Invasive Shrubs In The Northwoods

Sep 28, 2020

What started out as routine yardwork became a battle against unwelcome invaders. The Masked Biologist shares his tale of woe in this week’s Wildlife Matters.

As summer drew to a close, my wife and I were doing some cleanup work in the backyard. We had some trees that were growing up into the fence, and although I wasn’t excited about doing it, I had to cut them down to save the fence. As we worked to beat back the brush along the lot line, I was very disappointed by my discovery. Amidst the basswood, red maple and other various woody shrubs, I spotted an unwelcome familiar sight—buckthorn.

If you are not familiar, we have two types of buckthorn here, common and glossy buckthorn. Both are aggressive invasive species. Buckthorn is a large woody shrub or medium-sized tree sometimes up to 25 feet tall. If you don’t know what you are looking at or looking for, it may not be readily noticed, as it is so common. At one time it was planted as an ornamental shrub and considered good for wildlife because it bears small, dark little fruits that birds love. These fruits gave rise to one of its familiar names, autumn olive. It has sort of grayish-brown bark with lighter brown lenticels, or varying horizonal bulging lines (also seen on black cherry and serviceberry trees). The branches have thorns on them; in fact, if you look at the terminal or branch-ending buds in the spring, they are parallel and look almost like the track of a buck with a thorn protruding between them. Perhaps that is where the name buckthorn came from. Below the bark, the sapwood is a yellowish hue, and further into the heartwood it turns more of an orange color. The distinguishing characteristic for me though is the durable green leaves. This tree will be the first in the area to leaf out, and the leaves will stay green even when other trees have colored and shed some of their foliage. That’s why this time of year is really good for recognizing and removing these trees.

Buckthorn are bad for the environment, so much so that they are listed as a restricted species by the DNR under NR-40. This tree is an aggressive disperser, as are many successful non-native exotics. It spreads rapidly, and with early leaf-on and late leaf-off it is well equipped to capitalize on available sunlight and shade out competitors. If that isn’t enough, it is allelopathic, meaning it secretes a chemical that discourages the growth of other plant species in the immediate vicinity. Finally, remember when I mentioned those delicious fruits that birds love? Well, those fruits give birds the squirts, even worse than normal, meaning that it can spread very effectively through active seed dispersal anywhere birds land or take off. We don’t want these trees spreading in our forests; they can have devastating effects on our native tree species that are already struggling to keep a foothold due to changes in management, deer browse, and climate effects.

Cutting the buckthorn in my yard helped remove a couple of individual stems, but it felt like a drop in the ocean. When you drive around Rhinelander, look hard at the decorative shrubs around homes and businesses in town, and you will see it everywhere. For a while, I thought maybe it would be possible to remove the self-established trees, but we would need to work with local garden centers to come up with some kind of shrub swap program to help with all the landscaping. The more I tried to determine the scope and scale of the city’s infestation, the worse it appeared. It’s all around the downtown, and it lines the roads when I drive the kids to school. Even if landowners all worked together to remove every standing tree, there would be years of seed load to tackle to try to keep it from bouncing back in overwhelming fashion.

We have already lost the battle—and the war. These insidious trees have already made themselves at home. The best we can do is to try to protect our most valued natural habitats. We need to protect the riverways, the city parks and open spaces. If you have these trees in your yard, cut down and remove them. If you see them in your buddy’s yard, let them know that they are harboring a restricted invasive species. And the birds? Well, there isn’t really anything we can do about the birds. But take some time to get familiar with the various invasive species we have in the Northwoods, like spotted tansy, spotted knapweed, and the buckthorns, and you will be surprised at how well they have made themselves at home here.