Alder buckthorn (Frangula alnus, Mill. 1768)

Identity

Alder buckthorn (Frangula alnus Mill., 1768), also called glossy buckthorn or breaking buckthorn, is a deciduous shrub in the genus Frangula, family Rhamnaceae. Carl Linnaeus first described it in 1753 as Rhamnus frangula. Philip Miller separated it from Rhamnus in 1768 on the basis of its bisexual, five-petaled flowers, contrasting with the separate male and female, four-parted flowers of Rhamnus. ^{1 10}

The Latin verb frangere, meaning to break, gave Frangula its name, referring to the shrub's brittle wood. The epithet alnus recalls its waterside habitat, often shared with alder. Its French name, bourdaine, has an uncertain origin: the Centre national de ressources textuelles et lexicales traces it to an altered form of Old Norman borzaine, while leaving several competing explanations unresolved. ^{10 9}

Recognition

Thornless shoots immediately distinguish this deciduous shrub or small tree from its thorny relative, common buckthorn (Rhamnus cathartica). Unpruned alder buckthorn commonly reaches 2–6 m (7–20 ft), with specimens reported to 7 m (23 ft). Regularly clipped hedges rarely exceed 3 m (10 ft). ^{2 4}

Dark brown to blackish outer bark, scattered with small pale lenticels, conceals lemon-yellow wood just beneath the surface. A shallow cut reveals this contrast, unlike most neighboring hedgerow shrubs, whose cut bark remains pale or brownish. ^{7 10}

Alternate oval leaves measure 3–7 cm (1.2–2.8 in), with smooth, untoothed edges and 7–12 pairs of conspicuously grooved secondary veins. Tiny star-shaped flowers, 3–5 mm (0.12–0.2 in) across, have five greenish-white petals and open in small clusters in the leaf axils from May to July in its temperate Northern Hemisphere range. Berries 6–10 mm (0.24–0.4 in) across change from green through red to purplish black, with several stages often visible on one shoot. ^{7 10}

Useful taxonomy

POWO/WCVP accepts Frangula alnus Mill. The older name Rhamnus frangula L. remains in use, particularly in pharmacognosy, where the dried bark is recorded as Cortex Frangulae. The main synonyms are:

• Rhamnus frangula L. (older scientific name)
• Frangula dodonei Ard. ex Soldano (botanical synonym)
• Frangula vulgaris Hill (botanical synonym)

¹

Three visible garden features separate Rhamnus from the long-confused genus Frangula. Rhamnus has unisexual, four-parted flowers and frequently thorny shoots; Frangula has bisexual, five-parted flowers and consistently smooth shoots. Leaf arrangement also differs: opposite in Rhamnus, alternate in Frangula. ¹⁰

Habitat and range

Across its native range, Frangula alnus occupies almost all Europe, reaching the 68th parallel in Scandinavia, central Siberia and Xinjiang to the east, and northern Morocco, Turkey and the Caucasus to the south. In mainland France it occurs almost everywhere except the strictly Mediterranean region and Corsica, up to 1,000 m (3,280 ft). ^{1 6}

Acidic to neutral soils that stay moist or waterlogged suit alder buckthorn: damp hedges, wooded peatland edges, alder-willow woods and stream or pond margins. It tolerates temporary waterlogging that many hedging shrubs cannot withstand, explaining its regular association with wetlands. ^{7 3}

Nineteenth-century introductions for North American hedges were followed by naturalization and dispersal by birds eating the berries. Alder buckthorn is now considered invasive in the northeastern and midwestern United States, where dense thickets compete with native forest regeneration. ⁴

Life cycle and reproduction

Depending on climate, flowering extends from May to July in its temperate Northern Hemisphere range. Unlike dioecious common buckthorn, alder buckthorn bears bisexual flowers, pollinated by insects, especially bees. Each flower can therefore self-fertilize or receive pollen from a neighboring flower without an opposite-sex plant nearby. ¹⁰

Ripening berries change from green through red to purplish black in late summer and fall; each usually contains two or three brown seeds. Birds eating the fruit account for most seed dispersal, including well beyond the species' native range. ¹⁰

Garden propagation uses seed, cold-stratified in fall, or cuttings of semi-ripe shoots in summer. Cuttings reproduce the parent faithfully and root faster. Seeds germinate more slowly but produce genetically varied plants that are generally vigorous in moist soil. ³

Uses and risks

For centuries, alder buckthorn bark has served as a mild medicinal laxative after drying and aging for at least one year. Fresh bark is violently purgative and can cause severe abdominal pain. Its 3–7% anthraquinone content stimulates the colon wall roughly 8–12 hours after ingestion. ³

Unripe berries remain toxic if eaten by humans or dogs, causing vomiting and digestive upset. Charcoal from the wood was historically prized for gunpowder because it burns quickly and evenly. ^{2 10}

In gardens, alder buckthorn provides rural hedging for damp ground and feeds caterpillars of the brimstone butterfly (Gonepteryx rhamni), for which it is one of very few European host plants. ⁴

Varieties, look-alikes and related plants

Ornamental selections include 'Asplenifolia', with finely divided, fernlike foliage unrelated to variegation. 'Columnaris', known as tallhedge buckthorn, has a narrow upright habit suited to compact screening hedges. 'Fine Line', also sold as 'Ron Williams', is a cross between those two, combining columnar growth with divided foliage. ⁸

Common buckthorn, Rhamnus cathartica, is the closest source of confusion. It has thorny shoots and opposite, finely toothed leaves with veins curved toward the tip; alder buckthorn has thornless shoots and alternate, entire leaves. Common buckthorn's separate male and female four-petaled flowers complete the field distinctions. Both shrubs nevertheless belong to the same family, have similar purgative properties and bear berries that turn black, helping explain their historically confused common names. ¹⁰

Main sources

• Plants of the World Online / Kew, Frangula alnus Mill.
• Royal Horticultural Society, Frangula alnus (alder buckthorn)
• Plants For A Future, Frangula alnus
• NC State Extension, Frangula alnus
• IUCN Red List, Frangula alnus assessment
• INPN, Frangula alnus Mill., 1768 (Bourdaine)
• Info Flora, Frangula alnus Mill.
• Missouri Botanical Garden Plant Finder, Frangula alnus 'Asplenifolia'
• CNRTL, etymology of bourdaine
• Wikipedia (EN), Frangula alnus