The Buckthorns Rhamnus are a genus (or two genera, if Frangula is treated as distinct) of about 100 species of shrubs or small trees from 1-10 m tall (rarely to 15 m), in the buckthorn family Rhamnaceae. They are native throughout the temperate and subtropical Northern Hemisphere, and also more locally in the subtropical Southern Hemisphere in parts of Africa and South America.

Both deciduous and evergreen species occur. The leaves are simple, 3-15 cm long, and arranged either alternately or in opposite pairs. One semi-unique characteristic of many buckthorns is the way the veination curves upward towards the tip of the leaf. The plant bears fruits which are dark blue berries. The name comes from the fact that there is a woody spine on the end of each twig in many species.

Classification
The genus is divided into two subgenera, sometimes treated as separate genera:

• Subgenus Rhamnus: flowers with four petals, buds with bud scales, leaves opposite or alternate, branches with spines
  • Rhamnus alaternus
  • Rhamnus alnifolia
  • Rhamnus cathartica
  • Rhamnus crocea
  • Rhamnus infectoria
  • Rhamnus tinctoria
  • Rhamnus utilis
• Subgenus Frangula: flowers with five petals, buds without bud scales, leaves always alternate, branches without spines
  • Rhamnus californica (Frangula californica)
  • Rhamnus caroliniana (Frangula caroliniana)
  • Rhamnus frangula (Frangula alnus)
  • Rhamnus glandulosa
  • Rhamnus latifolia (Frangula azorica)
  • Rhamnus purshiana (Frangula purshiana)

Purging Buckthorn Rhamnus cathartica

The Purging Buckthorn or Common Buckthorn (R. cathartica) is a widespread European native species, in the past used as a purgative, though its toxicity makes this a very risky herbal medicine and it is no longer used. Introduced into the United States as a garden shrub, this has become an invasive species in many areas there. It has recently been discovered to be a primary host of the soybean aphid Aphis glycines, a problem pest for soybean farmers across the US. The aphids use the buckthorn as a host for the winter and then spread to nearby soybean field in the spring.

Another European species, Alder Buckthorn (R. frangula, syn. Frangula alnus) was of major military importance in the 15th-19th centuries, as its wood provided the best quality charcoal for gunpowder manufacture.

Other notable European species include Alaternus Buckthorn (R. alaternus), an evergreen species from the Mediterranean region, and Dyer's Buckthorn (R. tinctoria). This last species is used, together with the Asian Chinese Buckthorn (R. utilis), to produce the dye "china green". Another species, Avignon Buckthorn (R. infectoria) provides the yellow dye Persian berry, made from the berries.

Sanguinho (R. glandulosa) is endemic to the Macaronesian islands, where it is found in the laurisilva forests of the Madeira and Canary Islands.

The Cascara Buckthorn (R. purshiana, syn. Frangula purshiana), native to the western United States, was also used as a purgative by Native American tribes and early Spanish colonists (under the name cascara sagrada, Spanish for "sacred bark", referring to the bark extract consumed). It is the largest species of buckthorn, reaching 15 m tall on occasion.

Other American native species include Alder-leaf Buckthorn (R. alnifolia) right across the continent, Carolina Buckthorn (R. (F.) caroliniana) in the east, and the evergreen California Buckthorn or Coffeeberry (R. (F.) californica) and Hollyleaf Buckthorn (R. crocea) in the west.

Buckthorns may be confused with Dogwoods, which share the curved leaf venation, but the two plants are easy to tell apart because if a leaf is slowly pulled apart, in dogwood there are thin white latex strings. In the buckthorn these latex strings are not present.