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. Buckthorns are used as food plants by the larvae of some Lepidoptera species - see list of Lepidoptera which feed on Buckthorns.
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 : Italian Buckthorn
Rhamnus alnifolia : Alderleaf Buckthorn
Rhamnus arguta : Sharp-tooth Buckthorn
Rhamnus cathartica : Common Buckthorn
Rhamnus crocea : Redberry Buckthorn
Rhamnus davurica : Dahurian Buckthorn
Rhamnus globosa : Lokao
Rhamnus ilicifolia : Hollyleaf Redberry
Rhamnus japonica : Japanese Buckthorn
Rhamnus lanceolata : Lanceleaf Buckthorn
Rhamnus pirifolia : Island Redberry
Rhamnus saxatilis : Rock Buckthorn
Rhamnus serrata : Sawleaf Buckthorn
Rhamnus smithii : Smith's Buckthorn
Rhamnus utilis : Chinese Buckthorn
Subgenus Frangula: flowers with five petals, buds without bud scales, leaves always alternate, branches without spines
Rhamnus betulaefolia (Frangula betulifolia) : Birchleaf Buckthorn
Rhamnus californica (Frangula californica) : California Buckthorn
Rhamnus caroliniana (Frangula caroliniana) : Carolina Buckthorn
Rhamnus frangula (Frangula alnus)
Rhamnus latifolia (Frangula azorica)
Rhamnus purshiana (Frangula purshiana): Cascara Buckthorn
Rhamnus rubra (Frangula rubra): Red Buckthorn
Rhamnus sphaerosperma (Frangula sphaerosperma): West Indian Buckthorn
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 fields in the spring.
Another European species, Alder Buckthorn (R. frangula, syn. Frangula alnus) was of major military importance in
Alaternus Buckthorn (R. alaternus), an evergreen species from the Mediterranean region, has become a serious weed in some parts of New Zealandespecially on Hauraki Gulf islands.
Dyer's Buckthorn (R. tinctoria) 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 North American 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.
In South America, Rhamnus diffusus is a small shrub native from the Valdivian temperate rain forests in Chile.
Buckthorns may be confused with Dogwoods, which share the curved leaf venation; indeed, "dogwood" is a local name for R. prinoides in southern Africa, a plant used to make Ethiopian mead and known as "gesho" in Ethiopia. The two plants are easy to distinguish by slowly pulling a leaf apart; in dogwood thin white latex strings can be seen, strings not present in buckthorn.
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