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Control of the Invasive Exotic Rhamnus cathartica in Temperate North American Prairies.. Page 1 of 11 Control of the Invasive Exotic Rhamnus cathartica in Temperate North American Forests Samuel W. Gale Introduction Common or European Buckthorn (Rhamnus cathartica L.) is a deciduous shrub, or small tree, of the Rhamnaceae family that readily invades woodland, savannah, and prairie habitats. An endemic of Eurasia, R. cathartica was introduced to North America as an ornamental shrub in the mid 1800s and was originally used for hedges, farm shelter belts, and wildlife habitat. It has spread extensively since and has a range currently bound by Nova Scotia, Canada in the northeast, Saskatchewan, Canada in the northwest, northeastern Kansas, USA in the southwest, and North Carolina, USA in the southeast (Meader 1999). Disturbed deciduous forest edges and open oak woodlands seem to be most susceptible to invasion by R. cathartica. Most alarming is the speed with which it is able to invade an area and displace native vegetation. The purpose of this paper is to provide an overview of R. cathartica's biology and life history, the ecosystem it is invading, and the various control methods currently being used by restoration practitioners to halt its invasion. Biology R. cathartica grows from two to eight meters tall (six to 25 feet) (Figure 1) (MNDNR 2000). It has an irregular spreading crown and a dark gray to brownish rough, scaly bark at maturity. Leaves are usually opposite, elliptic or oblong shaped with fine, round toothed or nearly smooth margins and dark green on the upper side contrasting with a lighter green underside (Samuels 1996). The three to four main branching veins on each side of the midrib that strongly curve or arch forward towards the tip of the leaf are one of the more recognizable morphological features (Figure 2) (OVMA 2000). Identification of R. cathartica is especially easy by the end of the growing season as the leaves persist on the branch well into early winter, maintaining a dark green appearance even after several frosts (Meader 1999). The tips of branches as well as branch forks are typically spiked with spines ranging from 0.5 cm to 5.5 cm (Figure 3) (Zomlefer 1994). R. cathartica is dioecious. In the spring, small (1.5 -2.5 mm), greenish - yellow, four -lobed flowers emerge. Staminate (male) flowers grow in clusters of two to eight flowers with four stamens in each flower. Carpellate (female) inflorescence usually lack petals and grow in clusters of two to 15 flowers (USGS 2000). Beginning around five years of age, R. cathartica is capable of producing fruit classified as drupes (Archibold et al. 1997). Fruit develop in early summer and mature in mid to late fall. Each drupe typically contains four hard, round seeds and turn from green to dark purple to black at maturity. The seeds and fruit are considered poisonous to humans and other animals. If eaten, the drupes cause severe stomach cramps (Converse 1985). Although R. catharica is capable of asexual reproduction, its primary means of reproducing is via seed. Figure 1. Five meter tall mature male R. cathartica tree (foreground and center) with foliage still persisting on branches after three weeks of consistent freezing temperatures (all photos in this report were taken Nov. 27, 2000 in south Minneapolis, MN near Minnehaha Creek). http: / /www.hort. agri. umn .edu /h5015 /00papers /gale.htm 2/15/01