UNIVERSITY PARK, Pa. – The most pervasive environmental threat of the 21st century may be cloaked in the guise of ordinary and often attractive plants, according to a horticultural scientist in Penn State’s College of Agricultural Sciences.
Hidden problem. “Most people aren’t aware of the ecological damage invasive plants are inflicting on our biodiversity, our wildlife habitat and our native plant species,” said Larry Kuhns, professor of ornamental horticulture.
“Compared to invasive plants and weeds, the chemical pollutants of the ’60s, ’70s and ’80s will be considered an easy cleanup.”
Kuhns said an invasive plant is defined as a plant that grows aggressively, spreads and displaces other plants in an ecosystem.
In Pennsylvania. A small number of invasive species are native to Pennsylvania, which means they grew in the state before settlement by Europeans. Most invasive species, Kuhns explains, were brought to Pennsylvania from other continents. These species often are called “alien,” “exotic,” “introduced” or “non-native.”
“These plants have been introduced into environments where there are no diseases or pests to control them,” Kuhns said. “They often are silent threats because most people aren’t aware of invasive plants, or don’t notice them spreading into a new environment.”
Kuhns said many species now considered invasive originally were introduced intentionally as landscape plants or for other purposes.
Top 10. Kuhns lists 10 invasive plant species most commonly found in Pennsylvania ecosystems.
* Tree of heaven. This tree has invaded many states. It produces a huge amount of seed, which is spread easily by the wind along roadways.
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 as a wildlife 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 as a wildlife cover. Its seeds are spread by birds.
“This plant has a biological control,” Kuhns said. “A virus affecting it has spread in the Midwest and has reached Ohio.”
* Purple loosestrife. Another Pennsylvania noxious weed, purple loosestrife 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.
* Japanese knotweed. Also called Japanese bamboo, this plant spreads along rivers and in soil. It grows to nearly 10 feet, but provides little cover or food for wildlife.
Its spread by seed is limited, but it has an extensive root system, which makes it difficult to control.
* Phragmites, or the common reed. Common to wetland areas, this plant has native and introduced varieties. It forms huge colonies, pushing out native plants and wildlife.
* Canada thistle. This is a Pennsylvania noxious weed that spreads by airborne seeds. It creates serious problems in farm crops.
* 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 said.
* Japanese stiltgrass. An annual grass, it has invaded woodlands across the state. It has been a hindrance to forest regeneration, Kuhns said.
* Mile-a-minute vine. A Pennsylvania noxious weed, this plant’s range is expanding through the state. It can grow six 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.
Major threat. “Invasive plants threaten the habitat of two-thirds of all threatened and endangered wildlife species,” Kuhns said.
“The best way to control these plants is to familiarize yourself with them and work with local officials and agencies to start eradication programs.”
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