Rowan - Botany

Botany

Rowans are mostly small deciduous trees 10–20 m tall, though a few are shrubs. The leaves are arranged alternately, and are pinnate, with (7-)11-35 leaflets; a terminal leaflet is always present. The flowers are borne in dense corymbs; each flower is creamy white, and 5–10 mm across with five petals. The fruit is a small pome 4–8 mm diameter, bright orange or red in most species, but pink, yellow or white in some Asian species. The fruit are soft and juicy, which makes them a very good food for birds, particularly waxwings and thrushes, which then distribute the rowan seeds in their droppings. Due to their small size the fruits are often referred to as berries, but a berry is a simple fruit produced from a single ovary, whereas a pome is an accessory fruit.

Rowan is used as a food plant by the larvae of some Lepidoptera species; see Lepidoptera that feed on Sorbus.

The best-known species is the European Rowan Sorbus aucuparia, a small tree typically 4–12 m tall growing in a variety of habitats throughout northern Europe and in mountains in southern Europe and southwest Asia. Its berries are a favourite food for many birds and are a traditional wild-collected food in Britain and Scandinavia. It is one of the hardiest European trees, occurring to 71° north in Vardø in Arctic Norway, and has also become widely naturalised in northern North America.

The greatest diversity of form as well as the largest number of Rowan species is in Asia, with very distinctive species such as Sargent's Rowan Sorbus sargentiana with large leaves 20–35 cm long and 15–20 cm broad and very large corymbs with 200-500 flowers, and at the other extreme, Small-leaf Rowan Sorbus microphylla with leaves 8–12 cm long and 2.5–3 cm broad. While most are trees, the Dwarf Rowan Sorbus reducta is a low shrub to 50 cm tall. Several of the Asian species are widely cultivated as ornamental trees.

North American native species in this subgenus include the American mountain-ash Sorbus americana and Showy mountain-ash Sorbus decora in the east and Sitka mountain-ash Sorbus sitchensis in the west.

Numerous hybrids, mostly behaving as true species reproducing by apomixis, occur between rowans and whitebeams; these are variably intermediate between their parents but generally more resemble whitebeams and are usually grouped with them (q.v.).