You might have noticed a dense hedge, large shrub or small tree in your yard. Does it have glossy, broad green leaves that open early in spring? It might be buckthorn, one of the most troublesome invasive plants in the Chicago region.
According to the recently released 2020 Chicago Region Tree Census from The Morton Arboretum, 36% of all the trees in Chicago and the seven surrounding counties are a single species, European buckthorn (Rhamnus cathartica).
Very few of those trees were planted on purpose, according to Melissa Custic of the Chicago Region Trees Initiative, or CRTI, a partnership based at the Arboretum in Lisle. Most grew from seeds spread by birds that eat buckthorn berries. The plants can then expand into dense thickets by sprouting from underground roots.
“Buckthorn spreads very easily, and it’s able to crowd out most other kinds of trees,” Custic said. “That’s what makes it an invasive plant. It takes over any area it can get into.”
Why is buckthorn so destructive? It crowds out and outcompetes other trees and shrubs for water, nutrients and space. In natural areas such as forest preserves, it overpowers wildflowers and other native plants. Buckthorn has a longer growing season than most native trees, leafing out earlier in the spring and keeping its leaves longer in the fall. “That gives it an advantage,” Custic said.
Another problem is that a toxic chemical in buckthorn leaves, released when they decay, is washed into streams and ponds, where it is harmful to wildlife. Buckthorn also alters the soil, increasing its nitrogen levels to encourage other aggressive invasive plants such as garlic mustard (Alliaria petiolata). Although birds do eat buckthorn berries, the fruit acts as a diuretic and they get little nutrition from it, Custic said.
During the late 19th and early 20th centuries, buckthorn was deliberately planted in the United States to create hedges, imitating the way it is used in Europe. On that continent, where it is not invasive, buckthorn is a popular hedge shrub because it can tolerate the frequent shearing required to maintain the geometric shape of a formal hedge.
On this continent, where buckthorn has few plant competitors and no natural enemies, the plants quickly expanded beyond where they had been deliberately planted.
Buckthorn tends to sprout and form thickets along fences or under power lines where birds sit and drop the seeds. Since those accidental plants often follow property lines, homeowners may appreciate the privacy they create without realizing the plants are damaging to the environment.
CRTI encourages homeowners to remove buckthorn and replace it with other, noninvasive trees, shrubs and other plants that can form attractive privacy screens and beautiful gardens that are less harmful and support wildlife. You can find detailed advice about removing buckthorn and choosing replacement plants at chicagorti.org/healthy-hedges.
It can take persistence to remove buckthorn plants. “You can’t just cut them back,” Custic said. “They’ll resprout like a hydra.” Usually, repeated applications of herbicides are needed to completely kill off the root system. Some landscape contractors offer buckthorn removal as a service.
To check if your shrub is buckthorn, look at a branch. Buckthorn’s leaves line up nearly opposite each other on each side of a branch. Each oval leaf will be deep green, about 11/2 to 3 inches long, with tiny teeth, or serrations, around the edge. The tip of the leaf is usually pointed. The top side of each leaf is shiny and dark green, while the underside is lighter. The ovalberries are green in summer and dark purple to black when ripe.
The Plant Clinic at the Arboretum (mortonarb.org/plantadvice) can also help you identify a shrub you suspect might be buckthorn from photographs.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (mortonarb.org/plantadvice, 630-719-2424 or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
