(Editors note; this article was originally published June 6, 2009. Your questions and comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Many northern gardeners are familiar with Sorbus, known as mountain-ash in North America or rowans in Europe. These relatively small trees are wonderful additions to the garden for their floral display of white flowers in spring, attractive crop of orange-red berries in autumn and if you want to attract fruit-eating birds to your garden, then mountain-ash are one of the best woody plants to cultivate. However, what you may not know is that this genus of about 50 species shows considerable variation. While the common garden varieties are trees up to 12 m, there are some species that barely reach a foot! We typically think of mountain-ash as having orange-red fruit but among the many species that exist, this is a rare colour; white coloured berries are far more common, but they also come in yellow, pink and many subtle, in-between shades. And as common as mountain-ash are in North America and Europe, we, as a whole, have relatively few native species; the vast majority hail from the Himalayas.
Most recently, the genus Sorbus has been split into 4 genera. Today, the genus Sorbus is retained for those species with pinnately compound leaves. Among these, the most commonly cultivated species are S. aucuparia from Europe and S. decora and S. americana from North America. As it happens, these are among the largest growing Sorbus and are the reddish-fruited species. Before I get into the various species in more detail, I will provide some cultural details.
Mountain-ash prefer full sun and relatively moist soils. They will tolerate some shade but flower and fruit production will be reduced. As well, those species with notable fall foliage will be less than impressive if grown in too much shade. Most species are not fussy about the soil pH although some lime chlorosis may be evident under very alkaline soils. The commonly grown species are quite hardy and will survive into zone 2 but many of the smaller, Himalayan species are only rated to zone 5. The commonly grown species are mostly self-fertile but individual plants may vary in their fruit production from year to year. In regards to attracting birds, the reddish-fruited species are best, followed by the yellow-fruited forms and lastly the white and pink forms. The showy mountain-ash, S. decora appears to be the overall favourite Sorbus of fruit-eating birds. Mountain-ash are generally purchased as container-grown specimens but rarer species may be grown from seed. Seeds need stratification so sow them in pots in autumn and leave outside. Alternatively, seeds may be sown on damp paper towel on a petri-dish placed in a refrigerator. Check to make sure the seeds are kept damp and pot them as the seeds start to germinate (even at 34 F the seeds will germinate!)
Unfortunately, mountain-ash do have a number of insect pests and diseases. Mountain-ash sawfly can defoliate a tree within a few days. There are many moth larvae that also feed on the foliage of mountain-ash. Fire blight is perhaps the most serious disease as there is no easy cure. Bark canker is also evident but usually attacks older trees. It should also be noted that mountain-ash are not particularly long-lived compared to other common garden trees like ash, maple and oak.
From Europe comes the most commonly cultivated species, S. aucuparia. There are many subspecies that are offered as species including S. maderensis, S. pohuashanensis, S. amurensis and S. kamtschatcensis. When you consider that S. aucuparia extends from the British Isles to eastern Russia, there are bound to be some geographical differences. The two most common species in North America are the American mountain-ash, S. americana and the showy mountain-ash, S. decora, both native to eastern North America. These three species all reach about 12 m (40 feet) and have the typical reddish-orange fruit we associate with mountain-ash. The European mountain-ash is not always easy to distinguish from the American species S. decora and S. americana but if you check the winter buds, you will see grey hairs along the margins of the bud scales. The winter buds of the American species are generally hairless, black and somewhat sticky. In the autumn, the fall foliage of S. aucuparia is rather bland while the American species are often more colourful, especially S. decora.
Flower details of S. americana, S. decora and S. aucuparia
Fruit details from S. americana, S. decora and S. aucuparia
From western North America come shorter, shrubby species. These are also red-fruited and can exhibit some wonderful fall colour. They are not as hardy as the eastern American species, being rated for zone 5 or 6, but are ideal for gardeners in milder regions who have space limitations. These western species include S. cascadensis (2-4 m), S. scopulina (to 4 m), S. sitchensis (1-2 m), S. occidentalis (3 m) and S. californica (1-2 m).
Fruit details of S. sitchensis and S. scopulina
There are relatively few cultivated species from the Himalayan region, which is surprising since they are among the most attractive mountain-ash with colourful fruit, smaller-size, more fern-like foliage and often fantastic fall colours. Two noteworthy red-fruited Asian species are S. sargentiana (China) and S. commixta (Japan, Korea). Both reach about 10 m. Sorbus sargentiana is reputed to have some of the largest and most showy clusters of fruit of any mountain-ash species. Another larger Chinese species is S. hupehensis. This one is distinct in having bluish-tinted foliage! The fruit are white, tinted pink. Sorbus discolor and S. pseudohupehensis also have pink-tinted berries. With snow-white fruit are the Chinese species S. forrestii and S. bulleyana. These have a growth pattern and foliage not unlike the European rowan.
Details of S. commixta (flower and fruit) and the fruit of S. hupehensis
Next we come to some smaller Himalayan species which mature at about 5m. Both S. koehniana and S. vilmorinii have pink-tinted white fruit. Sorbus cashmeriana has pinkish flowers and quite large white berries. Both S. setschwanensis and S. rehderiana have glossy foliage, white to pink-tinted berries and spectacular fall foliage. The smallest species of all are S. reducta and S. poteriifolia. The former will reach to 1 m while the latter is only 15 cm! Both have very fine foliage, white to pink-tinted fruit and excellent fall foliage. They also sucker, forming nice-sized clumps over time. These are ideal shrubs for a rock garden setting. It should be noted that all of these Asian species are only rated for zone 5 but as they are so rare in cultivation in North America, the hardiness ratings may be subject to change!
Flowers and fruit from S. cashmeriana (above) and S. vilmorinii (below)
Lastly, there are a few notable mountain-ash hybrids. ‘Joseph Rock' is perhaps the most popular. This hybrid has yellow fruit and wonderful orange-red fall foliage. ‘Pink-Ness' has pink to peach-coloured berries and somewhat blue-tinted foliage, evidence of its S. discolor parentage. Sorbus 'Ghose' has large clusters of smallish berries that are burgundy-coloured.
Details of S. 'Joseph Rock' and 'Ghose'
I have seen all these Himalayan species offered in seed exchanges from other botanical gardens (primarily European) and I am currently growing most of them, although many are only a few years old and still quite small. However, already I can see some great potential in these more unusual specimens. Their finer foliage and great fall colour are a precursor to their potentially attractive fruit displays. If you ever get the opportunity, try to obtain some of these uncommon species. In the meantime, whether you want a tree for decorative flowers, fruit or an attraction to wildlife, you can still enjoy the classical orange-red fruited species of mountain-ash commonly offered in our local nurseries.
I would like to thnak the following people for the use of their pictures: Grasmussen (S. sitchensis), growin ('Ghose', 'Joseph Rock', S. vilmorinii fruit, flower and fruit of S. commixta), Kelli (S. scopulina) and philomel (S. hupehensis).