A species of small tree in the flowering plant family Rhamnaceae. It is native to Europe, northwest Africa and western Asia, from the central British Isles south to Morocco, and east to Kyrgyzstan. It was introduced to North America as an ornamental shrub in the early 19th century or perhaps before, and is now naturalized in the northern half of the continent, and is classified as an invasive plant
Buckthorn is allopathic. 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.
Allelopathic effects of exudates from R. cathartica leaf litter, roots, bark, leaves and fruit may reduce germination of other plant species in the soil. Soils in buckthorn-dominated areas are higher in nitrogen and carbon than normal soils, which speeds up decomposition rates of leaf litter. This can result in bare patches of soil being formed and R. cathartica performs well in such disturbed habitats, so this may be adaptive for the setting of its seed.