A medium-sized columnar tree with simple, green leaves that turn an orange-red in autumn. This species is always floriferous and in spring is covered in small, white flowers, giving rise to clusters of small, matt, pink-red fruits. 15m. RHS H5, USDA 3a-7b†.
Show me a garden that wouldn’t benefit from a rowan or a whitebeam, and I’ll be very surprised. They’re readily available in nurseries, fairly easy to grow and they are hard-working trees, with a range of seasonal attributes including interesting leaf shape and colour, highly scented corymbs of white flowers, an array of differing fruit shapes and colours, and various forms of habit and shape. Add to that their value for wildlife and you have a winner.
The genus Sorbus, to which these trees belong, contains around 200 species and numerous cultivars of mainly small- to medium-sized trees growing up to 20m. They’re found growing across the northern hemisphere, from western Europe through Asia to the Himalayas and Japan, and in North America from Alaska to Arizona, and are at home in a variety of habitats, from lowland woodlands to high elevation limestone rocky outcrops in free-draining acid to alkaline soils. The red-fruiting American rowan, S. americana grows in marshland and moist soils but will also happily grow on dry rocky hillsides.
The genus has a complicated taxonomy but can easily be broken down into two main groups. The largest group is the subgenus Sorbus, commonly known as the rowan or mountain ash. These have pinnate leaves, terminal corymbs of flowers and small, round fruits called pomes, which contain two to ten soft seeds. They grow across the northern hemisphere from North America to temperate Asia. Sorbus aucuparia is our native rowan, with several good cultivars that have been selected for gardens, such as ‘Asplenifolia’ with deeply cut leaves, ‘Besissneri’, with a narrow upright habit, and ‘Sheerwater Seedling’, a popular upright tree with a perfect oval-shaped crown and large trusses of orange berries. Sorbus x kewensis is a hybrid of two subspecies of S. aucuparia, and an outstanding fruiter. A popular cultivar, once sold as S. x kewensis, but most likely a seedling of S. ‘Pagoda Red’, is S. ‘Chinese Lace’, a beautiful tree, with lace-like, cut-leaf foliage. One of my all-time favourites in this group is, Sorbus ulleungensis, which I first encountered in 1989 on the island of Ulleungdo in the East Sea off the coast of South Korea, where I was collecting seed for the Arboretum at Kew. The tree was multi-stemmed on the summit of Seonginbong (984m elevation), the highest mountain on the island above the Port of Dodong. The cultivar S. ulleungensis ‘Olympic Flame’ has possibly the most electrifying autumn colour of any rowan with large trusses of bright-red berries hanging from its very stout branching habit.
The second group is the subgenus Aria, known as whitebeam. This group has entire leaves, usually with a silvery underside and terminal corymbs of larger fruits with a persistent calyx. The species in this group range naturally on alkaline soils through temperate Europe and Asia. Sorbus aria, the common whitebeam can be found growing naturally in free-draining, alkaline soil in open woodlands in southern England and throughout much of Europe, but in cultivation there are two particularly good cultivars. S. aria ‘Lutescens’, the silver-leaved whitebeam, is a medium-sized tree with a compact crown and silvery hairs on both sides of the new leaves, and S. aria ‘Magnifica’, with large leaves and clusters of creamy-white flowers, liked by bees, in late spring and large clusters of red fruits in autumn. There is a third group, the subgenus Micromeles, which differs slightly from the others in taxonomic terms in that its fruits have deciduous calyces and narrower leaves.
All grow naturally in Asia, with one of my favourites, both in spring for its flowers and in autumn for the persistent clusters of matt pink fruits, is the Korean whitebeam, Sorbus alnifolia. Another Sorbus in this group and always the first to flush into leaf in my garden, with its bronze foliage in January and February before the frosts have finished, is the Asian whitebeam, Sorbus caloneura from China. It must have some form of antifreeze in the leaves as it never ever gets frosted, even in the coldest London gardens. Sorbus are a must for planting in the treescape.
• Author Tony Kirkham is head of Arboretum, Gardens and Horticulture at the Royal Botanic Gardens, Kew. His recommendations for the best Sorbus can be found over the next five pages.
nPLANT PROFILE
Deciduous, hardy small- to medium-sized trees, occasionally shrubs, with ornamental flowering, fruiting and autumnal colour attributes.
Commonly known as rowan (or mountain ash) and whitebeam.
Northern hemisphere, from temperate North America to temperate Asia.
Flowering March to May and fruiting from September to November. 1-20m high.
Requires a moist but free-draining acid or alkaline soil, not heavy wet clay.
Sorbus are hardy in the UK (to temperatures from -15º C to -20 º C), with an RHS hardiness rating of H5 to H6, and are suitable for gardens in
USDA zones 3a to 7b.
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*Holds an Award of Garden Merit from the Royal Horticultural Society. Hardiness ratings given where available.
The Swedish whitebeam is a medium-sized tree with a dense, rounded crown and dark, glossy green leaves that are grey-downy on the underside with white flowers and clusters of orange-red fruits. 15m. RHS H6.
aff. filipes
A slow-growing rowan, often grown as a large shrub with fine, pinnate, dark-green leaves that turn a dark redgreen in summer. Despite its size, it has large trusses of fruits that start a delicate shade of pink, turning white when mature. 3m.
xThis small tree, a hybrid of two subspecies of
is an attractive rowan that has green pinnate leaves and offers highly scented flowers in spring. The bunches of bright-red autumn fruits are so abundant that they often weigh down its lateral branches. 6m.
Known as Vilmorin’s rowan, this is a small, elegant, spreading tree that has beautiful, delicate, fine, fern-like pinnate leaves with lots of small leaflets. The hanging clusters of white flowers are followed by small, deep-crimson-pink fruits. 6m. AGM. RHS H6.
Sorbus ulleungensis
This small tree has an extremely stiff and erect branching habit and stout buds with large green pinnate leaves. It has the best orange-red autumn colour of any rowan and large clusters of bright-red berries.
6m. AGM*. RHS H6.
‘Chinese Lace’
A seedling of either
‘Pagoda Red’ or a similar plant once grown at Kew, this ornamental rowan makes a small tree with deep-cut, lacy pinnate leaves turning a rich red-purple in autumn. The delicate white flowers produce red-orange fruits in autumn. 6m.
‘Gibbsii’ This is a vigorous whitebeam with an upright branching habit and a dense, columnar crown. The leaves are large, deep green and lobed.
The abundant white flowers in spring give rise to very large green fruits that turn to a coral red. 12m. AGM. RHS H6. ‘John Mitchell’
A medium to large-sized upright whitebeam with a broad, rounded crown. The enormous leaves, which are up to 20cm round with a silvery underside, are the special attribute of this cultivar. The fruits are also large and russet-brown in bunches. 20m.
The common whitebeam is a medium-sized tree with a compact, domed crown. Its wedge-shaped leaves are a shiny green above, with a silvery-white underside and turn a russet colour before they fall. Clusters of white flowers in spring are followed by scarlet-red fruits. 15m. RHS H6.
A small, flat-topped
Chinese species that is often wider in spread than upright. Its doubly toothed leaves emerge bronzetinted in early spring, before turning green. White flowers are borne in dense clusters, followed by persistent brown pear-shaped fruits. 5m.