European buckthorn Rhamnnus cathartica L. [RHACT, nerprun commun, Common buckthorn, nerprun purgatif, nerprun cathartique] Perennial, reproducing only by seed. Grows as a bushy small tree or several-stemmed shrub up to 6 m (20 ft) high. Most branches older than one year are tipped with a short, sharp thorn, hence "thorn" in its English name); this thorn often present in the fork of 2 branches; leaves usually opposite (2 per node), but sometimes alternate (1 per node), elliptic or oblong usually with fine rounded teeth but sometimes nearly smooth, dark green above and lighter green or yellowish-green below; usually with 3 main branching veins (sometimes 2 or 4) on each side of the midrib that strongly curve or arch forward towards be tip of the leaf ; flowers small, with 4 sepals and 4 petals, greenish to yellowish. shortstalked, in small clusters from the axils of leaves or on short twigs along the stem; each flower unisexual with either 4 stamens or 1 pistil but not both, and the sexes usually on different plants; berries round, 5 - 6 mm (1/5 - ¼ in.) in diameter, purplish-black and very juicy with 4 very hard seeds (stones). Flowers during May and June but the berries, and often the dry leaves, persist on the tree long into winter.

European buckthorn was introduced as an ornamental shrub, but its seeds have been widely scattered by birds and other animals so it is common in fence lines. woodland, pastures and abandoned farmyards throughout southern Ontario. Because European buckthorn is an alternate host for the fungus which causes leaf and crown rust of oats, it must be destroyed to minimize this disease on oats.

It is distinguished by the sharp, thorn-tipped branches in contrast to the simple or compound thorns growing from the sides of branches in the Hawthorns, by the prominent forward-curved side of the leaves, the clusters of purplish-black berries along the stems and short twigs, and each berry usually with 4 hard seeds. These juicy berries are very bitter and cathartic, and usually cause severe stomach cramps if eaten.

For additional information, see Ontario Ministry of Agriculture and Food Factsheet, Common Barberry and European Buckthorn. Alternate Hosts of Cereal Rust Diseases. Agdex 110/632.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)