Invasive Plant Species: Non-Native Shrubs Remain Priority, but Others Can Cause Problems in Chicago Region

For decades, land managers, gardeners and landscape firms that focus on native plants in the United States have been removing invasive plant species and replacing them with alternatives that are healthier for the environment.

“We’re never going to be done with keeping these invasive plants in check,” says Sharon Yiesla, plant knowledge specialist for the plant clinic at the Morton Arboretum. “But if homeowners and managers of public gardens and natural areas and natural area landscape firms work together, they can control these species and improve the health of ecosystems.”

A recent study co-authored by the Morton Arboretum found that the biggest invasive plant problem in the Chicago region is non-native shrubs, with common buckthorn (Rhamnus cathartica) being the No. 1 culprit and various non-native honeysuckle species in the Lonicera genus close behind (ScienceDirect.com/science/article/abs/pii/S1618866725000986).

“Invasive shrubs are a serious threat to our forests,” said report co-author and the Arboretum’s Center for Tree Science Fellow Lindsay Darling, Ph.D. “As quick and aggressive spreaders, these invasive shrubs can significantly alter and threaten our native habitats, including our keystone oak ecosystems, which support a large diversity of life.”

Other invasive species in the region include Callery pear (Pyrus calleryana) and wildflowers such as garlic mustard (Alliaria petiolata) and lesser celandine (Ficaria verna). All of these, along with the invasive shrubs, grow in urban and suburban yards as well as in natural areas.

They arrived from other countries through various means, both intentionally—to help farmers and provide new plants for gardeners—and unintentionally, such as through ship ballast and other pathways.

INVASIVE PLANTS DEFINED

Invasive plants are those introduced from outside their natural region, but not all introduced plants become invasive. “When we talk about invasive species, we’re looking at plants that are getting out to natural areas and becoming a problem for native species, and in turn the ecosystem,” Yiesla says. “It’s a much more serious issue than people realize.”

“Fruits and seeds from these invasive plants get transplanted from birds, animals, wind and rain, and even spread underground,” Yiesla explains. In the case of common buckthorn, birds eat the fruits and spread them very quickly. “The fruits pass through their systems so rapidly that they’re not getting any nutrition,” she says.

“When invasive plants take over, that sets off a chain reaction, because the native plants, insects and other wildlife rely on are not there anymore,” she adds.

WAYS INVASIVE PLANTS ARE KEPT IN CHECK

Many hours of volunteer work and funds are  required to keep these species at bay. For example, Chicago region forest preserve districts spend more than $1 million annually to eradicate buckthorn and non-native honeysuckle, according to the Chicago Region Tree Initiative at the Morton Arboretum.

Depending on the species, employees and volunteers perform periodic controlled burns, cut down invasive plants, pull them out of the ground and at times, trained staff carefully apply herbicides. Then they replace the landscape with more suitable plants and monitor for the return of invasives.

At the Morton Arboretum and other natural areas and public gardens, staff and volunteers periodically remove buckthorn and other non-native shrubs by cutting them down and burning the branches.

“Anyone working in natural areas knows buckthorn is such a big problem that we also have to use specific herbicides,” Yiesla says. “But we only treat the stump of buckthorn and use small amounts. It’s not like spraying the yard with chemicals.”

“It’s good to be concerned about using chemicals to get rid of invasive plants, and often it can be done in garden landscapes without using pesticides,” Yiesla adds.

Home gardeners can get rid of the buckthorn before it gets large and produces berries. “If you have a lot of it, you might have to hire a landscaper to remove as much as possible,” Yiesla says, adding that afterward, gardeners need to be diligent about watching for new sprouts of buckthorn in their yards.

GARLIC MUSTARD, INVASIVE PEAR, LESSER CELANDINE AND TEASEL

Yiesla says she has seen state park trails lined with garlic mustard. “Nothing else was growing there,” she says. Pulling garlic mustard is time consuming but fairly easy when the ground is moist. The best time to remove the plant, including its taproot, is in spring before it flowers and sets seed. Various natural area volunteers pull garlic mustard each spring, then put it into garbage bags so seeds cannot get carried away to create more stands of the invasive.

A newcomer to the state list of exotic weeds is the Callery pear, sometimes called Bradford pear. The sale of this species will be banned in Illinois beginning in January 2028.

“Look anywhere in a wooded area in spring where you see trees flowering white,” Yiesla says. “People think it’s a native cherry blooming, but it’s likely a Callery pear. They’re thorny, and it takes a lot of work to dig them out.” She’s seen open fields in the Chicago region “just filled with Callery pear.” 

Even if it’s still for sale, she recommends not buying or planting it.

The invasive lesser celandine is becoming more common and worrisome, Yiesla adds. This low-growing ground cover with yellow blooms forms dense carpets that smother native plants, riverbeds and lawns. Removing the plants is time consuming, doesn’t always work and requires digging the bulbs and tubers out of the soil.

Woodlands and other spots where invasive shrubs and other plants have been removed become a sea of diversity again. On a recent spring day, Yiesla visited a woodland at the Arboretum where volunteers keep the invasive plants in check.

“I saw so many wildflowers, Dutchman’s breeches and trillium, hepatica and bloodroot and spring beauty, and native shrubs and trees such as buckeye and wild cherry,” she says. These plants in turn attract native pollinators, which also serve as food for native birds.

Though typically not found in gardens, a plant called teasel (Dipsacus sp.) is invading wood edges, shrublands and roadsides. Forest preserve and natural land area volunteers remove teasel, which, at first glance, looks similar to thistle, by digging the basal rosettes in spring and cutting the seed heads. But it’s a tough plant to eradicate, and land managers have carefully used herbicides in some instances. One teasel plant drops thousands of seeds, overcrowding and smothering native plants, grasses and wildflowers that help feed natural wildlife, according to the Long Grove Park District, which works to eradicate it from its properties and asks homeowners to do the same.

“It’s important for people to know what plants they have in their yards and what they are buying,” Yiesla says. “Some of the introduced plants are fine, but people need to be educated on which ones are not. More people are asking questions and want to learn more about invasive plant species and how to get rid of them and what to replace them with.”

 Some examples for replacing non-native honeysuckles and buckthorn include native blackhaw viburnum, spicebush and bush honeysuckle, a low-growing native shrub with yellow flowers blooming in early summer. Yiesla notes the bush honeysuckle is in the Diervilla genus and is not related to the invasive honeysuckles in the Lonicera genus.

Editor’s note: While woods and shrublands are being invaded by destructive introduced plants, wetlands are also experiencing similar problems. Read more about this in an upcoming issue.

To learn more about identifying invasive plants and suitable replacements, visit Mortonarb.org/plant-and-protect/chicago-region-trees-initiative/how-to-deal-with-invasive-woody-plants.

Sheryl DeVore has written six books on science, health and nature, as well as nature, health and environment stories for national and regional publications. She is co-author of an upcoming book, Birds of the Midwest, to be published by Timber Press. Learn more at SherylDeVore.WordPress.com.

SIDEBAR

Ways Invasive Plants Were Introduced

Invasive plants came to the region in various ways, including intentional introduction to help farmers or gardeners, while others were inadvertently transported via boat ballast and other means. Many were sold for decades at garden centers.

Common buckthorn was introduced to North America from Eurasia in the early to mid-1800s for farm windbreaks and privacy hedges for homeowners. By the early 1900s, this shrub, which could tolerate many growing conditions, spread beyond gardens and farms into natural areas, where it shaded out native plants and added toxins to the soil.

Callery pear, thought to be sterile, was imported to the U.S. by the Department of Agriculture in hopes of saving the common pear tree from blight. The Callery pear soon hybridized with other ornamental pear tree species and began setting fruit and seeds. By the 1980s, it was the second most popular tree in the U.S. and was being sold at many garden centers. It, like buckthorn, adapted well to various conditions including drought and pollution.