If you've ever hacked buckthorn out of a patch of woods only to watch it come roaring back a few years later, University of Minnesota researchers have an explanation and a fix. A new study out of the College of Food, Agricultural and Natural Resource Sciences tracked what actually stops the invasive shrub from reclaiming restored land, and the short answer is: don't stop at removal.
Lead author Andrew Kaul, a former UMN postdoctoral researcher now at the Missouri Botanical Garden, and Mike Schuster of the Department of Forest Resources found that buckthorn comes back through resprouting, surviving seedlings, and the sheer volume of seeds already sitting in the forest floor. Schuster described it to MPR News as basically a "Chia Pet of buckthorn seedlings" waiting to sprout.
On the chemical side, the researchers found that applying the herbicide fosamine ammonium (sold as Krenite) to resprouting leaves the year after initial removal kills off much of what's left, and a second application two years later can nearly wipe buckthorn out of a treated area. Wait too many years before that follow-up treatment, though, and the results get a lot worse.
The bigger finding is about what goes in the ground afterward. Planting native grasses and wildflowers right after removal starves buckthorn seedlings of the sunlight, water and nutrients they need to get established, especially in spots without heavy tree cover. Wild rye grasses, in the genus Elymus, came out as the most cost-effective option — they grow fast, tolerate shade, and build a "thatchy layer" that blocks light to buckthorn seedlings during the fall months when that light matters most.
Schuster put the core logic simply: "The problem isn't that there is necessarily too much buckthorn, but that there isn't something else there to soak up those resources." Kaul, in comments to Minnesota Ag Connection, was blunt about the tradeoff involved: "There is no silver bullet for defeating buckthorn, but our work shows that control is possible with hard work and patience."
For anyone doing restoration work around the Twin Cities — including the volunteer crews who show up for buckthorn pulls along places like Minneapolis's East River Trail — the takeaway is straightforward: removal alone isn't the finish line. Seeding natives early and following up with targeted herbicide treatment on a set schedule appears to be what actually keeps buckthorn from taking the land back.