Written collaboratively by Peter Schaefer and David Graper.
When considering weed control in tree plantings, the focus is generally placed on the control of herbaceous vegetation (grasses and forbs), particularly during the establishment phase. This focus is appropriate since control of herbaceous weeds is generally critical to establish a successful planting. As these plantings mature, providing perching sites for birds, another weed problem develops – the establishment of competing woody vegetation. These woody weeds are often left unchecked for many years because they look “natural” in a windbreak or other area of trees. While having a variety of woody vegetation and forbs in the understory may be desirable, problem woody species can entirely occupy the understory, to the exclusion of all else, and provide serious competition with the trees for moisture and nutrients. Not surprisingly, the most problematic species are non-native and have become highly invasive. This is the first of several articles discussing background information, identification, and control measures for these unwanted plants. The focus of this article is common buckthorn (Rhamnus cathartica), which poses the most significant threat to tree plantings and native woodlands in eastern South Dakota and much of the Midwest.
Common (or European) buckthorn was brought for ornamental uses from Europe to North America prior to the 20th century. It has proven to be a tough and highly adaptable species, growing in full sun to shade and acidic to alkaline soils. It tends to leaf out earlier in the spring and hold leaves later in the fall, lending a competitive advantage over native understory species, particularly after the buckthorn matures to form a dense understory canopy. Once established, it is common for buckthorn to completely occupy the understory of wooded areas.
Of importance in agricultural areas, buckthorn is also the alternate host for oat crown rust (Puccinia coronata), and is an overwintering host for soybean aphid. In fact, the crown rust is an aid in identification, the rust pustules on the leaves make it easier to distinguish the buckthorn plant from the branches of other shrubs in which they may be growing.
Buckthorn is a prolific producer of small, blue-black fruits that are consumed by birds, which then distribute the seeds in their droppings. Many individuals have mistaken buckthorn fruit for chokecherry fruit, and even used the fruit to make jam or jelly. (Individual buckthorn berries are directly attached to a twig while those of chokecherry are borne in clusters that are attached to the twigs.) You do not want to make that mistake because the fruit is not edible for humans and has a laxative effect which likely assists in the seed dispersal by birds. Not surprisingly, the seeds are often heavily deposited where birds perch, so buckthorn infestations tend to be concentrated in fencerows, hedges, windbreaks, woodlots and forests. Infestations are also common in urban areas that tend to receive little or no care. They are often allowed to grow and accepted as an ornamental plant by unknowing homeowners. Buckthorn seed will germinate from spring through late summer, although many seeds may lay dormant in the soil for one or more years.
Common buckthorn is a small tree or large shrub, generally not exceeding 20 feet, with a broad crown when grown in the open, much the same as a crabapple or hawthorn. It has simple leaves which are elliptical or ovate, 1 ½ to 3 inches long, and have small rounded teeth along the leaf margin. The upper side of the leaves is a dark, glossy green, with the lower side lighter in color. Leaf arrangement is sub-opposite.
As with most unwanted plants, the best time to control buckthorn is when it is small. Very small plants can be pulled by hand, while plants up to 2 inches in diameter can be pulled with a wrenching tool specifically designed for that purpose (sometimes referred to as a weed wrench). Hand pulling of small plants is practical for small areas, such as fence lines, hedgerows or alleys in residential areas, however it is generally not the solution for control in rural areas. Seedling removal in large, rural landscapes is best accomplished with fire, where appropriate, or with herbicides. Where a broadcast application is necessary, a broadleaf herbicide such as triclopyr would be used in most instances. Remember to read herbicide labels before each use, and follow label directions.
Larger stems require treatment of individual plants, either through mechanical removal (cutting), girdling or basal herbicide application. Cutting or girdling the tree will result in vigorous sprouting from the stump, which requires several cycles of follow-up treatment. As such, it is more effective to immediately apply herbicide (triclopyr or glyphosate) to the cut stump or girdled area. This will greatly reduce or eliminate sprouting. Another method is to simply spray the base of the stem with herbicide. In this case the product concentration must be higher than for foliar applications (25% for glyphosate; 8% for triclopyr).
Regardless of the method used to remove buckthorn, it would be rare to achieve adequate control with one treatment. Stumps will sprout and seeds in the soil will germinate, sometimes prolifically, so close monitoring of treated areas is a must for several years. In fact, it is difficult to actually eliminate buckthorn from a site, but it can be kept to tolerable levels with ongoing monitoring and site management.