Plant Focus: Sorbus
Two distinct groups of trees sold under one familiar name, and a long season of berries for birds
Sorbus belongs to the rose family, Rosaceae, alongside apples, pears, hawthorn and cotoneaster, and the family resemblance shows in the flat heads of creamy-white flowers that open in late spring and in the small pome fruits that follow them. Two members of the group are native to Britain and both have been planted here for centuries, the rowan through the uplands and the north, the whitebeam on the chalk and limestone of the south.
The name rowan covers the species with pinnate leaves, made up of paired leaflets along a central stalk, which is why the tree picked up the folk name mountain ash. The resemblance is superficial, since ash sits in an entirely different family, but the name has stuck for long enough to cause occasional confusion on plant schedules. Whitebeam takes its name from the leaf, simple rather than divided and densely white-felted underneath, so that a breeze turns the whole canopy silver.
Both groups stay small to medium in size, flower and fruit reliably from a young age, colour in autumn, and hold their berries into the part of the year when very little else is feeding anything. They also tolerate the conditions that finish off more demanding trees, including exposure, wind, thin soils and urban air.
That tolerance is the reason Sorbus appears so often in car parks, verges, housing schemes and street planting, where the growing conditions are rarely generous and the space available is rarely large. The berries matter most in those settings, because they carry thrushes and winter migrants through months when other food has gone.
Key Characteristics:
✓ Deciduous trees of the rose family, Rosaceae
✓ Flat heads of creamy-white flowers in late spring
✓ Clusters of red or orange berries from late summer into winter
✓ Reliable autumn colour in red, orange, russet or gold
✓ Small to medium size, suited to confined planting positions
✓ Tolerant of exposure, wind and atmospheric pollution
✓ Two leaf types, pinnate in the rowans and simple in the whitebeams
Did You Know?
The species name aucuparia comes from the Latin for bird-catching, from avis meaning bird and capere meaning to take. Rowan berries were once used as bait in bird traps, which tells you how dependably birds strip them.
Why some of these trees are no longer Sorbus
Botanists have spent the last few years taking the genus apart, and the version of Sorbus now accepted is a good deal smaller than the one most plant lists still use. As currently defined, Sorbus holds only the pinnate-leaved species, which means the rowans. Everything with a simple leaf has been moved out.
The whitebeams now sit in the genus Aria, and the type species of that genus is Aria edulis, the tree sold everywhere as Sorbus aria. The wild service tree has become Torminalis torminalis, the true service tree has gone to Cormus, and a group of narrow-leaved east Asian species has gone to Micromeles. Because whitebeams hybridise so freely, several further genera exist to hold the crosses, among them Hedlundia, Karpatiosorbus and Scandosorbus.
None of this has reached the trade, and there is a reasonable argument that it should not be rushed. Plant schedules, tender documents and nursery catalogues all still say Sorbus, and a tree ordered as Aria edulis would puzzle most suppliers. The names used below are therefore the ones you would use to order the plant, with the reclassification noted here once rather than repeated throughout.
The split is worth understanding even so, because it follows something you can see standing in front of the tree. A pinnate leaf and a head of scarlet berries is a rowan and still a Sorbus. A simple leaf, white and felted underneath, is a whitebeam and botanically an Aria. The two groups behave differently in the ground as well, which matters more than the label.
Sorbus in landscaping
Very few of these trees outgrow the space they are given, and several are naturally narrow, which makes them workable in streets, courtyards, car park islands and the strips of planting between a building and a footpath where a forest species would become a liability within fifteen years. Scale is one reason they are specified as widely as they are.
One tree also gives blossom in May, fruit from late summer, autumn leaf colour, and in several cases berries that hang on well past leaf fall, a sequence few plants of this size manage on their own. The berries also make the clearest biodiversity case of any small ornamental tree, since they feed birds directly at the hardest point of the year.
Rowans we recommend
Sorbus commixta ‘Olympic Flame’
Sorbus commixta ‘Olympic Flame’
A small, columnar tree reaching around 6m in height with a 4m spread, grown above all for its autumn colour, when the leaves turn vivid red and orange after emerging copper-bronze in spring. Creamy-white flowers in late spring are followed by glossy red berries that hold into winter. The narrow shape suits courtyards and front gardens on housing schemes, where autumn colour is wanted without the width. Prefers acid to neutral, well drained soil in full sun or partial shade, and is also listed as Sorbus ulleungensis ‘Olympic Flame’.
Sorbus aucuparia
Commonly known as rowan or mountain ash, a native tree with pinnate leaves, flat heads of creamy-white flowers in May and hanging bunches of orange-red berries from late summer. It copes with thin, poor and acid soils and real exposure, which makes it a dependable choice for verges and windy open schemes where little else will grow. The berries feed thrushes, blackbirds, redwings, fieldfares and waxwings through autumn, and the flowers give pollen and nectar to bees.
Sorbus aucuparia ‘Sheerwater Seedling’
A vigorous selection of the native rowan with a narrow, upright habit, reaching around 8m to 12m in height with a spread of 2.5m to 4m. Creamy-white flowers in April and May are followed by large clusters of red berries that last well into winter, and the foliage turns yellow and gold in autumn. The narrow crown makes it a common street tree, giving height and berries without spreading into the carriageway. Grows in full sun on any well drained soil, takes exposure well, and holds the RHS Award of Garden Merit.
Sorbus aucuparia
Sorbus aria
Whitebeams we recommend
Sorbus aria
Commonly known as common whitebeam, a native tree with oval leaves that are dark green above and white-felted beneath, so the canopy turns silver whenever the wind moves through it. It reaches around 15m in height with an 8m spread, and grows wild on the chalk and limestone of southern England. Few small trees handle as wide a range of ground, from chalk and clay to acid sand, and it tolerates pollution and coastal exposure well, which suits it to roadsides, car parks and seaside sites. Spring blossom feeds pollinators, and birds take the berries through autumn.
Sorbus aria ‘Lutescens’
A compact selection reaching around 10m, upright when young and spreading with age. The young leaves open creamy silver-white and stand upright on the shoots like candles, then settle to grey-green and finish russet and gold in autumn. Creamy-white flowers in May are followed by orange-red berries for birds. Its size suits courtyards, front gardens and streets where a full whitebeam would outgrow the space, and it copes with urban pollution and coastal wind. It holds the RHS Award of Garden Merit.
Sorbus aria ‘Magnifica’
A German selection grown above all for its large, glossy, dark green leaves with the usual white felt beneath, reaching between 8m and 15m in height with a crown of 4m to 8m. It holds its leaves late and avoids the early leaf drop that can leave the species looking thin by September. The scale suits parkland, avenues and larger open schemes, in full sun on chalk or clay. It is often confused in catalogues with ‘Majestica’, a separate large-leaved selection.
Sorbus aria ‘Magnifica’
How to grow and care for Sorbus
Both groups perform best unshaded. Full sun gives the heaviest flowering, the best berry set and the strongest autumn colour, and while all of these trees accept partial shade, the display suffers in proportion to the shade. Rowans are happiest on neutral to acid ground and can look tired on thin chalk, while whitebeams are native to chalk and limestone and grow perfectly well on alkaline soil. Soil is where the two groups part company, and it makes the choice between them a reading of a soil test rather than a matter of taste.
Neither group enjoys ground that stays waterlogged through winter, though both perform on soils that would be considered poor for most ornamental trees, so drainage is the thing to check rather than fertility. Once established they cope with dry spells and with wind, and the whitebeams in particular tolerate salt-laden coastal air and urban pollution.
Spacing should follow the mature spread rather than the size of the tree on delivery, which is where narrow selections earn their place. ‘Sheerwater Seedling’ and ‘Olympic Flame’ hold a crown of three or four metres and can sit close to a building line or a carriageway, while Sorbus aria and ‘Magnifica’ need several times that and become a problem where they are planted as though they were the small trees they appear to be in the nursery.
Water through the first two or three summers, keep a vegetation-free circle around the base so that turf is not taking the moisture first, and mulch the root area while leaving a gap at the trunk. Most young Sorbus that fails is lost in those early years rather than later, as with any newly planted tree.
Remove dead, damaged or crossing wood and lift the crown where clearance is needed, and otherwise leave the tree alone, since the natural shape of most Sorbus is the reason for planting it in the first place. Prune in late summer or early autumn rather than in cool, wet conditions, which reduces the risk of silver leaf entering the cuts.
Pests and diseases
Sorbus is a reliable group and most trees give no trouble at all, but there are a few things worth knowing. Fireblight, caused by the bacterium Erwinia amylovora, is the significant one. It shows as sudden wilting and blackening of flower trusses and shoot tips, with necrotic patches spreading in from the leaf margins, and young trees are more vulnerable than established ones. Affected growth should be cut out well below the visible damage, with tools disinfected between cuts. Apple canker and coral spot also occur, most often on trees already under stress.
Silver leaf is a fungal disease that enters through wounds in cool, wet weather and blocks the vessels that carry water, which is the reason for pruning in dry conditions rather than through the winter. Among the pests, aphids, scale insects and sawfly larvae all appear from time to time, rowan sawfly on the fruit and pear slug and leopard moth caterpillars on the foliage, and none of them usually needs intervention on a healthy tree in open ground.
The best protection is a tree that arrives clean and in good condition. Greenwood is Plant Healthy certified following independent audit across all our sites, with a structured biosecurity policy behind it, because a pest or disease that arrives with the stock is far harder to deal with than one that turns up later.
Rowan for neutral and acid ground and for real exposure, whitebeam for chalk, for the coast and for the silver flash of the foliage, and either one where a small tree has to give blossom, autumn colour and a winter berry crop from a position that will never be generous. Specifying Sorbus comes down to the soil before anything else.
If you would like more information on the Sorbus varieties we grow and sell, speak to the G Team today.