LISLE, Ill. (May 14, 2025)—Invasive shrubs have overtaken the understory of an estimated 77% of Chicago region forests, impacting the structure of forests and threatening their ecosystems and biodiversity, shows a new study co-authored by researchers at The Morton Arboretum and Purdue University.

The paper, published in this month’s volume of the peer-reviewed, international journal Urban Forestry and Urban Greening, coincides with Illinois Invasive Species Awareness Month in May.

“Invasive shrubs are a serious threat to our forests,” said report co-author and the Arboretum’s Center for Tree Science fellow Linsday Darling, Ph.D. She is also the geographic information system (GIS) administrator for the Arboretum’s urban and community forestry program, the Chicago Region Trees Initiative (CRTI). “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.”

Healthy urban forests support greater biodiversity—including flowers, birds and wildlife—while also being more resilient to climate change and potentially storing more carbon.

Common buckthorn as well as Amur and Tatarian honeysuckle are some of the most prevalent invasive shrubs threatening local forests, making up over 42% of trees across the Chicago region, according to the 2020 Chicago Region Tree Census conducted by the Arboretum and its partners.

For the Urban Forestry and Urban Greening study, researchers detected the shrubby invasive species using data from airborne LiDAR, a type of laser scanning technology, and spectral imagery to create multi-dimensional maps of the forests and see into the understory. Their groundbreaking research model has an accuracy rate of 93% for detecting invaded forest patches.

Invasive shrubs, which were originally introduced to the region as ornamental garden plants and escaped cultivation, negatively impact forests by creating a dense sub-canopy that shades out native plants and trees. They also leaf out earlier and lose their leaves later than native species, giving them another competitive advantage.

“Our predictive map, mapping areas of non-native shrub species invasion, could serve as an invaluable tool for regional managers,” the report reads. “It offers the potential to direct efforts towards the eradication of these invasive shrub species.” CRTI has made an online, interactive version of this map available.

This Illinois Invasive Species Awareness Month and beyond, individuals, homeowners and land managers can help combat invasive plant species by removing existing invasives and selecting non-invasive trees and shrubs as replacements. The regionwide map of shrubby invasives can help natural areas managers identify patches of invasives outside of their boundaries and work with homeowners to remove them, Darling said.

Those seeking to learn more about invasive species can visit the Arboretum’s Problem Plants webpage, which provides in-depth information about invasive plants and exotic, noxious and dangerous weeds, among other non-recommended plants. For non-invasive plant recommendations, people can visit the Arboretum’s online Tree Selector or contact its free Plant Clinic, which can help identify invasives either in person (with paid admission to the Arboretum), by phone or by sending a photo via email. Plant samples or photos of the plant can be brought in person to the Plant Clinic for assessment. Plant samples that could be poisonous should be sealed in a bag.

CRTI also has an online invasive species hub with resources for homeowners, volunteers and land managers on how to identify and eradicate invasive woody plants and recommendations on what to plant in their place. Those interested in helping to remove invasive species locally can sign up for volunteer opportunities through CRTI and its partners.

Additionally, the Arboretum offers invasive species-related adult programs throughout the year. Upcoming classes include Natural Areas Management (online and in person) (June 4, 14 and 21) and Invasive Species (on demand, anytime).

Arboretum visitors with paid admission can also stop inside the Visitor Center from 11 a.m. to 2 p.m. Wednesdays and noon to 3 p.m. Saturdays in May for free information about potential invasive plants and insects and how to combat them.