Sorbus

This species grows as a shrub along the west coast of North America from Alaska to New Mexico. Further inland it is very much a species of the Rocky Mountains, though its range also extends to the eastern valleys of the Cascades. It particularly favours the lower forested slopes of foothills.

Like other American mountain ash species, this one resembles the European equivalent (S. aucuparia) but does not grow into a tree, remaining a thick-branched shrub. It thrives in southern Finland, at least, and seems to survive comparative drought.This species has been grown at Mustila from seed collected by the Helsinki University Botanic Garden expedition to British Columbia in 1995. It can be seen growing both on Pohjoisrinne, or Northern Slope, and at the western end of Etelärinne, or Southern slope, in the Sorbus (i.e. mountain ash) collection.

The Swedish whitebeam is a sturdy mid-sized tree reaching about 10m, with a broad round crown. It grows naturally along the coasts of the Baltic Sea, and sparsely in the south-west archipelago. The leaves are the main difference from the hybrid whitebeam (S. hybrida); the Swedish whitebeam’s leaf is shiny dark green above, sometimes even olive green, with shallow-lobed edges; on non-flowering shoots the leaves may have a single pair of leaflets near the base. The leaf undersides are grey or yellowish-felted.

Originally Linné grouped the Swedish whitebeam with the hawthorns (Crataegus), like the other mountain ashes with entire leaves. The tree can be grown from seed, and the seedlings closely resemble each other. According to current knowledge, the Swedish hornbeam is a cross between three mountain ashes: the rowan (S. aucuparia), the wild service tree (S. torminalis) and rock whitebeam (S. rupicola). The wild service tree was an ancient native of the Baltic area.

The Swedish whitebeam is highly valued as a street tree, being undemanding and tough. In very severe winters some older specimens may be destroyed, as happened in Helsinki in the mid-1980s due to the cumulative effect of consecutive extreme winters.

This hybrid mountain ash was first discovered in Finland, so can be regarded as a native. However, in addition to south-west Finland, it also grows naturally on Swedish islands and in Norway. Pietari Kalm called the tree S. fennica but nowadays it is called S. hybrida, the name given to it earlier by Linné, which acknowledged its hybrid status. Linné believed its parents to have been the rowan (S.aucuparia) and the Swedish whitebeam (S. intermedia), but it has later been shown that this second parent was the whitebeam S. aria. Its ancestry thus includes both native and exotic genes, so the tree has been adopted as the emblem of the Finnish Dendrological Society.

Hybrid mountain ash is a healthy tree well suited to streets and parks, and can even be used for hedging. The leaves on flowering shoots have one or two pairs of separate leaflets at their base, and the undersides are covered in grey-green felt. As with other mountain ash species, the flowers are creamy white and scented. The red, round, sweet-tasting berries ripen in September.

Slight variations between individual trees are an indication that crossings between the parent species have happened several times in different areas. Plants raised from seed are usually clones of the mother tree, but sometimes crossings with other mountain ash species do occur.