European Mountain Ash (Sorbus aucuparia)

Sorbus aucuparia, also known as european mountain-ash, is a deciduous tree native to Europe. It grows a slim trunk with upward-curving branches as it matures. Its admirable upright and dense branching gives the plant an oval crown. The juicy and soft clustered fruits are an excellent source of food for birds, while its yellow to plum foliage adds to its overall aesthetic appeal.

I. Appearance and Characteristics 

Sorbus aucuparia, commonly called rowan and mountain-ash, is a species of deciduous tree or shrub in the rose family. It is a highly variable species, and botanists have used different definitions of the species to include or exclude trees native to certain areas. A recent definition includes trees native to most of Europe and parts of Asia, as well as northern Africa. The range extends from Madeira, the British Isles and Iceland to Russia and northern China. Unlike many plants with similar distributions, it is not native to Japan.

The common name mountain ash dates from the 16th century. It was first used by John Gerard in 1597, translating it directly from the then botanists’ Latin Montana fraxinus

S. aucuparia was previously categorized as Pyrus aucuparia.

Sorbus aucuparia is a small tree or shrub that grows up to between about 5 and 15 metres (16 and 49 feet) in height. The crown is loose and roundish or irregularly shaped but wide and the plant often grows multiple trunks. A trunk is slender and cylindrical and reaches up to 40 centimetres (15+1⁄2 inches) in diameter, and the branches stick out and are slanted upwards. The bark of a young plant is yellowish gray and gleaming and becomes gray-black with lengthwise cracks in advanced age; it descales in small flakes. Lenticles in the bark are elongated and colored a bright ocher.

The plant does not often grow older than 80 years and is one of the shortest-lived trees in temperate climates. The wood has a wide reddish white sapwood and a light brown to reddish brown heartwood. It is diffuse-porous, flexible, elastic, and tough, but not durable, with a density of 600 to 700 kilograms per cubic metre (1,000 to 1,200 pounds per cubic yard) in a dried state. The roots grow wide and deep, and the plant is capable of root sprouting and can regenerate after coppicing.

The compound leaves are pinnate with 4 to 9 pairs of leaflets on either side of a terete central vein and with a terminal leaflet. The leaves are up to 20 cm (8 in) long, 8 to 12 cm (3 to 4+1⁄2 in) wide. They have paired leaf-like stipules at the base of the petiole, and are arranged alternately along a branch, distinguishing them from those of ash, Fraxinus excelsior, which are opposite and without stipules. The leaflets are elongated-lanceolate in shape, 2 to 6 cm (3⁄4 to 2+1⁄4 in) long, and 1 to 2.5 cm (1⁄2 to 1 in) wide with a sharply serrated edge, and have short stems or sit close to the central vein except for the outermost leaflet. Leaflets are covered in gray-silvery hairs after sprouting but become mostly bare after they unfold. Their upper side is dark green and their underside is a grayish green and felted. Young leaflets smell like marzipan when brayed. The leaflets are asymmetrical at the bottom. The foliage grows in May and turns yellow in autumn or a dark red in dry locations.

The buds are often longer than 1 cm (3⁄8 in) and have flossy to felted hairs. These hairs, which disappear over time, cover dark brown to black bud scales. The terminal buds are oval and pointed and larger than axillary buds, which are narrow, oval and pointed, close to the twig, and often curved towards it.

The species is monoecious. It reaches maturity at age 10 and carries ample fruit almost every year. The plant flowers from May to June (on occasion again in September) in many yellowish white combs that contain about 250 flowers. The combs are large, upright, and bulging. The flowers are between 8 and 10 millimetres (3⁄8 and 3⁄8 in) in diameter and have five small, yellowish green, and triangular sepals that are covered in hairs or bare. The five round or oval petals are yellowish white and the flower has up to 25 stamens fused with the corolla to form a hypanthium and an ovary with two to five styles; the style is fused with the receptacle. The flowers have an unpleasant trimethylamine smell. Their nectar is high in fructose and glucose.

Its berries are round pomes between 8 and 10 mm (5⁄16 and 3⁄8 in) in diameter that ripen from August to October. The fruit are green before they ripen and then typically turn to orange or scarlet in color. The sepals persist as a black, five-pointed star on the ripe fruit. A corymb carries 80 to 100 pomes. A pome contains a star-shaped ovary with two to five locules each containing one or two flat, narrow, and pointed reddish seeds. The flesh of the fruit contains carotenoids, citric acid, malic acid, parasorbic acid, pectin, provitamin A, sorbitol, tannin, and vitamin C. The seeds contain glycoside.

The species has a chromosome number of 2n=34.

The species is pollinated by bees and flies. Its seeds are not digested by birds and are thus propagated by being passed intact in their droppings. The fruit are eaten by about 60 bird species and several mammals. They are liked particularly by thrushes and other songbirds, and are also eaten by cloven-hoofed game, red fox, European badger, dormouse, and squirrel. The fruit are eaten by migratory birds in winter, including Bohemian waxwing, spotted nutcracker, and redwing. Cloven-hoofed game also excessively browse foliage and bark. The plant roots can be found in symbiosis with arbuscular mycorrhizal and less commonly with ectomycorrhizal fungi.

It is usually later superseded by larger forest trees. In Central Europe it often grows in association with red elderberry, goat willow, Eurasian aspen, and silver birch. The plant is highly flammable and tends not to accumulate plant litter.

II. How to Grow and Care

Sunlight

This tree requires full sun at least six to eight hours per day. Some partial sun is better than scorching sun, which can make the tree decline.

Temperature and Humidity

It will not grow well south of USDA Zones 5 and 6. As the common name suggests, this tree prefers cool mountain climates and dislikes hot and humid summers.

Watering

The tree needs a medium amount of water to keep the soil moist. However, it may not need any supplemental water if there is normal rain. If there is not an average rainfall, it can be given 1 inch of water every week.

Soil

Perform a soil test to ensure the soil is acidic, in the range of 4.5 to 6.0, which this tree prefers. You can add amendments to acidify the soil if the soil pH is too high. This tree does best in moist, well-drained soils. It will not thrive in compact or salty soils.

Fertilizing

This tree does not need any fertilizer if the soil is rich, moist, or it’s amended with compost. The tree may benefit from a feeding if the soil is extremely poor, which would mean it’s dry, crumbling, cracked, and compact without moisture or nutrients.

Pruning

Fire blight can be spread by pruning if the conditions are right (very warm). Prune this tree at winter’s end during the tree’s dormant period.

Propagation

Propagate this tree with semi-hardwood cuttings. Cuttings will only work with this tree if it’s young. A cutting from an older European mountain ash tree may not reproduce well. If you have a newer tree and would like to grab a cutting, do so in the spring when there’s new growth so you directly plant the cutting. Here’s how:

• Find a branch with new growth but one that is somewhat woody, but that is not old or brown.
• With a sterile cutting tool, cut a 6 to 8-inch long piece right below a node. Remove all the leaves on the cutting but leave the pair on top of the stem. (Cut large leaves in half if they are in the way.)
• Create a couple of vertical wounds on the bottom of the cutting, about 1 inch long. Dip the wounded end in rooting hormone.
• Place the cutting directly into the prepared ground of peat moss, sand, perlite, and water.
• Keep the ground moist (not soggy) and roots should take hold in four to six weeks if there is enough sun to keep it warm.

How to Grow From Seed

It’s very easy to propagate a European mountain ash tree from the seeds of the fruit. The seeds will need to go through the stratification process. Here’s how to grow this tree from seed:

• Harvest the berries in August or early September to extract the seeds.
• Stratify the seeds by putting them in a plastic bag and into a freezer for 90 days. Thaw them to room temperature and put into your refrigerator for 60 days.
• Plant seeds in a small pot no more than 1 inch deep in potting soil.
• Keep soil evenly moist, but not soggy.
• If necessary, repot into larger containers with a peat moss, compost, and sand mixture until the seedlings are ready to be placed in their permanent outdoor spot in the spring.

Pests and Diseases

Common Pests and Plant Diseases

European mountain ash trees can attract several pests that can cause problems, including:

• Aphids: This pest sucks sap from young leaves and weakens the plant, but they can be knocked off with a blast of water or insecticidal soap.
• Apple fruit moth (Argyresthia conjugella): Larvae bores into the berries but good air circulation can help the problem.
• Blister mites: Pests produce small galls or blisters on foliage and fruit, often attacking a neglected tree.
• Mountain-ash sawfly larvae (Pristiphora geniculata): Devours foliage and can defoliate the tree but specific pesticides control the problem.
• Oystershell scale (Lepidosaphes ulmi): This pest attacks stressed trees and cause branch dieback. Insecticides or organic options like horticultural oil or insecticidal soap will help.

In addition to pests, this tree can be afflicted with various bacterial and fungal diseases, including:

• Apple scab (Venturia inaequalis): This fungal disease occurs in high humidity and rainfall, affecting foliage and fruit with olive-green blotches.
• Botryosphaeria (Botryosphaeria dothidea): Fungal problem that causes canker and branch dieback. Cultural control, disposal of dead material, and fungicides help.2
• Cedar-quince rust (Gymnosporangium clavipes): This fungal problem attacks plants in the rose family, causing flaky branch coverings and infected fruits. Remove infected portions of the tree and apply fungicides at the correct time.3
• Cytospora canker (Cytospora spp.): Though this airborne fungal disease is more prevalent in spruce trees, it can also get into wounds of the European mountain ash tree and cause cankers and branch dieback in the canopy. There is no cure except to manage the disease by pruning out dead parts of the tree.4
• Bacterial fire blight: (caused by Erwinia amylovora): This condition happens when the tree is growing in conditions that are too hot for the plant. Prune affected branches right away.
• Honey fungus: Shows as white growth between bark and wood at the ground, sometimes honey-colored mushrooms accompany the problem. There is no control or prevention for this deadly disease that spreads underground.5
• Powdery mildew: This common fungal issue that deposits dusty particles on foliage occurs in high humidity and is treated with fungicides.

Common Problems 

This tree can be very tough and enduring in spite of all the pests and diseases that may be problematic if it’s not planted in the cool conditions it prefers. You’ll also find deer grazing on the tree’s leaves. But it’s best to be on the lookout for some signs that can indicate the tree may need some help.

Brown/Black Leaves

The tree may have leaf spots, which show up as brown spots on leaves that develop black spores and leaf drop. Fire blight shows as black leaves and browning flower clusters.

Splitting Bark

Usually sloughing or splitting bark is a normal regenerative process for trees. Excessive splitting may be the result of too many crossed branches stressing the bark. However, for a European mountain ash, it’s best to peel away some of the bark and take a whiff. If there is a mushroom-like smell, the tree may be afflicted with a “honey” fungus.

No Flowers or Berries

The tree could be waterlogged or sitting in very compact or salty soil.

Blue Growth on Branches

This is likely lichen (composite fungal and algae-like organisms that grow on trees to form a symbiotic relationship), which tends to needlessly worry tree owners. But this is only a surface issue and indicates that the air around the tree is healthy.

III. Uses and Benefits 

• Ornamental uses

It is also used as an ornamental plant in parks, gardens, or as an avenue tree. Ornamental cultivars include ‘Asplenifolia’, which has divided and sharply serrated leaflets, ‘Blackhawk’, which has large fruit and dark green foliage, ‘Fastigiata’, which has an upright columnar form, ‘Fructu Luteo’, which has orange yellow fruit, ‘Michred’, which has brilliant red fruit, ‘Pendula’, which is a weeping tree, and ‘Xanthocarpa’, which has orange yellow fruit.

‘Sheerwater Seedling’, an upright and slender cultivar, and ‘Wisley Gold’ with yellow fruits, have received the Royal Horticultural Society’s Award of Garden Merit.

Cultivars are vegetatively propagated via cuttings, grafting, or shield budding.

• Culinary and Medicinal uses

The fruit of S. aucuparia were used in the past to lure and catch birds. To humans, the fruit are bitter, astringent, laxative, diuretic and a cholagogue. They have vitamin C, so they prevent scurvy, but the parasorbic acid irritates the gastric mucosa. Pharmacist Mannfried Pahlow wrote that he doubted the toxicity of the fruit but advised against consuming large amounts. The fruit contains sorbitol, which can be used as a sugar substitute by diabetics, but its production is no longer relevant. Sorbus aucuparia fruits have been used in the traditional Austrian medicine internally (as tea, syrup, jelly or liqueur) for treatment of disorders of the respiratory tract, fever, infections, colds, flu, rheumatism and gout.

Fresh fruit are usually not tasty, but they can be made into compote, jelly, jam, a tangy syrup, a tart chutney, or juice, as well as wine and liqueur, or used for tea or to make flour. Fruit are served as a side dish to lamb or game. Debittering can be accomplished by freezing, cooking, or drying, which degrades the parasorbic acid. The fruit are red colored in August but usually only harvested in October after the first frost by cutting the corn. The robust qualities of S. aucuparia make it a source for fruit in harsh mountain climate and Maria Theresa, ruler of the Habsburg monarchy, recommended the planting of the species in 1779. The oldest Finnish candy still commercially produced, Pihlaja, is named after and originally contained rowanberries.

A more palatable variety, named Sorbus aucuparia var. dulcis Kraetzl, or var. edulis Dieck, or var. moravica Dippel, was first discovered in 1810 near Ostružná in the Hrubý Jeseník mountain range of Northern Moravia and became widespread in Germany and Austria in the early 20th century. Its leaves are larger and pointed, only the front part of the leaflets is serrated, and they have darker bark, larger buds and larger fruit. Similar non-bitter varieties found in Southern Russia were first introduced in Central Europe in 1900 as ‘Rossica’ and ‘Rossica Major’, which have large fruit up to 1.5 cm (5⁄8 in) in diameter.

Two widespread cultivars of the Moravian variety are ‘Konzentra’ and ‘Rosina’, which were selected beginning in 1946 by the Institut für Gartenbau Dresden-Pillnitz, an agricultural research institute in Saxony, from 75 specimens found mostly in the Ore Mountains, and made available in 1954. Fruit of the more widely used ‘Konzentra’ are small to medium-sized, mildly aromatic and tart, easier to transport because of their thicker peel, and used for juicing, while fruit of ‘Rosina’ are larger, sweet and tart, and aromatic, and candied or used in compote. The two cultivars are self-pollinating, yield fruit early, and the sugar content increases while the acid content decreases as the fruit ripen. ‘Beissneri’ is a cultivar with reddish foliage and bark and serrated leaves. Other edible varieties originate in and are named after Klosterneuburg, Lower Austria.

Russian botanist Ivan Vladimirovich Michurin began in 1905 to crossbreed common S. aucuparia with other species to create fruit trees. His experiments resulted in the cultivars ‘Burka’, ‘Likjornaja’, ‘Dessertnaja’, ‘Granatnaja’, ‘Rubinovaja’, and ‘Titan’. Other S. aucuparia hybrids planted in Western Europe beginning in the 1980s include ‘Apricot Queen’, ‘Brilliant Yellow’, ‘Chamois Glow’, ‘Pink Queen’, and ‘Salmon Queen’.

The leaves were fermented with leaves of sweet gale and oak bark to create herb beer. Fruits are eaten as a mash in small amounts against lack of appetite or an upset stomach and stimulate production of gastric acid. In folk medicine they are used as a laxative, against rheumatism and kidney disease, and as a gargled juice against hoarseness.

• Timber

The wood is used for cartwright’s work, turner’s work, and woodcarving. Wood can be used from trees as young as 20 years. The sapwood is golden and white, while the heart-wood is brown. In almost treeless regions it is used as firewood. The leaves are sometimes used as fodder for livestock while the fruit are used against erysipeloid infections in domestic pigs and goats. Bark of the plant was used to dye wool brown or red. Honey from the flowers is strongly aromatic and has a reddish color.

The species is planted in mountain ranges to fortify landslide and avalanche zones.