Wisconsin gardening comes with many obstacles: rabbits, deer, cold winters, and invasive non-native plant species. In the war against invasive plants, one of the worst enemies is Buckthorn, specifically Rhamnus cathartica (Common Buckthorn) and Rhamnus frangula (Glossy Buckthorn). Many times I'll be at a customer's site and they want to know what are the shrubs creating a screen on the edge of their property? More often than not, it's Buckthorn. Probably the worst example I've seen is someone's neighbor pruned their Buckthorn up and put twinkling lights on it to enjoy from the deck. The tree was the source of loads of Buckthorn seedlings every year.
So what's the big deal about battling Buckthorn anyway? Europeans brought Buckthorn over with them to use for hedges and screening. It grows fast, hardly any maintenance (unless you want to get rid of it), is easily sheared into a hedge, and rabbits and deer don't eat it.
Problem #1: Buckthorn can quickly establish in sun or shade. The aggressive plant will push out native plants, especially in the understory of forests where native plants don't receive enough sunlight to thrive. We will be seeing more Buckthorn popping up in forests as Ash Trees are being lost to Emerald Ash Borer. The greedy buggers will gladly move into the spaces that Ash Trees leave behind.
Problem #2: Female plants produce a multitude of black berries. Birds will eat these, but receive little nutritional value from them. The species name of Common Buckthorn, cathartica, refers to how quickly the fruit and seeds go through the system of an animal. This way, the plant has ensured that it is quickly dispersed through the landscape. I call this the "evacu-ate" propagation method.
So what do you do about it? The best time for battling Buckthorn is in late fall/early winter when it is easily recognizable because it holds onto its green leaves much longer than the other deciduous trees and shrubs. You'll also be able to identify it from the bold "lenticels" that create light, horizontal stripes on the dark stems. Larger, mature specimens will have long thorns on the branches. Scraping the bark reveals a bright orange/yellow color.
November is the perfect time to ramp up on battling Buckthorn, whether in your yard or natural areas. Plants are moving water and nutrients into their roots for the winter, so pesticide applications are most effective. Larger plants (1/2" caliper or more) that are too difficult to pull out of the ground or too large to be sprayed, can be destroyed with the "cut and paint" method. Easier with two people doing the work, one person cuts the woody plants to the ground, the other will come behind with a container containing the herbicide. Look for pesticides labeled for woody plants like Bonide Stump Out or glyphosate concentrates (like Round Up) mixed with a little water. Be aware that some herbicides require a minimum temperature to be effective; check labels. Use a sponge brush to apply the herbicide to the freshly cut stump. I like to add a little red food coloring to the mix so it's easy to tell which stumps have already been painted. Make sure to thoroughly coat the outer cambium ring (don't worry about the center on larger stumps). The sponge brush will give you more control so you don't accidentally drip onto desirable plants. Burning the debris of branches you've cut down is the best way to destroy the seeds so you aren't spreading them to another area.
If the task of personally removing large stands of Buckthorn is too daunting, contact a company like Wachtel Tree Science. When the Buckthorn is gone you can start replacing them with native plants like Gray Dogwood, American Filbert, Fragrant Sumac, Elderberry, and Bladdernut. Once you've won the battle, it'll be easier to spread the word about Buckthorn eradication to your neighbors, and we can finally win the war.
Find more information about Buckthorn from the Southeastern Wisconsin Invasive Species Consortium.