common buckthorn

Rhamnus cathartica is a deciduous, dioecious shrub or small tree growing up to 10 metres (33 ft) tall, with grey-brown bark and often spiny branches. The leaves are elliptic to oval, 25–90 mm (1–3+1⁄2 in) long and 12–35 mm (1⁄2–1+3⁄8 in) broad; they are green, turning yellow in autumn, have toothed margins, and are arranged somewhat variably in opposite to subopposite pairs or alternately. The flowers are yellowish-green with four petals; they are dioecious and insect pollinated and bloom in April–June. The fruit is a globose black drupe, 6–10 mm (1⁄4–3⁄8 in) across, and contains two to four seeds.

In addition to the kindred Frangula alnus, Rhamnus cathartica may be visually confused with a plant of another family, Cornus sanguinea. The species was originally named by Carl Linnaeus as Rhamnus catharticus, but this spelling was corrected to cathartica as the genus name Rhamnus is treated as being of feminine gender.

Rhamnus cathartica is shade-tolerant, moderately fast-growing and short-lived. It is a food plant of the brimstone butterfly, Gonepteryx rhamni. The sulphur-yellow males are indicative of the presence of this species or of its alternative host, Frangula alnus. Rhamnus cathartica is the alternate host for the rust disease of cereals caused by Puccinia coronata. Crown rust fungus results in leaf damage and reductions in photosynthesis during its transit in this species. Rhamnus cathartica is also the primary overwintering host in North America for a significant agricultural pest of soybeans, the soybean aphid.

Allelopathy. Secondary compounds, particularly emodin, have been found in the fruit, leaves, and bark of the plant, and may protect it from insects, herbivores and pathogens. The emodin present in R. cathartica fruit may prevent early consumption, as it is found most in unripe fruits, which allows seeds to reach maturity before being dispersed. Birds and mice significantly avoid eating unripe fruits, and if forced to ingest emodin or unripe fruit, the animals regurgitate the meal or produce loose, watery stools. Rhamnus cathartica is a constitutive isoprene emitter Allelopathic effects of exudates from R. cathartica leaf litter, roots, bark, leaves and fruit may reduce germination of other plant species in the soil. …

The bark and fruit of Rhamnus cathartica were used as a purgative in traditional medicine, but is generally no longer used for this purpose on account of the fruit's violent action and potentially dangerous effects on the body. It also appeared to be routinely used as a purgative in some monastic settings, as was clear from the latrine pits of the Benedictine Abbey at St Albans excavated in the 1920s, where great numbers of buckthorn seeds were found mixed up with the fragments of cloth used by the monks as lavatory paper. The wood is hard and dense, and the bark yields a dye.