People often don’t notice invasive plants spreading into a new environment. That’s why ornamental horticulturist Larry Kuhns believes Pennsylvanians must be taught about the risks posed by invasive species and shown how to eliminate them before they get too well established.

“Invasive plants threaten the habitat of two-thirds of all threatened and endangered wildlife species,” Kuhns says. “The best way to control these plants is to familiarize yourself with them and work with local officials and agencies to start eradication programs.”

Here is Kuhns’ list of 10 invasive plant species most commonly found in Pennsylvania ecosystems.

Tree of heaven. This tree produces a huge amount of seed, which is spread easily by the wind. It grows very fast—as much as 10 feet in a single season—and can grow more than 60 feet tall. It also spreads by producing new shoots from its extensive root system.

Autumn olive. Introduced for wildlife food and cover, this shrub produces large amounts of seed that is widely spread by birds.

Multiflora rose. This shrub, classified as a noxious weed by the Pennsylvania Department of Agriculture, also was introduced for wildlife food and cover. Its seeds are spread by birds. “This plant has a biological control,” Kuhns says. “A virus affecting it has spread throughout the Midwest, into Ohio and West Virginia, and has reached the Pennsylvania border.”

Purple loosestrife. This noxious weed invades wetlands and sites that have been disturbed or cultivated. Its seeds are easily transported by animals or vehicles. Its vibrant purple flowers and long growing season make it attractive to uninformed gardeners.

Canada thistle. This noxious weed spreads by airborne seeds and creates serious problems in farm crops.

Japanese knotweed. Also called Japanese bamboo, this plant spreads along rivers and through root pieces in soil that is moved. It grows to nearly 10 feet, but provides little usable cover or food for wildlife. Its spread by seed is limited, but its extensive root system makes it difficult to control.

Phragmites, or the common reed. Common to wetland areas, native and introduced varieties of this plant form huge colonies, pushing out native plants and wildlife.

Poison hemlock. Unrelated to the hemlock tree, this herbaceous plant is what the philosopher Socrates mixed into a drink to commit suicide. “It’s related to water hemlock, which happens to be more poisonous than poison hemlock,” Kuhns says.

Japanese stiltgrass. This annual grass has invaded woodlands across the state, hindering forest regeneration, Kuhns says.

Mile-a-minute vine. Another noxious weed, this plant’s range is expanding through the state. It can grow 6 inches a day and more than 25 feet in one growing season. It can smother small saplings and seedlings in developing forests or deforested land.

—Jeff Mulhollem