Rhamnus cathartica L.
Rhamnus cathartica (Buckthorn, Common Buckthorn or Purging Buckthorn), is a species in the family Rhamnaceae, 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.
Rhamnus cathartica is a deciduous shrub or small tree growing up to 10 m tall, with grey-brown bark and spiny branches. The leaves are elliptic to oval, 2.5–9 cm long and 1.2–3.5 cm broad; they are green, turning yellow in autumn, 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. The fruit is a globose black drupe 6–10 mm diameter containing two to four seeds; it is mildly poisonous for people, but readily eaten by birds, which disperse the seeds in their droppings.
The Common Buckthorn, Rhamnus cathartica, is shade-tolerant, moderately fast-growing and short-lived. This species is a tough, durable tree which adapts to urban or suburban environments, and virtually any area it is dispersed in. It is widely regarded as a major invasive species whose shade prevents the establishment of native trees or shrubs. It has become the target of efforts to eradicate it from home sites, parks and woodland areas. It is difficult to control because it sprouts vigorously and repeatedly from the root collar following cutting, girdling, or burning, though it can be controlled by applying concentrated herbicide to the cut stem.
The species was originally named by Linnaeus as Rhamnus catharticus, but this spelling was corrected to cathartica as the genus name Rhamnus is of feminine gender.
Cultivation and uses
The bark and fruit were used as a purgative in the past, though their potentially dangerous violent action and side effects means they are now rarely used.
The wood is hard and dense, but little-used.
It is a food plant of the Brimstone butterfly. The sulphur-yellow males are indicative of the plant's presence.
This species is the alternate host for the important rust disease of cerals caused by Puccinia coronata. R. cathartica is also the primary overwintering host in North America for an important agricultural pest of soybeans, the soybean aphid.
Invasive species - North America
The species is naturalised and sometimes invasive in parts of North America. R. cathartica has a competitive advantage compared to native trees and shrubs in North America because it leafs out before native species. Soil in woodlands dominated by R. cathartica was higher in nitrogen, pH, and water content that soil in woodlands relatively free of R. cathartica, probably because R. cathartica has high levels of nitrogen in its leaves and these leaves rapidly decompose.
R. cathartica is also associated with invasive European earthworms (Lumbricus sp.) in the northern Midwest. Removing R. cathartica led to a decrease of invasive earthworm biomass of around 50%. 
^ Flora Europaea: Rhamnus cathartica
Source: Wikipedia, Wikispecies: All text is available under the terms of the GNU Free Documentation License