January–February 2024

A Thorny Problem

Invasive buckthorn is damaging to landscapes and notoriously hard to control. New research may help manage the pesky plant.

Emily Sohn

Mike Schuster veers off a leaf-strewn path into a patch of woods near the St. Croix River, about 45 minutes northeast of Minneapolis. Stomping down prickly brambles and climbing over downed logs, the University of Minnesota ecologist points to a burr-covered stickseed plant and recommends avoiding it. He stops near a cluster of colored flags marking one of his team’s many experiments, each designed to tinker with the composition of the forest to reach one overarching goal: Get rid of buckthorn.

The shrub, which was imported from Europe in the 19th century, has since spread into every county in Minnesota. It has invaded urban parks, timber stands, and oak forests in Minnesota’s north woods. It has taken over public lands and private ones. And everywhere it goes, it wreaks havoc, damaging native ecosystems and reducing biodiversity. It has been a scourge for decades, despite ongoing efforts to control it.

Schuster’s buckthorn projects are part of a new wave of efforts to manage buckthorn in Minnesota by understanding—and targeting—its biology. From strategic cutting and planting protocols to fungal biocontrols, multiple strategies are being leveraged to manage buckthorn. And all ideas are welcome, says Sascha Lodge, terrestrial invasive species program coordinator for the Minnesota Department of Natural Resources’ Forestry division.

“Buckthorn is a really tough cookie,” says Lodge, whose job is to coordinate the management of invasive plants on state forest lands. “We’d love to have more tools in the toolbox. We are very excited for anything that researchers can come up with.”

Ornamental Origins. Buckthorn was originally brought to the United States as an ornamental hedge, and for good reason. It grows quickly into dense walls that offer privacy from neighbors. It holds onto its leaves until late in the season, staying nice and green well into winter. And it produces lots of berrylike fruits called drupes that attract birds. 

But those same features also make buckthorn a major pest. Its long growing season means that it can rapidly outshade native plants. Some species of birds disperse its prolific seeds far and wide. And although deer nibble on its leaves, there aren’t many predators or pathogens here to keep it in check. 

“Buckthorn is as tenacious as they come,” says restoration ecologist Alex Roth, conservation director at Friends of the Mississippi River, a nonprofit group focused on protecting and restoring habitat along the watershed.

Consequences can ripple through ecosystems and economies. When buckthorn replaces native plants, the invader eliminates food and habitat for mammals, birds, and pollinating insects. It reduces production in timber stands and can harm agricultural crops by hosting pests and diseases affecting soybeans, oats, and alfalfa. Classified as a noxious weed by the Minnesota Department of Agriculture, buckthorn may also influence erosion and nutritional availability in the soil in ways that scientists are still working to understand. 

“The impacts are widespread and may not be evident at first glance,” Schuster says. “But they can be quite far-reaching.”

Getting rid of buckthorn has often relied on two main strategies: mechanical (mowing, yanking, cutting) and chemical (herbicides). It’s not hard to get rid of a single buckthorn plant, Schuster says. But trying to fight it on a larger scale is like playing whack-a-mole. A cut stem, untreated by herbicide, quickly grows back. Even if you get out the roots, the plant’s plentiful fruits settle into the soil and quickly replenish what was lost—and then some. 

“In invasive ecology, we call this a Lazarus effect,” Schuster says. “You think that you have managed the problem, but it sprouts back to life.”

To transition from removing individual plants to taking the upper hand in controlling buckthorn, Schuster is part of a team that has been focusing on what happens after buckthorn is removed. In this brief period of time, when light and nutrients become available, the only plants typically available to soak up those resources are more buckthorn. The goal of Schuster’s work through the University of Minnesota’s Minnesota Invasive Terrestrial Plants and Pests Center is to quickly give a foothold to other plants instead. Called Cover It Up, the project encompasses a variety of approaches, all on display at the field site in Marine on St. Croix. 

Native Plants to the Rescue. On a warm September morning at the study area, Schuster shows off a forest once dominated by buckthorn that is now mostly free of it. Back in 2017, a team at the St. Croix Watershed Research Station, which is affiliated with the Science Museum of Minnesota, mowed down buckthorn in about 40 acres of woods. Within a couple acres of the removal area, Schuster and colleagues took what he calls a “shotgun approach,” setting up basically any kind of experiment they could think of to see what would work best for keeping buckthorn out.

In every direction, Schuster points to experimental plots marked with PVC piping and colored flags. There are big plots and little ones. In some, researchers spread native plant seeds after buckthorn removal. In others, they transplanted nursery-grown natives. Experiments have included different densities and combinations of seeds or plants: elderberry, sugar maple, rye grasses, and more. In some places, they followed mowing with herbicide spraying to kill buckthorn resprouts. In others, they erected wire fences to keep out deer, which are ubiquitous in the area. 

Soon after the experiments started, promising results began accumulating, all showing that careful revegetation can make a huge difference in buckthorn’s seedling survival. Planting shrubs and trees in high densities has been particularly effective. Careful placement of elderberry, balsam fir, and sugar maple, in particular, reduced the invasion of buckthorn by more than 80 percent over four years. After that, buckthorn that managed to grow back in revegetated plots were half as tall and produced a third fewer leaves compared with buckthorn that grew in plots that were left alone. It’s not logistically practical to bring in large numbers of shrubs and trees to revegetate big areas, but careful seeding might be enough to make a dent, Schuster says. In work yet to be published, he found that spreading a mix of wildflower seeds and grass, especially wildrye, after removal reduced growth and survival of buckthorn by as much as 77 percent. The researchers are also experimenting with lower density planting.

And time is of the essence, Schuster adds. As part of their experiments, he and his colleagues have monitored more than 13,000 buckthorn seeds in the soil, where 97 percent of the seeds that are going to germinate do so in the first year, they reported in 2023. That finding contradicted long-held—but untrue—wisdom that buckthorn seeds could stay viable in soil for five years or more. The results suggest that the most effective buckthorn control will involve immediate revegetation. 

“There are huge dividends for acting more strongly earlier,” Schuster says. “As we wait, those buckthorn that are there get larger and larger and harder and harder to control.”

Death by 1,000 Cuts. The Cover It Up project isn’t the only ongoing effort to turn buckthorn’s strengths into weaknesses. Along the Mississippi River gorge in Minneapolis, staff and volunteers with Friends of the Mississippi River are returning to the same buckthorn plants over and over again to employ a protocol called critical period cutting. The idea is to cut buckthorn stems at a height of 4-5 feet and to hack off growing buckthorn resprouts repeatedly at key times over a couple of years to interrupt the replenishment of sugar stores from the roots to the rest of the plant.

For FMR, the approach arose out of necessity after the Minneapolis Park Board placed a moratorium on certain herbicides in city parks starting in 2019. Before that, the group used to control buckthorn with a one-two punch of cutting with chainsaws, then applying herbicides to the cut stems to keep them from growing back, before seeding with native understory species. Critical period cutting is labor-intensive, but it eliminates the need for herbicides and appears to be effective. After volunteer efforts made a dent in buckthorn numbers in 2019, the group started hiring contractors to do the work in 2021. “After about four cuts, that stem dies and can just be pushed out of the ground,” Roth says.

One of most tantalizing approaches to battling buckthorn, experts say, is the potential for recruiting fungi into the fight. Although researchers have been trying to find a natural enemy that might work as a biocontrol for years, University of Minnesota forest pathologist Robert Blanchette has started making new inroads into the effort by tracking down places where buckthorn was reported to be dying naturally from a fungus. In 2023, he and colleagues at the U of M’s Minnesota Invasive Terrestrial Plants and Pests Center started collecting samples of canker, wilt, and root rot fungi from dying buckthorn in sites around Minnesota and Wisconsin. They identified and grew the samples in their lab. Then they started applying fungal concoctions to buckthorn in field trials. Now they are waiting to see what happens.

The work is just getting started, but Blanchette sees promising signs in the rotted roots that other researchers, like those at Friends of the Mississippi River, are finding after employing critical period cutting. “Something’s going on there and it appears that fungi are responsible,” Blanchette says. 

In Canada, researchers have developed a paste, based on a fungal pathogen, that is used to kill unwanted woody vegetation. But for use in Minnesota, it is important to find a native option that won’t carry the risk of unintended consequences. Fungal biocontrol would be environmentally friendly, reducing the need for herbicides, Blanchette says. The strategy could also help speed up critical period cutting protocols, making it unnecessary to go back and cut so many times. The team is now collecting fungal samples from FMR sites that are using the new cutting method, illustrating the collaborative nature of buckthorn-fighting efforts. “Anything that we can do to cut down on the amount of buckthorn is important,” Blanchette says. “We’ve got to do something to control this invasive plant.”

Every Tool Helps. There is never going to be one silver bullet for ridding the landscape of buckthorn, experts say, and it’s likely impossible to eradicate the plant altogether. But, with an ever-growing roster of strategies—a list that also includes grazing goats and stump-smothering plastic bags—every new tool is welcome in the effort to relegate the plant to a less damaging role.

After walking around the research plots in Marine on St. Croix and returning to the start of the main path, Schuster points to the other side of the dirt road, where the researchers haven’t mowed and the forest has been allowed to grow without interference. It is all buckthorn.

The invaders tower over the road and darken the understory, where most of the smaller plants are also buckthorn. Behind him in the study area, sunlight penetrates the forest, which is filled with native shrubs, trees, grasses, and flowering plants in shades of green, yellow, and purple.

They are two sides of the same road, but they are completely different. “It is day and night,” Schuster says. “Literally, dark and light.” 

Want tips for controlling buckthorn on your own property? See mndnr.gov/buckthorn.