Nature in my Backyard: Non-native, Invasive Buckthorns
Robin Schachat
Let’s start with the exception – the rare exception. Yes, there is in fact an Ohio native Buckthorn that need not be removed from your garden, Rhamnus alnifolia, the alder-leaved buckthorn, a small (2’ - 3’ tall), noninvasive little shrub that grows in fens or low, damp areas with calciferous soil across the northern US and southern Canada. Dead giveaways if you think you may have found this happy little shrub, and not one of its rambunctious, hateful, foreign cousins: the native buckthorn has a five-petalled flower (nonnatives tend to have four petals) and the native has no thorns! If you find one of these on your property, it is entirely acceptable, useful for our native birds and pollinators, and develops bright, attractive autumn coloration.
As for the foreign cousins – that’s another story completely. The Buckthorns native to Europe and Asia are destructive in many ways to our native habitats, and do not present healthy food sources for native pollinators and birds. Because they have no native predators, and because they leaf out earlier in spring than natives in the same areas, they are able to out-compete desirable native plants in our woodlands, where they create a thick undercover that chokes natural woodland undergrowth. In more open areas they also shade out the natives there and, as relatively heavy feeders, deplete soil nutrient levels. Worse still, invasive Buckthorns are allelopathic: they release chemicals into the soil around their roots that deter the growth of other varieties of plants. Thickets of Buckthorn establish quickly and are almost impenetrable by wildlife and (take note!) humans trying to eradicate them, so the sooner such well-minded humans get to eradicating invasive Buckthorns, the easier it will be to eradicate them. Meanwhile, their leaves are a terrific food source for earthworms, and this is a problem here in northeast Ohio.
“Why”, you ask, “I thought earthworms are a sign of healthy soil?” In South Carolina, absolutely. But earthworms have not been native to our part of the world since the last Ice Age. Glaciers here killed off native earthworms then, and when the glaciers began to recede about 20,000 years ago the earthworms had been shoved many hundreds of miles to the south. They have begun their inexorable wiggling northward, but it takes an earthworm and its food sources many millennia to recapture hundreds of miles of territory. The key to that sentence is “their food sources”: the plant materials that feed earthworms no longer occur here naturally either. If earthworms advance ahead of their native foods, as they have done because humans have assisted their migration north in order to use them for fishbait, they must find other native plants to eat, and they are now decimating low-growing native plants in the duff layer of our Ohio woodlands. In the Cleveland area, earthworms, like Buckthorns, are invasive non-natives that harm our environment.
Back to the bad Buckthorns. Non-native varieties were imported to North America for use as hedging plants. Thick, thorny hedges were deemed useful by settlers “taming farmland”. The foliage of the hedges is shiny and deep green, leafing our early in spring and holding on late in fall, making for a long growing season. The berries mature to a glossy black, which can appear very attractive. Indeed, our native birds will eat them, and here another problem arises. Our native birds did not co-evolve with these alien Buckthorn varieties, and need different levels of nutrients to thrive than non-native Buckthorns can produce. The berries of the non-natives are low in protein and have a notable laxative effect on our birds, even killing small ones. Sadly, deer do not find alien Buckthorns attractive, neither the fruits nor the foliage. If Buckthorns are competing for habitat with native plants, the deer will decimate the native plants and leave the Buckthorns untouched.
Rhamnus cathartica is known as Common Buckthorn, and it is the plant most of us here in upland NE Ohio think of when we think of a “Buckthorn”. It is a taller shrub than other Buckthorns, growing to 25’ tall or sometimes more, with a spread at full growth of 10’ – 15’. Leaves are oval, 1” – 3” long with finely toothed edges, markedly veined with 3 or 4 veins per side, and held sub-opposite on twigs. Thorns project from joints between the twigs. The plant blooms over multiple months in spring, small yellow/green flowers (a side note – the plants are dioecious), and the flowers on the female plants mature through summer and into fall to become round, shiny, black, berry-like drupes. Late fall leaf color is yellow. (See photos 4 and 5) The bark closely resembles that of native cherry; it is silvery, with horizontal lenticels. Common Buckthorn grows rapidly in shade or sun, and requires no special care or watering. It is also an alternative host for rust diseases of oats, for soybean aphids, and for alfalfa mosaic virus – not at all helpful to farmers, as it turns out.
The other invasive Buckthorn horror in our world is the Glossy Buckthorn, Frangula alnus, formerly known as Rhamnus frangula. This one tends to grow in wetter areas – although it will grow anywhere, given the chance. Its liking for wetlands and stream borders gives it the opportunity to do even more damage than Common Buckthorn. When it out-competes native undergrowth, it leaves the soils around its root zones uninhabited; water runoff near stream beds then causes heavy erosion, degrading water quality.
Glossy Buckthorn closely resembles Common Buckthorn in many ways. The leaves differ in that Glossy Buckthorn’s leaves have many more parallel veins, up to 10 per side, and the top surface of the foliage is very brightly shiny. The margins of the leaves tend to be smooth, not serrate. The plants are monoecious, which means all will fruit, and the drupes go through a red phase before maturing to a purple-toned black. Glossy Buckthorn does not usually grow higher than 20’ tall, and its flowers may have 5 petals, like the native. Bark on this plant is darker and grayer than that of Common Buckthorn, but may still be mistaken for cherry.
When you encounter non-native Buckthorns on your property, you must be determined to remove them, lest your garden become one impenetrable thicket of poisonous plants. There are many ways to attack these undesirables, but please know that they want to survive. They WILL fight back. Rather than try to give specific training here, we suggest you research the various methods detailed on the internet. You may find one method more to your liking, or to your tolerance, than another. Better yet, if you have the opportunity, help a more experienced friend or a wildland or park expert one day when he or she takes on an eradication effort – experience will definitely help. Now, we will leave you with two photos of the same area, before and after clearing of understory invasive Buckthorns. Job well done!
