Buckthorn
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. Buckthorns are used as food plants by the larvae of some Lepidoptera species  see list of Lepidoptera which feed on Buckthorns.

# 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  Italian Buckthorn
Rhamnus alnifolia  Alderleaf Buckthorn
Rhamnus arguta  Sharp-tooth Buckthorn
Rhamnus cathartica  Common (or Purging) Buckthorn (syn. R. catharticus)
Rhamnus crocea  Redberry Buckthorn (ssp. crocea), Hollyleaf Buckthorn (ssp. pilosa)
Rhamnus davurica  Dahurian Buckthorn
Rhamnus diffusus
Rhamnus globosa  Lokao Buckthorn
Rhamnus ilicifolia  Hollyleaf Redberry
Rhamnus japonica  Japanese Buckthorn
Rhamnus lanceolata  Lanceleaf Buckthorn
Rhamnus libanotica
Rhamnus lycioides
Rhamnus petiolaris
Rhamnus pirifolia  Island Redberry Buckthorn
Rhamnus prinoides  Shiny-leaf Buckthorn
Rhamnus saxatilis  Rock Buckthorn, Avignon Buckthorn, Avignon Berry (syn. R. infectoria, R. infectorius)
Rhamnus serrata  Sawleaf Buckthorn
Rhamnus smithii  Smith's Buckthorn
Rhamnus staddo  Staddo (syn. R. rhodesicus)
Rhamnus tinctoria - Dyer's Buckthorn (syn. R. saxatilis ssp. tinctorius)
Rhamnus utilis  Chinese Buckthorn
- Rhamnus alaternus  Italian Buckthorn
- Rhamnus alnifolia  Alderleaf Buckthorn
- Rhamnus arguta  Sharp-tooth Buckthorn
- Rhamnus cathartica  Common (or Purging) Buckthorn (syn. R. catharticus)
- Rhamnus crocea  Redberry Buckthorn (ssp. crocea), Hollyleaf Buckthorn (ssp. pilosa)
- Rhamnus davurica  Dahurian Buckthorn
- Rhamnus diffusus
- Rhamnus globosa  Lokao Buckthorn
- Rhamnus ilicifolia  Hollyleaf Redberry
- Rhamnus japonica  Japanese Buckthorn
- Rhamnus lanceolata  Lanceleaf Buckthorn
- Rhamnus libanotica
- Rhamnus lycioides
- Rhamnus petiolaris
- Rhamnus pirifolia  Island Redberry Buckthorn
- Rhamnus prinoides  Shiny-leaf Buckthorn
- Rhamnus saxatilis  Rock Buckthorn, Avignon Buckthorn, Avignon Berry (syn. R. infectoria, R. infectorius)
- Rhamnus serrata  Sawleaf Buckthorn
- Rhamnus smithii  Smith's Buckthorn
- Rhamnus staddo  Staddo (syn. R. rhodesicus)
- Rhamnus tinctoria - Dyer's Buckthorn (syn. R. saxatilis ssp. tinctorius)
- 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, Coffeeberry
Rhamnus caroliniana (Frangula caroliniana)  Carolina Buckthorn, Indian Cherry (syn. R. carolinianus)
Rhamnus frangula (Frangula alnus)  Alder Buckthorn, Glossy or Breaking Buckthorn, Black Dogwood
Rhamnus glandulosa - Sanguinho
Rhamnus latifolia (Frangula azorica)
Rhamnus purshiana (Frangula purshiana)  Cascara Buckthorn (syn. R. purshianus)
Rhamnus rubra (Frangula rubra)  Red Buckthorn
Rhamnus sphaerosperma (Frangula sphaerosperma)  West Indian Buckthorn
- Rhamnus betulaefolia (Frangula betulifolia)  Birchleaf Buckthorn
- Rhamnus californica (Frangula californica)  California Buckthorn, Coffeeberry
- Rhamnus caroliniana (Frangula caroliniana)  Carolina Buckthorn, Indian Cherry (syn. R. carolinianus)
- Rhamnus frangula (Frangula alnus)  Alder Buckthorn, Glossy or Breaking Buckthorn, Black Dogwood
- Rhamnus glandulosa - Sanguinho
- Rhamnus latifolia (Frangula azorica)
- Rhamnus purshiana (Frangula purshiana)  Cascara Buckthorn (syn. R. purshianus)
- 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 the 15th to 19th centuries, as its wood provided the best quality charcoal for gunpowder manufacture.
Italian 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. saxatilis) 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.
North American species include Alder-leaf Buckthorn (R. alnifolia) right across the continent, Carolina Buckthorn (R. (F.) caroliniana) in the east, Cascara Buckthorn (R. (F.) purshiana) in the west, 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.