New exhibit focuses on invasive plants

It’s hard to imagine life in North America without the influences of Europe and Asia. The Beatles, lasagna, lo mein — there’s a lot to be grateful for.

But Europe and Asia have also brought thousands of species of invasive plants to North America. Some European and Asian plants were brought to North America for aesthetic or medicinal purposes, while others made it to the country after their seeds clung to the clothes and suitcases of travelers. While these plants are often beautiful and easy to maintain, they are also the most likely to pose major threats to the ecosystem.

Backyard Invasion, a new exhibit at the Carnegie Museum of Natural History, features images of invasive and endangered Pennsylvania plants. The display — both informative and elegant — is a combination of drawings, paintings, etchings, and giclées (digital prints).

The illustrators obtained much of their information from the extensive library of records on Pennsylvania plants. “If a plant is misfiled, it might be lost for a hundred years,” explained Bonnie Isaac, collection manager for the museum’s herbarium.

Non-native plants — also known as “exotic” plants — can be classified as “invasive” when they exhibit rapid and aggressive growth. According to the Pennsylvania Landscape and Nursery Association, Pennsylvania has approximately 3400 species of plants, 1300 of which are exotic.

Though some invasive plants are native to North America, the vast majority come from other continents, primarily Europe and Asia. When they take over a garden, park, or planting area, invasive plants endanger the surrounding ecosystem.

The invasive plants are likely to encroach upon regions previously occupied by native plants, many of which are rare, if not at risk for extinction. By the year 2000, an estimated five percent of plants native to Pennsylvania had been depleted. That same year, about one-fourth of those remaining were categorized as endangered.

Invasive plants flourish where other plants are likely to dwindle: on bare soil or disturbed ground. Exotic plants are known to thrive off the nitrogen in chemical fertilizer, so it is recommended that people fertilize their gardens with organic substances, such as compost and mulch.

One example of an attractive — yet harmful — invasive plant is purple loosestrife (Lythrum salicaria). “[The loosestrife] is starting to grow and take over our native species,” said Cynthia Morton, associate curator and head of botany for the museum, and a professor of biology at both the University of Pittsburgh and Carnegie Mellon.

According to a report by the Plant Conservation Alliance, purple loosestrife first made it to North America in the 19th century, when it was desired for medicinal and decorative purposes. Traced back to both Europe and Asia, loosestrife is still sought after for its looks, though it has been made illegal in Minnesota, Wisconsin, and Illinois in an attempt to protect native plants.

Another more common example of an invasive plant is ivy. “This is one all people can recognize,” Morton said. “It grows everywhere.”

English ivy (Hedera helix) has the ability to grow quickly and densely. When attached to a tree, the ivy will spread to cover increasing portions of its host, which will eventually die from lack of sunlight. Ivy can also cause damage to brick walls or other structures, as its tiny roots deteriorate the surface.

Able to adapt quickly to new environments and produce seeds in short cycles, invasive plants can be extremely hard to eliminate. Any removal effort is certain to cost money as well as time.

When the problem becomes sufficiently large, however, it is no longer practical to pull out invasive weeds by hand. At this point, it becomes the job of a professional to handle the situation, either through mechanical removal (digging or cutting) or chemical removal (herbicide). Frequently, one must uproot invasive plants for years in a row before they stop growing back. Still, Morton insists, it’s worth the effort.

Though many invasive plants made it to North America through accidental means, sometimes people go right out and buy them. Wal-Mart, K-Mart, and many local garden shops sell invasive plants to customers who probably don’t know any better, Morton explained.

“If you can’t control it, don’t plant it,” she said. “It takes work.” The primary goal of Backyard Invasion is to educate visitors on the differences between invasive and endangered plants, and why it matters to protect the latter.

One endangered plant depicted in Backyard Invasion is the Maryland meadow beauty (Rhexia mariana). “There’s only two rhexias in Pennsylvania,” Morton said; one is endangered, and the other close.

What’s important is to replace invasive plants with native ones. If the ground remains bare, invasive plants are more likely to return. To choose a native plant that is sure to flourish, the Landscape and Nursery Association recommends observing which species are present in neighboring areas.

For gardeners only interested in plants with “exotic” features, another option exists: cultivars. Cultivars are native plants developed for specific qualities, taking into account aesthetics, size, blooming-period, and the presence of thorns.

Cultivars can satisfy a consumer’s wants without harming the environment.