into our region from forestry plantings, displacing indigenous forest trees.
Control: (1); (7), (8), (9), or (10); (11) on small trees and re-growth.
RECOMMENDED NATIVE SHADE TREES:
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EMPRESS TREE, PRINCESS TREE (Paulownia tomentosa): Large panicles of lavender flowers, like upside-down wisteria,
identify this tree in spring; the large brown seed capsules remain all year. The leaves are
very large and heart-shaped. Winged seeds allow it to spread deep into undeveloped areas, though it
needs some sunlight and is most common along trails and waterways. It grows very rapidly and sprouts
readily from roots and cut stumps.
Control: (1) - seedlings and small saplings only; (7), (8), (9), or (10)
- use 50% solution, anytime the ground is not frozen; (11) on re-growth and small trees.
MIMOSA (Albizia julibrissin) has rather garish pink flowers in summer and feathery compound leaves. It
spreads slowly by wind-borne seedpods, or in water or fill-dirt. It re-sprouts when cut or burned. Needs
some sunlight.
Control: (1); (7), (8), (9), or (10).
SIBERIAN ELM (Ulmus pumila), a fast-growing medium-height tree also sold for hedges, displaces our native
elms, which are already under pressure from Dutch elm disease. It forms dense thickets under which
nothing else grows. Its small oval leaves have a single tooth.
Control: (1); (7), (8), (9), or (10).
RUSSIAN OLIVE, AUTUMN OLIVE (Eleagnus angustifolium, E. umbellata): Formerly recommended for
erosion control and wildlife value, these have proved highly invasive and diminish the overall quality of wildlife habitat.
Control: (1) - up to 4" diameter trunks; (7) or (10) or bury stump. Do not mow or burn.
FLOWERING FRUIT TREES: These displace our native fruit trees.
RECOMMENDED SMALL NATIVE ORNAMENTAL TREES:
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RECOMMENDED NATIVE TREES FOR HEDGES:
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MULTIFLORA ROSE (Rosa multiflora), formerly recommended for erosion control, hedges, and wildlife habitat, becomes a huge shrub that chokes out all other vegetation and is too dense for many species of birds
to nest in, though a few favor it. In shade, it grows up trees like a vine. It is covered with white flowers in
June. (Our native roses have fewer flowers, mostly pink.) Distinguish multiflora by its size, and by the
presence of very hard, curved thorns, and a fringed edge to the leaf stalk.
Control: (1) - pull seedlings,
dig out larger plants at least 6" from the crown and 6" down; (4) on extensive infestations; (10) or (11). It
may remain green in winter, so herbicide may applied when other plants are dormant. For foliar application, mix Rodeo with extra sticker-spreader, or use Roundup Sure Shot Foam on small plants.
BUSH HONEYSUCKLES (Lonicera spp.), including Belle, Amur, Morrow's, and Tatarian honeysuckle. (In our
region, assume that any honeysuckle is exotic unless it is a scarlet-flowered vine). Bush honeysuckles
create denser shade than native shrubs, reducing plant diversity and eliminating nest sites for many forest
interior species.
Control: (2) on ornamentals; (1); on shady sites only, brush cut in early spring and again
in early fall (3); (4) during the growing season; (7); or (10) late in the growing season.
OTHER ORNAMENTAL SHRUBS:
JAPANESE SPIRAEA (Spiraea japonica). Control: (1); (2); (3), (7), (10), or (11).
PRIVET (all Ligustrum species). Control: (1); (7) or (10); or trim off all flowers. Do not cut back or mow.
BURNING BUSH, WINGED EUONYMUS, WINGED WAHOO (Euonymus alatus), identified by wide,
corky wings on the branches. [There is another species called burning bush, E. atropurpureus, which
is indigenous to the Appalachians, and a piedmont euonymus called strawberry bush (E. americanus).]
Control: (1); (7) or (10); or trim off all flowers.
JAPANESE BARBERRY (Berberis thunbergii), red and green varieties.
Control: (1); (7) or (10); or trim off
all flowers.
RECOMMENDED NATIVE SHRUBS
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All of these vines shade out the shrubs and young trees of the forest understory, eventually killing them, and changing the open structure of the forest into a dense tangle. DO NOT PLANT NEXT TO OPEN SPACE.
KUDZU (Pueraria lobata), the vine that smothered the South, is now spreading through the Northeast and
Midwest. It has large lobed leaves in groups of three, thick stems, flowers that resemble wisteria, and
hairy, bean-like seedpods in fall. It grows extremely rapidly both above and below ground, and can pull
down trees.
Control: Small patches may be eliminated by repeated weeding (1), mowing (2), or grazing;
established infestations can only be controlled with herbicide (10) or (11) - expect re-growth, but wait a
full year and re-treat in the third year. Herbicide is most effective in early fall. Controlled burning (4) of
the dead plants the following spring allows native vegetation to return.
JAPANESE HONEYSUCKLE (Lonicera japonica), including Hall's honeysuckle, has gold-and-white flowers
with a heavenly scent and sweet nectar in June. This is probably the familiar honeysuckle of your childhood. It is a rampant grower
that spirals around trees, often strangling them.
Control: (1); (3); (10); (11)
in fall or early spring when native vegetation is dormant. Plan to re-treat repeatedly.
WISTERIA, CHINESE AND JAPANESE (Wisteria sinensis, W. floribunda) both become heavy, woody vines that
can pull down a large tree.
Control: (1); cut back and deadhead ornamental plants (2); (10).
ORIENTAL BITTERSWEET (Celastrus orbiculatus) has almost completely displaced American bittersweet (C.
scandens). The Asian plant has its flowers and bright orange seed capsules in clusters all along the stem,
while the native species bears them only at the branch tips.
Control: (1); keep ornamental plants cut
back, remove all fruits as soon as they open, and bag or burn fruits; to eradicate use Garlon 3a (10).
PORCELAIN BERRY (Ampelopsis brevipedunculata) has small, hard fruits in a loose, flat cluster that turn from
white to yellow, lilac, green, and finally a beautiful turquoise blue.
Control: (1) before fruits appear; keep
ornamental plants cut back, and bag or burn fruits before they ripen; to eradicate use Garlon 3a (10).
ENGLISH IVY (Hedera helix) grows up trees and can eventually pull them down. It spreads along the ground
and occasionally by fruits.
Control: Clip off flowers or fruits if any are seen (2), and (1) pull any seedlings. To eradicate ivy climbing trees, cut stems as high above ground as you can reach, then pull down
and paint lower portion of stems and foliage with Garlon 3a (10), taking care not to wet the tree bark.
Ground cover: pull up as much as you can, dig out the roots as well as you can, and repeat until it no
longer re-sprouts; or treat re-growth with Garlon 3a.
WINTERCREEPER (Euonymus fortunei). Control: Same as for English Ivy, but Garlon is not effective; glyphosate mixed with extra sticker-spreader may be.
VINCA, PERIWINKLE (Vinca minor). Control: With persistence, you can dig out vinca (1); plan to remove re-growth. If digging is not feasible, cut to the ground and treat re-growth with glyphosate (11).
RECOMMENDED NATIVE ORNAMENTAL VINES:
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CROWN VETCH (Coronilla varia) has striking pink flowers. Its bare woody stems are unattractive in winter.
Often planted along highways, its seeds spread invasively.
Control: (1); (10) or (11).
CREEPING BUGLEWEED (Ajuga reptans)MINTS, including SPEARMINT (Mentha spicata),
GROUND IVY, GILL-OVER-THE-GROUND, CREEPING
CHARLIE (Glechoma hederacea), HENBIT (Lamium amplexicaule), and PURPLE DEAD NETTLE (L.
purpureum), spread by wind-borne seed as well as by runners. They grow in sun and shade and are
common lawn weeds which have spread to woods and wetlands. Recognize mints by square stems and a
minty smell when crushed. PLANT CULINARY AND ORNAMENTAL MINTS IN CONTAINERS; PREVENT
FROM SPREADING OUT DRAINAGE HOLES OR OVER THE TOP.
Control: (1) (difficult); (2); (6); (11).
INDIAN STRAWBERRY (Duchesnea indica). From India, this shade-tolerant ground cover spreads by fruit and
runners.
Control: (1), taking care to remove each crown; (6).
RECOMMENDED NATIVE GROUND COVERS:
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RUNNING BAMBOOS (many species and genera; Phyllostachys, Bambusa, and Pseudosasa are the most
destructive). Many bamboos send runners great distances, under pavement and edging. Once established, they form impenetrable thickets that are almost impossible to eradicate. PLANT BAMBOOS ONLY
IN CONTAINERS, NEVER IN OPEN SOIL. PREVENT FROM SPREADING OUT DRAINAGE HOLES.
Control: (1) - an enormous job; (10) or (11).
RECOMMENDED ALTERNATIVES TO BAMBOO:
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A number of ornamental plants once recommended for water gardens or moist garden soil have spread to our riverbanks, floodplains, and wetlands. They are extremely difficult to eradicate once established - up to 10 years of repeated treatment may be needed to remove purple loosestrife or Phragmites. These plants propagate by seed and by fleshy root parts which break off easily. Both are spread by water, feet (human, animal, bird), and tires, including those of mowers. They are also found in dredge spoil, fill dirt, and compost. It is not clear whether seeds may be transported by wind. Do not plant exotic water garden plants unless they are not hardy, and never dump plants from fish tanks or water gardens into toilets, storm drains, lakes, or streams.
COMMON REED (Phragmites australis, formerly P. communis) looks like a tall ornamental grass with lovely
plumes, usually white or tan. Although the species is indigenous, a particularly aggressive strain, probably introduced or a hybrid, has escaped from natural controls and taken over many formerly diverse wetlands. It is also seen in roadside ditches.
Control: (10) or (11), using Rodeo when the plant is flowering.
If possible, follow up with a controlled burn of the dead plants, to allow native plants to return. DO NOT
DIG PHRAGMITES - THE ROOTS WILL BREAK, RE-SPROUT, AND SPREAD. If herbicide cannot be used,
cut annually in late July to reduce spread.
GIANT REED (Arundo donax) chokes waterways from Virginia south. It can grow 20' tall.
Control: Same as
for Phragmites or mow several times a season.
JAPANESE KNOTWEED, MEXICAN BAMBOO (Polygonum cuspidatum) can grow in shade. The stems have
knotty joints, reminiscent of bamboo. It grows 6-10' tall and has large pointed oval or triangular leaves.
Control: Cut at least 3 times each growing season and/or treat with Rodeo (10) or (11). In gardens,
heavy mulch or dense shade may kill it.
PURPLE LOOSESTRIFE (Lythrum salicaria, L. virgatum), a handsome garden plant, has tall spikes of magenta
flowers over a long bloom season. Often marketed as sterile, it is at best self-sterile, i.e., it can be pollinated by plants you may not be aware of, growing nearby. A single plant can produce up to a million
seeds. Like Phragmites, it chokes out all competitors and has taken over millions of acres of wetland in
the US.
Control: Initial infestations may be hand-pulled (1) before flowering (DO NOT DIG). Bag and
burn or send to the landfill. Otherwise, use Rodeo (10) or (11) when plants begin to bloom (they continue to flower while setting seed). Expect to re-treat for several years until the seed bank is exhausted.
LESSER CELANDINE, CELANDINE BUTTERCUP (Ranunculus ficaria) has spread from gardens to carpet our
floodplains with small yellow flowers in spring. It comes up in winter, giving it a head start over most
native spring wildflowers.
Control: It is not yet known whether digging is effective - the small reproductive
corms break off very easily. Try digging (1) before the plants flower. Otherwise, use Rodeo (10 or 11),
preferably in February to protect native plants, frogs, and salamanders which become active in March.
RECOMMENDED NATIVE WETLAND PLANTS FOR WATER GARDENS:
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Often promoted as native plants, most ornamental grasses come from outside our region. Once established,
they are extremely tenacious. They are now spreading into our meadows.
So far, PAMPAS GRASS
(Cortaderia selloana and C. jubata),
JAPANESE SILVER GRASS (Miscanthus sinensis), and
REED CANARY GRASS (Phalaris arundinacea) have been the most invasive. Those with heavy seeds are less likely to
spread.
Control: (1); (2); or (11), using additional sticker-spreader.
RECOMMENDED NATIVE GRASSES
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GARLIC MUSTARD (Alliaria petiolata, A. officinalis), a white-flowered biennial with rough, scalloped leaves
(kidney-, heart- or arrow-shaped), recognizable by the smell of garlic and taste of mustard when its leaves
are crushed. (The odor fades by fall.)
Control: Pull before it flowers in spring (1), removing crown and
roots. Tamp down soil afterwards. Once it has flowered, cut (2), being careful not to scatter seed, then
bag and burn or send to the landfill. (11) may be appropriate in some settings.
JAPANESE or VIETNAMESE STILT GRASS, EULALIA (Microstegium vimineum) can be identified by its lime-green color and a line of silvery hairs down the middle of the 2-3" long blade. It tolerates sun or dense
shade and quickly invades areas left bare or disturbed by tilling or flooding. An annual grass, it builds
up a large seed bank in the soil.
Control: Easily pulled in early to mid-summer (1) - be sure to pull before it goes to seed. If seeds have formed, bag and burn or send to landfill. Mowing weekly or when it
has just begun to flower may prevent it from setting seed (3). Use glyphosate (11) or herbicidal soap
(less effective) on large infestations. Follow up with (5) in spring.
MILE-A-MINUTE VINE, DEVIL'S TAIL TEARTHUMB (Polygonum perfoliatum), a rapidly growing annual vine
with triangular leaves, barbed stems, and turquoise berries in August which are spread by birds. It quickly
covers and shades out herbaceous plants.
Control: same as for stilt grass.
JAPANESE PERILLA, BEEFSTEAK PLANT (Perilla frutescens). Sold as a salad plant, this member of the mint
family is extremely invasive by wind-borne seeds. Recognize it by the odd odor, supposedly like raw beef,
when you rub it.
Control: (1); (2); (10) or (11).
SPOTTED KNAPWEED (Centaurea maculosa), a biennial with thistle-like flowers.
CANADA THISTLE, BULL THISTLE (Cirsium arvense, C. Vulgare). Exotic thistles are far more common than
native ones. If you cannot identify the species, it is probably better to remove it.
Control: Do NOT pull
(1) unless the plant is young and the ground is very soft - the tap root will break off and produce several
new plants. Wear sturdy gloves. (2); (6); (10) or(11).
NOTE ON HERBICIDES: MNPS strongly recommends non-chemical methods of control wherever feasible. However, for large infestations, and for a few plants specified above, non-chemical methods are inadequate. Applied carefully to avoid non-target plants, glyphosate is the least environmentally damaging herbicide in most cases. Roundup contains a stronger concentration of glyphosate than Kleen-Up. Both contain a petroleum-based sticker-spreader. Rodeo, the glyphosate formulation for wetlands, does not contain any sticker-spreader and thus is safer for the environment. The smallest size of Rodeo available is one quart of concentrate, obtainable from farm supply stores for about $60 in 1999. Add food coloring for visibility, and a soap-based sticker such as Cide-Kick. For small applications, another choice is Roundup Sure Shot Foam, easier to see and control than liquid Roundup. Glyphosate is ineffective on some plants; for these, triclopyr (Garlon), a stump and brush killer, may be indicated. When using herbicides, read the entire label and observe all precautions listed, including proper disposal. If in doubt, call your state Extension Service.
March 1999
Maryland Native Plant Society
P. O. Box 4877; Silver Spring, MD 20914
E-Mail: info @ mdflora.org