Buckthorn was introduced in the eighteen hundreds as an ornamental hedge plant, and it has a lot of traits that make it really effective at that. It grows really quickly, has tons of vegetation, lots of leaf area. Those leaves stay late into the fall so it’s blocking line of sight and doing the whole hedge thing later than it might be otherwise. It’s resistant to pruning and herbivory and it produces a ton of fruit. All these things made buckthorn extremely popular, and it was sold all over the country as a hedge plant. And those traits are great at making buckthorn a hedge, but they also make buckthorn extremely competitive in natural areas. And so when you translate all those traits into a setting where we have native species or other species that we want to survive, buckthorn just wins and it pushes them out. And so this is part of the reason why buckthorn is such a big problem and also part of the reason why buckthorn has spread so quickly. It just is able to dominate systems very easily because of those traits. My name is Mike Schuster. I’m a researcher at the University of Minnesota. I study buckthorn for a living and am an expert at invasive plant management. Buckthorn and other invasive species like it are problematic because they reduce biodiversity, they harm nature, and native species really need a leg up right now. There’s so much going on in the world that disadvantages native species that they really don’t need one more thing to keep them down. Buckthorn has a lot of direct impacts on humans in terms of things like increased erosion, reduced value of natural areas, and exclusion of important species like timber. We don’t know exactly why buckthorn is invasive here. When we do comparisons between buckthorn in Europe and buckthorn in North America, it doesn’t seem like there’s some, like, really specialist herbivore or something like that that’s keeping it in check over there, but not here. The switch that was flipped is unclear. One of the things that seems to make buckthorn so invasive here is its extended fall phenology. It keeps its leaves late into the fall, allowing it to capture light, perform photosynthesis, and grow later into the fall than many of our native species. It has that trait not as something new that showed up once it was introduced in North America, but as a consequence of the more mild climate of the European continent compared to places like Minnesota. Covered Up started in twenty sixteen, and it was originally conceptualized as a way of investigating whether or not native species could effectively compete against buckthorn and other woody species, but primarily buckthorn, and how that could be utilized in a management context. So, we noticed that there are places, particularly places like sugar maple forest where buckthorn doesn’t grow very well, or other places where there’s dense vegetation like grasslands where buckthorn doesn’t grow very well. Are those species potentially things that could outcompete buckthorn and prevent it from causing such a persistent issue for land managers? So part of the work that we’ve done is involved aging buckthorn in different places in Minnesota. And the oldest buckthorn that we found were about eighty years old, which is pretty old for a buckthorn. And those are stems that get to be real, real big. Those are trees that that are also pretty gnarly, you know, multi stems, multi stem plants that have seen their fair share of damage and just continually are resprouting and regrowing. However, those plants are not necessarily fruiting. Right? Buckthorn is a dioecious plant, meaning that it has both males and females, and so only the females are gonna carry fruit. And the propensity for any given female to fruit is going to be related to how many resources it has. So things like light, water. Right? Those are gonna be important predictors of how much an individual can fruit. So any one buckthorn is gonna fruit a varying amount depending on how large it is and how many resources it can capture. Larger plants are gonna be able to capture more light, more water. They’re gonna be able to produce more fruit. However, if we’re dealing in a management context where we go and remove those fruiting trees, that releases some of those resources for other nearby plants. And so you can kind of get this domino effect of taking out the big fruters and then giving extra resources to some nearby plants that might be fruiting now, but will be fruiting much more in the future because of those extra resources. There used to be this idea that buckthorn seeds lasted five or six years in the soil, and that would suggest that at any time over a five to six year window, you could have this kind of like surprise attack of buckthorn and have to deal with that. But that’s not what we see in nature. Right? Like when we do management, we see a flush of buckthorn germination the following year and not really much after that. And so, like, inherently, I think we know that this was inaccurate in some ways. Through our experiments, we were able to disprove this because associated with our different experiments that we’ve been doing for a while now, we have all sorts of different bud thorn seeds that are growing out in these fields. So we know when those seeds were dispersed, we know when they germinated, we know how they survived, all these things. Right? And we can use those data to evaluate, how long it took any given seed to germinate. And ninety seven percent of them show up in the first year. We get a little bit in year two and basically nothing after year three. And so that shows us very clearly that germination doesn’t last five or six years. It happens right away and these seeds don’t persist in the soil like we once thought they did. Quadthorn spreads by a couple different ways. One is that it drops seeds and, yeah, tons of seeds fall off of these trees, especially for the, you know, really heavy fruiters. You get a lot of seeds that are dropping just below or right next to the tree, and that can create these, you know, layers of seedlings once you remove those trees. Those are the things that are responsible for the carpet of seedlings that you see after buckthorn removal. But you also get spread by birds, although that’s probably not happening for very long distances because if you think about the birds that are moving buckthorn things like waxwings or American robins, they don’t have a behavior that, you know, entails them eating a berry and then flying as fast as they can in the opposite direction. They kinda hang out in one spot and are moving things in that journal area. And so those birds are probably doing more short to mid distance dispersal, whereas other things that are eating them, maybe deer, might be responsible for that longer distance transport. So because the buckthorn germinates in year one, year two, we really need to have some sort of native cover there immediately. It has to be done concurrent simultaneously with our removal efforts in order to have that layer of vegetation there to suppress any new buckthorn seeds that are going to pop up. Grasses do that. Wild rye do that. They germinate very quickly. They form those dense layers of vegetation very quickly. But those layers don’t last forever. They last, you know, maybe five years, and then they start to peter out a bit. And so the timing of buckthorn germination with that robust growth of wild rice really lines up very well. And as those wild rice start to fade out, as you start to lose your grass cover, that’s when those wildflowers really start to come into their own. And so it’s a natural succession when you pair grasses with forbs, grasses and wildflowers, that as the grasses start to fade out, the wildflowers are there to take over and continue that layer of cover going forward. But then we did a couple other different experiments alongside that. One was looking just at the basic light requirements for buckthorn, because if we think that native plants are able to suppress buckthorn, one of the key suspects in that is competition for light. And so we grew buckthorn under a bunch of different canopy conditions up at Cloquet Forestry Center where we had a bunch of different experimental forests. And in that experiment, we looked at growth and survival of little buckthorn, and what we found was that buckthorn didn’t really care how much light it had as long as it was more than ten percent canopy openness. So if you look up at the sky, you see more than ten percent blue sky. Buckthorn is gonna do absolutely great. But once you get below that, then buckthorn performance starts to decline a little bit, and you really have to get down to like two or three percent canopy openness to get buckthorn into a place where it’s going to start doing poorly and having really low survival rates, which is a tall order. Having that much shading is tough to do. And so we looked at how we could potentially do this using native species in a variety of different contexts. One was some intense plantings of things like sugar maple and balsam fir or elderberry species. Another experiment was looking at a more practical approach of using just native grasses and wildflowers and trying to create that shade. And we found that both strategies were effective in different contexts. So on average that canopy openness window is, you know, about two to three percent light availability averaged over the growing season. But if we break it down into light availability in different seasons, it does matter. Buckthorn really is pretty insensitive to light availability in the middle of summer. But what actually is determining buckthorn survival is light availability in the spring and fall. Really the key here is that places that have a lot of shading in the fall are going to be better able to resist buckthorn invasion than places that don’t have a lot of shading in the fall. And this is a big issue for places like oak canopies or other deciduous forests where that canopy thins earlier in the fall than, let’s say, a place that has a lot of long lived leaves or an evergreen. Native grass seeding is effective because it creates this quick dense layer of vegetation that can shade buckthorn throughout the growing season, but it also leaves a layer of thatchy debris that shades the plants into the fall. So there’s a couple things going on with with buckthorn in disturbance. One is that disturbance, whether that’s kids or deer or something else, is going to be reducing competition and increasing resource availability. So we tend to see more invasion in those areas because it’s just a better place for any plant to grow, but especially buckthorn given how aggressive it is and how many seeds it produces. So it’s present and has the opportunity to exploit those resources, and it will. The other thing that’s going on with buckthorn and disturbance is earthworms. So many people don’t know that earthworms in Minnesota, in the upper Midwest, are not native. They’re also an invasive species. And earthworms tend to turn up the soil, make it looser, and they also eat up the litter layer that would normally block seeds from getting in contact with the soil. And so those two things open up forests, whether we’re talking about backyard or somewhere a hundred miles away from the nearest person, opens up those those forests for invasion. And buckthorn actually has a mutualistic relationship, a positive interaction with earthworms where buckthorn produces a ton of leaf litter, a ton of dead leaves that are really high in calcium, and earthworms love that. Buckthorn feeds the earthworms, and earthworms prime the soil for more buckthorn. So they help each other out, and that can be really challenging to deal with. We have an experiment where we’re comparing a bunch of different seed mixes at different rates. So we have the M and L mix. We have a couple state seed mixes. We have a seed mix based on our earlier work, and the M and L mix is is doing really, really well because of its of its emphasis on grass species. So if we’re looking at which species are best suited for revegetation based on the work that we’ve done, certainly the wild rye grasses are going to be really strong competitors early on after management, and so we want to make sure that they’re present. So species like Virginia waterleaf, columbine, bee balm, any of the Rudbeckia species are all gonna be really strong, competitors over longer time periods and produce some really nice flowers and foliage for, your forest. So if someone has a lot of buckthorn in their backyard and they wanna remove it, it’s probably best to set reasonable goals for yourself to begin with. Oftentimes, people can find success with going through and taking out the largest plants first. Those are also gonna be likely the ones that are producing the most seed. And so getting those large plants out first is probably a good first step. And then picking targeted areas to work through. So not trying to do everything all at once, but taking a smaller piece of the puzzle and doing that part well. So that means getting rid of as much of the buckthorn as possible, even the little guys if you can, and then immediately replacing that vegetation with native species is gonna give you your best bet. Doing that sequentially year after year, step by step, will eventually lead to a better outcome than trying to bite off more than you can chew and ultimately having to do it all over again in a few years.