The PLANTS project: Sorbus

Commonly known as rowan, mountain ash and whitebeam, Sorbus trees are widespread in the UK, including in National Trust for Scotland gardens. The PLANTS project audit has identified over 40 species and 60 cultivars under the Trust’s care. Sorbus trees are popular for their year-round interest, wildlife benefits and easy availability.

The PLANTS project primarily audits the beautiful gardens within the Trust’s historic gates and walls. In the areas between these more formally managed areas and the wider estate, there is a vast array of specimen plants of note and interest that greet you at our sites. In these welcoming ‘in-between’ spaces I have often been delighted to find a huge range of various Sorbus trees – they blur the edges of the managed and the wild, and provide me with a wonderful reminder that I’m lucky to live in Scotland.

Sorbus aucuparia

Whilst travelling to the gardens as part of the PLANTS North team I have enjoyed seeing many wild Sorbus aucuparia. Commonly known as the rowan, Sorbus aucuparia is a variable species with a wide endemic range across Europe and into Asia. It’s also known as the mountain ash, due to its preference for mountainous habitats and its superficial similarities to the ash (Fraxinus ssp). Both produce odd-pinnate compound leaves – comprising pairs of lateral leaflets (leaf-like parts of a compound leaf) attached directly to a central stalk with a single terminal leaflet. In Sorbus aucuparia the leaves are alternately arranged on the branch, each leaf having 6–8 pairs of leaflets, oblong to oblong-lanceolate in shape, with sharply toothed margins that extend nearly to the base of the leaflet.

Sorbus aucuparia subsp. aucuparia is a pioneer species, alongside Populus tremula (aspen), Betula spp (birch) and Pinus sylvestris (Scots pine). In exposed sites it typically grows as a multi-stemmed tree. Often found at woodland margins, open hillsides and riverside slopes, it generally reaches a maximum height of 15–20 metres, but is smaller and more shrub-like at higher altitudes. In densely shaded sites, it grows very slowly, awaiting an opportune opening in the canopy and accelerating its growth as it captures more sunlight.

Abundant on the thin acidic soils of the Scottish Highlands, where they have adapted to the short growing season, rowans enjoy the higher rainfall and cooler climes. They use their tough, fibrous roots to tenaciously grip to the rocky substrates, up to the edge of the tree line at around 600m. Although they generally produce the best autumnal leaf colours in more northern climes, rowans are adaptable to many different growing conditions. With their neat slender habit and upright branches, rowan cultivars and related species are often chosen as ornamental street trees across the UK.

The leaflets can help us identify different related species, hybrids or cultivars, especially if fruit isn’t present. For instance, Sorbus aucuparia var. edulis has slightly larger, wider-spaced leaflets that are generally only toothed to half their length. The winter dormancy of Sorbus aucuparia subsp. aucuparia’s buds is broken only after a period of cool/cold temperatures. With increasingly warmer winters and hotter, drier summers, the range of this tree is contracting to its northern reaches. The inflorescences (clusters of flowers) of the rowan comprise up to 250 tiny creamy white flowers produced in frothy abundance in late spring/early summer. The flowers are strongly sweet, but not wonderfully scented. Sometimes described as foetid, they are particularly attractive to flies who don’t seem to mind! A wide range of other pollinating insects also enjoy the protein-rich pollen, even beetles, which prefer the slightly flat-topped flower heads.