Invading plants from beyond our borders are steadily expanding the areas of Pennsylvania they occupy, displacing native plants and altering the native ecosystem.

Among the watchlists for these exotic invasive plants is one maintained by the Pennsylvania Department of Conservation and Natural Resources for plants that “have been determined to be invasive on state forest and state park lands and may act aggressively in other parts of Pennsylvania.”

DCNR has 3 rankings for invasive plants. The following are Rank 1, posing the most severe threat: “Exotic plant species that possess characteristics of invasive species, spread easily into native plant communities and displace native vegetation.”

Don't Edit

Common buckthorn

Native to Europe and Asia, common buckthorn (Rhamnus cathartica) was introduced to North America as an ornamental shrub in the mid-1800s. Birds and small mammals eat the plentiful fruit and then spread the buckthorn. But most of the fruit falls directly beneath the parent plant and begins new plants there in a dense understory that chokes out all competitors.

The common buckthorn forms dense, even-aged thickets that crowd out native plants and prevent them from regenerating. It also changes the ecosystem of any site it colonizes.

Don't Edit

Common reed

Although the reed is indigenous to North America, the more aggressively invasive genotype, the common reed (Phragmites australis) from Europe and Asia was introduced to North America in the late 18th or early 19th century, most likely in contaminated ballast material from ships. It has since colonized the entire continent.

Common reed spreads by seed and rhizome runners. It can rapidly take over new wetland sites, crowding out native plants, altering hydrology and wildlife habitat, and increasing the potential for wildfire.

Don't Edit

Garlic mustard

Adapted to spread quickly across disturbed habitats like trails, roadsides, fields and backyards, garlic mustard (Alliaria petiolate) advances across the landscape at an average 20 feet per year, forming dense clusters that crowd out native vegetation.

Native to Asia and Europe, garlic mustard was intentionally introduced to North America as a food plant and for its medicinal applications. Its first occurrence in the U.S. appears to have been in 1868 on Long Island, New York.

Don't Edit

Giant hogweed

A member of the carrot and parsley family, giant hogweed (Heracleum mantegazzianum) is native to Europe and Asia, but was introduced to North America as a showpiece for arboreta and Victorian gardens. Bee keepers also favored the plant for its huge flowers.

Powered by the 100,000 tiny, winged seeds produced by each plant, giant hogweed escaped into the wild. The seeds can remain viable in the soil for as much as 10 years. Giant hogweed forms dense colonies and its large leaves block smaller plants from getting the sunlight they need.

The sap of the giant hogweed contains a psoralen, which on contact sensitizes human skin to sunlight and can lead to severe burns, blistering, painful sores, and blackened scars.

Don't Edit

Don't Edit

Glossy buckthorn

Native to Europe and Asia, glossy buckthorn (Frangula alnus) was introduced to North America in the mid-1800s as a hedge plant. It also was planted as a wildlife habitat element. The plant has colonized much of the northeastern and northcentral U.S.

Glossy buckthorn spreads by seeds in its prolific berries, which are eaten by birds and are extremely buoyant in streams and rivers. In wet areas it rapidly produces a dense overstory that shades out native plants.

Don't Edit

Non-native honeysuckle

Several species of sweet-flowering honeysuckle have been introduced to North America from Europe and Asia, including sweet breath honeysuckle (Lonicera fragrantissima), Amur Honeysuckle (Lonicera maackii), Morrow’s honeysuckle (Lonicera morrowii), beautiful honeysuckle (Lonicera morrowii x bella), Standish honeysuckle (Lonicera standishii) and Tartarian honeysuckle (Lonicera tatarica). All of them have escaped our home landscapes and are now established as shrubs or vines in various parts of North America, from the Great Plains east.

Through a profusion of tiny, seed-bearing fruits that birds eat in large numbers, the honeysuckles spread rapidly and widely. And, when a cluster has established itself, it then spreads farther by vegetative sprouting. The honeysuckles successfully compete against native plants for sunlight, moisture and pollinators. Also, the fruits do not provide the same level of nutrition to the birds as fruits on native plants.

Don't Edit

Goatsrue

Native to Europe and Asia, goatsrue (Galega officinalis) was introduced to North America as a popular garden plant. It forms dense crowns and spreads along waterways.

Goatsrue was added to the federal list of noxious weeds in 1980 and the Pennsylvania list in 2000.

Don't Edit

Japanese angelica tree

A relatively new invasive species in Pennsylvania, Japanese angelica tree (Aralia elata) was introduced to North America as an ornamental in 1830. It’s native to Asia.

Spread through seed dispersal by birds, but also sprouts from the root to form dense thickets across relatively large areas. It replaces native plant life and establishes a monoculture of itself.

Don't Edit

Japanese honeysuckle

A trailing or twining vine, Japanese honeysuckle (Lonicera japonica) can grow to more than 18 feet in length, with stems as much as 4 inches in diameter. The leaves are simple ovals 1-5 inches long on opposite sides of the stems. Flowers are double-tongued, white fading to yellow, and scented with vanilla.

Introduced to North America in the late 1800s, Japanese honeysuckle grows rapidly over native shrubs and trees, choking and killing them, and forming a monocultural mat.

Don't Edit

Don't Edit

Japanese and European barberry

Enormously popular in the home-landscaping business, European barberry (Berberis vulgaris) and Japanese barberry (Berberis thunbergii) have revealed themselves to be aggressively invasive as well. The former was planted as hedgerows by European settlers to North America. When European barberry was found to be a host for the black stem grain rust, Japanese barberry was introduced as an ornamental plant in 1875.

Both spread quickly through the abundance of seeds in tiny fruits eaten by birds and small mammals and through rooting by branches touching the ground.

According to DCNR, barberry forms dense stands in natural habitats including forests, open woodlands, wetlands and meadows. Once established, it displaces native plants and reduces wildlife habitat and forage, increasing pressure on natives by deer. It has been found to alter the pH and biological activity of soil.

Barberry is also a human health hazard, not only because it has sharp spines, but also because it acts as a nursery for deer ticks, which can transmit Lyme disease.

Don't Edit

Japanese hops

Japanese hops (Humulus japanicus) is an annual, climbing or trailing vine that can grow up to 35 feet in length in one growing season. Leaves are opposite along the vine and average 5 lobes with toothed margins and a rough surface. The stem is covered with small thorns. Flowers appear in mid-summer. Female flowers are cone-shaped clusters that hang down, known as hops. Male flowers are upright and stem like.

The vine grows fast, spreads rapidly through a profusion and small seeds, and forms dense patches that outcompete and smother native vegetation.

Don't Edit

Japanese knotweed

Native to Asia, Japanese knotweed (Fallopia japonica) was introduced to North America as an ornamental in the mid-1890s. It was first reported in Philadelphia and Schednectady, New York.

It thrives in disturbed landscapes and spreads fast, formed dense monocultures of bamboo-like stems from 3-15 feet tall.

Don't Edit

Japanese spirea

Native to Asia, Japanese spirea (Spiraea japonica) was introduced to North America in the 1870s as an ornamental plant. Occurring in many varieties with many colors of bloom, the plant has colonized much of the eastern U.S. and the Midwest.

Spread through the hundreds of tiny seeds produced by each plant and dispersed by water, Japanese spirea grows rapidly into dense thickets that crowd out native plants. It also builds a seed bank that persists in the soil for many years.

Don't Edit

Japanese stiltgrass

Native to Asia, Japanese stiltgrass (Microstegium vimineum) arrived in North America in 1919, introduced accidentally as packing material. Spread by seed, with each plant producing as many as 1,000 seeds, Japanese stiltgrass has colonized most of the eastern U.S.

It crowds out native plants, while it is growing and through the thick thatch it deposits when it dies back each fall. It change soil nutrient cycling on a site to starve native shrubs and trees.

Don't Edit

Don't Edit

Jetbead

Introduced to North America from Asia as an ornamental in the 1860s, jetbead (Rhodotypos scandens) has become established through the northeastern U.S., mid-Atlantic and northern Midwest.

It spreads primarily by seed and suckering from roots, and is particularly invasive in forested areas, where it produces a thick, monoculture, shrub layer that shades out all other plants

Don't Edit

Kudzu

Fast-growing and able to cling to and climb nearly any surface, including all other plants, kudzu (Pueraria montana var. lobate) smothers native vegetation and forms dense mats of monoculture.

Native to Japan, it was intentionally introduced to North America as an ornamental, fodder for livestock and erosion control plants. It was first introduced in 1876 at the Centennial Exposition in Philadelphia and then in 1883 at the New Orleans Exposition.

Don't Edit

Lesser celandrine

Introduced to the North America as an ornamental, lesser celandrine is an aggressive, early-spring emerger, giving it an advantage over native, low-lying species. It forms a thick carpet that chokes out native spring ephemerals, which can interfere with the lifecycle of pollinators in search of nectar and pollen.

It’s native to Europe and Asia.

Don't Edit

Mile-a-minute

Growing as fast as 6 inches per day, mile-a-minute (Persicaria perfoliata) is a vigorous, thorn-covered, vine that forms a dense mat and smothers other plants by growing over them.

A native of Asia, mile-a-minute was introduced to North America through contaminated holly seed brought into York County in 1930.

Don't Edit

Multiflora rose

One of the most abundant and widespread invaders is the multiflora rose (Rosa multiflora), which was introduced to North America from Asia as ornamental rootstock in the 1860s and then again in the 1930s as a erosion-control and livestock-containment planting recommended by the U. S. Soil Conservation Service.

It now grows nearly everywhere across the eastern U.S. and farmers who planted it along the edges of their fields now fight a constant battle to remove it from their properties.

According to DNCR, “It is estimated that a single plant may produce a million seeds per year, which may remain viable in the soil for up to 20 years. The hips are readily eaten by birds, which are the primary seed dispersers. New plants can also be formed by rooting from the tips of canes touching the ground.

“Multiflora rose forms impenetrable thickets that exclude native plant species. This shrub grows very prolifically in riparian areas, where its inedible leaf litter can change the composition of the aquatic macroinvertebrate community. Its occasional habit of climbing can weigh down trees, making them susceptible to breaking.”

Don't Edit

Don't Edit

Oriental bittersweet

DCNR describes Oriental bittersweet (Celastrus orbiculatus) as a deciduous, climbing, woody vine that can grow up to 60 feet in length and up to 4 inches in diameter. Alternate, elliptical leaves are light green, finely toothed and 2-5 inches long. Fruits are round and yellow, splitting to reveal bright red berries in the fall through winter.

Oriental bittersweet spreads rapidly by its sprawling root system and by the seeds, which are eaten and carried over long distances by birds. The vines climb, girdle and kill native trees and shrubs.

Don't Edit

Poison hemlock

Native to Europe and Asia, poison hemlock (Conium maculatum) was introduced to North America as an ornamental but escaped cultivation and became naturalized. Each plant can produce more than 30,000 seeds, each of which can stick to everything from vehicles to clothes and fur for wide dispersal.

Poison hemlock is quick to colonize a site, displacing native plants.

Don't Edit

Porcelain berry

A deciduous, woody, perennial climbing vine, porcelain berry (Ampelopsis glandulosa) has deeply lobed, grape-like leaves, which are sometimes variegated, according to the U.S. Fish and Wildlife Service. Tiny greenish-yellow blooms in mid-summer are followed by clusters of shiny, hard berries in various shades of white, yellow, lilac or green that mature to a bright turquoise blue.

It grows well in dry or moist areas, especially along forest edges, pond margins, stream banks and waste places, from full sunlight to partial shade.

Don't Edit

Purple loosestrife

Native to Europe and Asia, purple loosestrife (Lythrum salicaria) was initially introduced to North America in contaminated cargo ship ballast dumped into Lake Ontario in 1869. However, the plant also became popular in the garden industry and as recently as the 1980s was sold widely to the public, even at butterfly festivals.

Purple loosestrife is an enthusiastic invader of wetlands, where it outcompetes native plants and formed dense monocultures with greatly reduced habitat value for wildlife. It also clogs waterways disrupts the natural nutrient cycle and collects debris.

Don't Edit

Swallow-wort

Growing as much as 8 feet in a single season, black swallow-wort (Vincetoxicum nigrum) and pale swallow-Wort (Cynanchum rossicum) invade fields and field edges, vining over native vegetation and choking it. In addition, the plant is toxic to many insect species.

It’s native to Europe.

Don't Edit

Don't Edit

Tree-of-heaven

Outside of its native China, where it has a host of pests, parasites and diseases to keep it in check, tree-of-heaven (Ailanthus altissima) fails to live up to its common name. It’s an aggressive invader, spreading quickly by root and seed, and crowding out native species. In addition, it gives off a chemical that kills other nearby plants.

Tree-of-heaven was introduced to North America in the late 1700s and spread quickly through home landscapes because of its ability to thrive in urban environments.

Don't Edit