Rhamnus, buckthorn, alder buckthorn: planting, pruning and care

Description and botany

Genus Rhamnus includes nearly 125 species of shrubs or small trees. It now incorporates genus Frangula (with naked buds without scales, flowers with 4 sepals and bisexual flowers), which notably designated bourdaine (Frangula alnus) also known as Rhamnus frangula. Buds can be naked (former genus Frangula) or scaly in true Rhamnus. Vernacular names such as purging buckthorn, alaternus buckthorn or simply alaternus are also applied to other main native species. These shrubs occupy temperate or subtropical regions of northern hemisphere, mainly eastern Asia, North America and China. Species cultivated locally belong to temperate and southern zones of Europe and North Africa and are reputed to tolerate drought and sea spray. The genus gave its name to family Rhamnaceae, which notably includes Céanothes and red date trees.

Bark is smooth, dotted with numerous white lenticels on a very dark background in alder buckthorn. Purging buckthorn (Rhamnus cathartica) bears dwarf branches ending in a thorn. Leaves are simple, oval and dentate, and may be positioned alternately (R. alaternus, R. frangula), more or less opposite (R. catharticus) or even fasciculate (clustered) on short shoots. They are sometimes coriaceous, persistent and with more or less cartilaginous toothed margins in alaternus but, more often,

Rhamnus frangula – botanical illustration

deciduous as in alder buckthorn and purging buckthorn. Prominent veins curve to follow leaf margin. Foliage with short petiole measures between 1 and 8 cm long. It is exceptionally finely divided and undulate in cultivars such as Rhamnus frangula ‘Asplenifolia’ nicknamed fern-bourdaine and Fine Line ®. Habit is compact and dense in alaternus, rather slender and open in alder buckthorn, columnar in cultivar Rhamnus frangula Fine Line ®. Young shoots are often tinged violet in alaternus. Leaves turn red in autumn in alder buckthorn and yellow in purging buckthorn, and remain dark green in alaternus.

The yellowish-green or white flowers are small, rather inconspicuous but very fragrant, attracting many pollinators between March and June depending on species. Flowers are solitary or grouped in small clusters and are often unisexual on dioecious plants (entirely male or female specimens), notably in alaternus. Calyx is cup-shaped or campanulate, with 4 or 5 sepals and corolla with 4 or 5 petals, absent in alaternus. Flowers contain 4 or 5 stamens the size of the petals or shorter, or pistils ending in 2 or 3 points. Fruits, from green through red to black, are drupes that resemble berries about 5 mm across because they contain several stones with or without a furrow. Fruiting extends throughout summer in alder buckthorn, giving a very decorative appearance of small green, red and black spheres.

Foliage of buckthorns: Rhamnus alaternus ‘Argenteovariegata’ (Photo Gartenknorze), Rhamnus frangula ‘Asplenifolia’ (photo Jonathan Landsman).

Main varieties of Rhamnus

Native species in France are found in temperate zones, on calcareous soil such as purging buckthorn or more often in damp heathland, woods or marshes like alder buckthorn, or on Mediterranean calcareous slopes even exposed to sea spray such as alaternus buckthorn. Rhamnus generally tolerate all these situations, ranging from dry understorey to wind-blown shores and rocky banks subject to drought. To introduce these shrubs into gardens, horticulturists have worked to select more ornamental forms with cream-variegated or laciniate foliage, and columnar habit so they can decorate terraces whatever the exposure and climate.

Rhamnus alaternus Argenteovariegata - Italian Buckthorn

An evergreen shrub both charming and very undemanding. Both dense and well-branched, with beautiful grey-green foliage edged with white, it quickly forms a bright mass, very luminous in semi-shaded areas of the garden, even in understorey in warm, dry climates. This all-terrain shrub is perfectly resistant to drought and sea spray, and laughs at stony, calcareous or clay soils. Its only weak point is limited hardiness, down to -10/-12°C in well-drained soil.

• Flowering time April, May
• Height at maturity 3 m

See the full description

Rhamnus frangula Fine Line - Alder Buckthorn

A variety of alder buckthorn whose particularly light foliage provides an elegant upright habit that gives a striking graphic effect to this shrub derived from a local species (hardy to -40°C).

• Flowering time June, July
• Height at maturity 2,50 m

See the full description

Rhamnus frangula Aspleniifolia - Alder Buckthorn

Better known as fern-leaved alder buckthorn or laciniate-leaved alder buckthorn, this shrub is perfectly suited to our soils and climates since it is native to our regions (hardy to -40°C). Very fine leaves in dense clusters give a delicate lightness to this rounded-habit shrub. In autumn they take on lovely golden tones. A highly dissected silhouette for a handsome Japonising effect without demanding site conditions.

• Flowering time June, July
• Height at maturity 2,50 m

See the full description

Planting

Where to plant Rhamnus?

Plant Rhamnus in ordinary soil, well worked and well drained. Italian buckthorn prefers rather calcareous soils, but is very undemanding and adapts to poor, clay‑limestone and stony soils. Alder buckthorn grows in sandy soils, rather neutral to acidic, tolerating conditions from marshy to dry.

Summer drought and high temperatures are not a problem once these shrubs are well established. Plant Italian buckthorn in well‑drained soil and sheltered from cold winds to ensure frost tolerance of -12 to -15°C. All tolerate sea spray well and also thrive in oceanic climates. In urban areas, they cope well with air pollution.

Plant them in full sun or, if necessary, in partial shade. Italian buckthorn even tolerates a shaded position in dry, warm climates.

When to plant?

Plant Rhamnus preferably in autumn to ensure deep rooting before summer drought. In cooler, wetter climates, planting in spring is also fine. Container‑grown shrubs can be planted at any time, but avoid periods of frost or severe drought.

How to plant?

For planting a Rhamnus :

1. Immerse the root ball in a bucket of water to soak it thoroughly.
2. Dig a planting hole, 50 cm in all directions or a trench for a hedge.
3. Add a 10 cm drainage layer (gravel, sand… ) if your soil is heavy clay.
4. Add well‑rotted manure or compost, or potting compost if soil is poor.
5. Place the plant in the planting hole so the top of the root ball is level with the soil.
6. Backfill with soil and firm lightly.
7. Water.
8. Spread a layer of gravel around the base to help keep the area around the roots cool. This will also reduce weed growth and improve drainage.

Establishment is easy and quick and requires one or two generous waterings per week.

Rhamnus frangula ‘Fine Line’ is suitable for container growing.

Care and pruning

Water only twice a month from third year onwards, and only during drought. It is a plant that requires very little maintenance and grows without difficulty once conditions are suitable. Fertiliser inputs are not essential (add just a little hornmeal to bottom of planting hole), but can be useful in very poor soil.

Pruning is not essential. You can cut back stems slightly after flowering to encourage plant to branch. Avoid severe pruning. On variegated forms, if you notice stems bearing green leaves, remove them.

Container culture:

Use a light substrate, mix of garden soil, coarse sand and potting compost. In summer, water only when soil is dry, thoroughly, but by spacing waterings. Plant can be overwintered in a cool, bright, barely or unheated place, sheltered from severe frosts. Apply a little slow-release fertiliser in spring and autumn.

Multiplication

Multiply Rhamnus by sowing, layering or by propagation by cuttings.

Propagation by cuttings

• Take tip cuttings from herbaceous shoots in spring or from semi-ripe shoots in late summer and stick them into a mixture of sand and potting compost.
• Overwinter under a cold frame.
• Plant out in the open ground in the following spring or autumn.

Layering

• Choose a long flexible twig, bury its central part and hold it in place with a peg in the soil.
• Slightly scrape the bark of the buried section to encourage rootlet growth.
• Wait 1 or 2 years before separating the layer from the parent plant.

Sowing

• Harvest buckthorn fruits as soon as they turn black in late summer to autumn.
• Separate the seeds from the pulp and rinse them.
• Sow them in a pot filled with sand placed outdoors, against a north-facing wall.
• Germination occurs the following spring or the year after. Plant out seedlings aged 6 months directly in their final position.

Uses and associations

Alder buckthorn is typical of fairly damp to marshy woodland understoreys, on deep, acidic, sandy-loam or clay soils, but it also grows on drier ground regardless of pH. It is a pioneer species on sandy and semi-peaty soils where it can form a useful windbreak.

Les formes horticoles comme Rhamnus frangula Fine Line et ‘Asplenifolia’ livrent un feuillage lacinié très original qui prend une jolie teinte dorée en automne. Elles peuvent s’installer dans un grand massif en dessous d’un arbre plus grand pour habiller élégamment le tronc de celui-ci. Leur silhouette très découpée offre un bel effet japonisant sans les contraintes de terrain habituelles de l’Erable du Japon qui requiert un sol acide.  Le port colonnaire de Fine Line lui donne fière allure dans un grand pot accompagné de vivaces basses ou simplement dans une haie libre ou un grand massif arbustif en compagnie de Physocarpus, de troènes, de fusains, amélanchiers, Elaeagnus commutata Zempin, lauriers tins…  De plus, les baies de ces arbustes attireront dans votre jardin les oiseaux qui en sont friands.

An example of a planting scheme: Euonymus alatus ‘Compactus’, Osmanthus heterophyllus ‘Variegata’, Osmanthus burkwoodii, Mahonia ‘Lionel Fortescue’ (Mahonia ‘Winter Sun’ is a similar variety with long soft yellow inflorescences), Rhamnus alaternus ‘Argenteovariegata’.

With Italian buckthorn, only female plants produce berries. The white-variegated foliage form of Rhamnus alaternus ‘Argenteovariegata’ is one of the loveliest shrubs for dry gardens in shaded spots. Both elegant, bright and dense, attractive all year and slow-growing, it is perfectly suited to areas that are particularly difficult to plant, such as root-strewn understoreys and coastal gardens exposed to sea spray. This variegated Rhamnus, a truly all-terrain shrub, is widely planted by landscape professionals in southern France, as well as in Portugal and Spain. Forming a bright silvery mass, with good hardiness down to -10°/-12°C in well-drained soil and perfectly self-sufficient once established, the variegated buckthorn makes a fine specimen to plant in a hedge or a border of easy-care shrubs in a mild climate, or even as a specimen plant, for example at the corner of a small tree clump. It is, together with Filaria, Pistacia lentiscus, laurustinus, Myrtus tarentina ‘Variegata’ and arbutus, one of those indispensable shrubs for creating the evergreen structure of a dry garden, even among the roots of oaks or pines.

An idea for a combination playing on leaf shapes and colours: Rhamnus frangula ‘Aspenifolia’ accompanied by golden foliage (Sambucus nigra ‘Aurea’ for example) and autumnal colours (Euonymus alatus, Corylopsis pauciflora, Cornus, etc).

Did you know?

Berries of alder buckthorn (Rhamnus frangula) and purging buckthorn (Rhamnus cathartica) have a purgative effect and are used in the manufacture of laxative medicines. However, consuming them raw is not recommended because effects can sometimes be severe.

Alder buckthorn honey is renowned for its delicate aromas and a fruity, balsamic and very subtly flavoured taste.

Flexible shoots of alder buckthorn were used in basketry, notably to make beehives, and its charcoal was used to produce gunpowder, hence its nickname ‘powder wood’. Alder buckthorn wood is used in cabinetmaking.

Fruits and shoots of purging buckthorn were harvested, notably in the Comtat Venaissin (Carpentras), to obtain yellow, yellow-green and brown dyes, such as bladder-green used in the fine arts or to dye fabrics. The Brimstone butterfly and the Buckthorn Azure are among buckthorn’s hosts, so the shrub is essential to their survival.

The species cathartica, introduced to North America, has become invasive in Quebec. Alder buckthorn has also naturalized in the region.