Buckthorns are various species of shrubs and small trees in the family Rhamnaceae, a mostly tropical and subtropical family of about 600 species. Most of the buckthorns are in the genus Rhamnus.
Buckthorns have a few economic applications, although none of these are very important. A dye known as sap green is made from the fruits of the European buckthorn (Rhamnus cathartica). Another pigment known as Chinese green is made from the bark of several Chinese species of buckthorn (Rhamnus globosus and R. utilis). A laxative and tonic known as cascara is made from the bark of the western buckthorn (R. purshiana) of the United States, and from a European species, the alder buckthorn (R. frangula). The wood of some buckthorns may also be carved, for example, as pipe stems.
There are a number of native species of buckthorns in North America. The Carolina buckthorn (Rhamnus caroliniana) occurs widely in the eastern United States. Cascara buckthorn (R. purshiana) occurs relatively broadly in forested areas of the west coast. Western species of more restricted distributions in the southwestern United States include birchleaf buckthorn (R. betulaefolia) and California buckthorn (R. californica).
The European buckthorn (Rhamnus cathartica) and glossy buckthorn (R. frangula) are European species that were introduced to North America through horticulture, and have now become invasive weeds in some areas. These shrubs can form dense stands that exclude native species of shrubs and other plants, and that represent severely degraded ecosystems. The European buckthorn is also an alternate host of oat rust, a fungus that causes an economically important disease of oats (Avena sativa).