Sorbus

These hardy plants offer year-round interest thanks to their changing leaf colour, bright berries and corymbs of flowers, says Bethan Norris.

S. cashmiriana - image: Floramedia
S. cashmiriana - image: Floramedia

Sorbus has many common names — including whitebeam, rowan, service tree and mountain ash — and is a genus of around 100 species of deciduous trees and shrubs in the Rosacea family.

It is widely distributed in northern temperate regions, where it is found in woodland as well as on hills, mountains and scree. Many varieties have also been introduced from Asia.

The genus is valued by garden designers for its ornamental leaves, which often colour well in autumn.

S. sargentiana Award of Garden Merit (AGM) has foliage that turns a blazing red in autumn, while S. ‘Chinese Lace’ has serrated edged leaves that turn a deep shade of purple in the autumn.

The trees are also grown for their panicle-like corymbs of white or pink flowers that appear in the spring and early summer as well as their berries, which can be red, white, orange, yellow or brown and prolong the trees’ interest in autumn and winter.

Generally, specimens that have white and pink berries hold onto them for the longest time and provide a good display well into winter. The trees are also very tolerant of atmospheric pollution, which makes them ideal street trees for inner cities.

Sorbus species grow best in moderately fertile, well-drained acid to neutral soil in full sun or dappled shade. They do not like damp conditions in winter and equally will not do well in drought conditions during the summer. S. aria will thrive on dry chalky soil as well as on acid soil. They are full to frost hardy and can tolerate exposed positioning.

Gardeners should, however, watch out for pests and diseases because sorbus are prone to quite a few. These include aphids, blister beetles, red spider mites, scale insects, sawfly larvae, canker, silver leaf, honey fungus and fireblight.

Fireblight is a particular enemy of sorbus, especially S. ‘Joseph Rock’. It is a bacterial disease that causes leaves to shrivel, branches to die back and sometimes a white slime is exuded.

The symptoms appear in late spring and autumn, and foliage looks scorched. The only solution once fireblight has been detected is to remove the tree.

Often slow-growing and with a compact, neat habit, sorbus make ideal specimen trees. Sympathetic planting of early-flowering bulbs and flowers — such as hellebores, crocus or Saxifraga fortunei — at the base of the tree can create a stunning display when combined with the white berries of some sorbus varieties.

The easiest variety to grow in the UK is S. aucuparia (the mountain ash or rowan). This British native bears orange/red berries that are very attractive to birds in the winter, which means that they do not last long. The edible berries can be used to make rowan jelly and the tree is very hardy. S. cashmiriana Hedl. AGM produces clusters of white berries on red stems that last well into the winter and early spring.

What the specialists say

Katie Hill, brand leader, Ornamental Tree Nurseries, Herefordshire

"All the varieties have different interest, some for their berries and some for their foliage. The most popular is S. aucuparia — the straight variety of the common mountain ash — and we sell most of that. ‘Chinese Lace’ has good autumn foliage and the colour is very pretty. Generally, the ones that give autumn colour are the ones people want because once the berries are not there customers don’t just want something green and boring.

"S. commixta ‘Embley’, S. ‘Eastern Promise’ and S. ‘Joseph Rock’ I should mention too as popular varieties. I’d say the popularity is going up. We sell more towards the autumn from October because of the autumn colour they offer. The business changed hands two or three years ago so data from the past two years shows popularity is rising, because of the autumn colour.

"They’re not evergreen so don’t offer leaf all year round, but they offer some interest most of the year, even when the leaves have dropped. The bark is a popular feature on some too. ‘John Mitchell’ is a new one and we’ve not sold many yet. It has green foliage and no autumn colour."

 James Alexander, tree manager, Hillier Landscape Contractors & Nurseries, Hampshire

"We like to recommend ‘Streetwise’ because we’re the only ones who do it. It’s just more upright than others. But they’re not really in vogue at the moment. Landscape architects aren’t keen on the berries for street planting. In more native planting they do get used, but it depends on the project really."

Simon Scarth, manager, Chew Valley Nursery, Somerset

"The best one is ‘Cardinal Royal’, which is pretty quick-growing and very uniform. It’s upright but slightly spreading and creates a good street or playground tree. It’s good and strong. For red berries, the ‘Cardinal Royal’ is best because the berries are very consistent and guaranteed.

"For yellow berries, ‘Sunshine’ is a very strong tree and a very prolific berrier, and a stronger tree than ‘Joseph Rock’, which is commonly specified. There’s quite a lot of new varieties around. ‘Olympic Flame’ is reasonably new. It got rebranded in time for the Olympics and has good autumn colour and quite a good shape, and is generally a good all-round tree.

"Sorbus comes and goes in cycles. A few years ago we sold a lot but that tailed off a bit over the past three or four years. It has very good ornamental value because of the berries and has white flowers in the spring. It’s a smaller tree so fits the smaller garden and is good for screening. The foliage is light so it gives a dappled shade and takes the eye away without blocking the view entirely."

Adam Dunnett, sales and marketing director, Wyevale Nurseries, Herefordshire

"We recommend ‘Olympic Flame’ for its autumn colour, berries and size. ‘Blackberry’ and ‘Pink Pagoda’ would also be in my top three. They’re not massively long-lived but they give plenty of interest with their berries, young foliage and great autumn colour. They’re an upright small tree that’s quite cheap and suitable for all soils. They bulk up well and they’re rock solid for transplanting, and disease-wise are fairly reliable though they might get a bit of fireblight."

In practice

Mark Gregory, managing director, Landform Consultants, Surrey

"It’s a good-value tree and I like it as a one-off specimen. There are issues with fireblight and it’s not a long-lived tree, so I can understand resistance from landscape architects to using it."

Helen Elks-Smith, garden designer, Hampshire

"I do include them sometimes, generally S. aria cultivars. They are lovely trees and S. aria ‘Lutescens’ is a useful size."

Species and varieties

S. aria (whitebeam) produces toothed dark-green leaves that are white and hairy underneath. White flowers are followed by brown-speckled dark-red berries 1cm across. Height: 10-25m. Spread: 10m.

S. aucuparia is a broadly conical to rounded tree with mid- to dark-green leaves that turn red or yellow in autumn. It bears corymbs of white flowers 12cm across then orange/red berries. Height: 15m. Spread: 7m.

S. cashmiriana Hedl. AGM (H6) is a spreading tree with pinnate leaves up to 20cm long that originates from the western Himalayas. It bears pink or white flowers in corymbs 12cm across followed by large 1.5cm-wide berries that are pink-tinged at first. Height: 8m. Spread: 7m.

S. ‘Chinese Lace’ is an upright tree with pinnate leaves that have deeply cut dark-green leaflets. Small white flowers are produced in late spring followed by orange/red berries. Height: 6m. Spread: 5m.

S. commixta is a compact, broadly conical tree with erect branches and pinnate leaves up to 25cm long, each with up to 17 elliptic dark-green leaflets that turn yellow to red and purple in autumn. It bears white flowers in late spring in corymbs 15cm across, followed by orange or red berries 8mm across. Native to Japan and Korea. Height: 10m. Spread: 7m.
S. commixta ‘Embley’ AGM (H6) has bright-red leaves in autumn and produces large quantities of bright-red berries. This variety also has distinctive pale speckled bark.

S. domestica (service tree) is a columnar tree with pinnate leaves up to 20cm long. It has dark-green leaflets turning yellow and red in autumn. The berries, yellow/green and red-flushed, are up to 3cm wide. Height: 20m. Spread: 12m.

S. hupehensis is a columnar tree with pinnate leaves up to 15cm long with blue/green leaflets that turn red in autumn. It bears white flowers in spring followed by spherical white berries that are slightly flushed pink. Height and spread: 8m.
S. hupehensis ‘Pink Pagoda’ produces pink berries that last throughout the winter.

S. ‘Joseph Rock’ is an upright tree with pinnate leaves up to 15cm long and up to 21 narrowly oblong toothed bright-green leaflets that turn orange, red and purple in autumn. It bears white flowers in corymbs 10cm across followed by pale-yellow berries that turn orange/yellow.

S. reducta is a suckering shrub with upright shrubs. It has pinnate leaves up to 10cm long composed of up to 15 dark-green leaflets that turn red and purple in autumn. It bears white flowers followed by crimson then white berries 1cm across. Native to China. Height and spread: 1-1.5m.

Thank you to Floramedia, which supplied the images for this article from its photo library
www.floramedia-picture-library.com


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