SCIENTIFIC NAME: Sorbus aucuparia
This tree in Colorado Best when sited properly; not drought or heat tolerant. Prefers moist, well-drained, acidic to mildly alkaline soils. A medium-sized tree that is used more as an ornamental than for shade. Not a true ash (Fraxinus), so written as Mountain-ash or mountainash.
Hardiness Best in Zones 4 to 6. Above 7500 ft. use Sorbus scopulina, a native shrub.
Growth rate and mature size Medium growth rate, to about 35 feet tall, width varies with cultivar.
Foliage Alternate, pinnately compound with serrated, medium to dark green leaflets. Fall color may be yellow to rust/orange to reddish purple.
Fruit 1/4″ diameter pome, orange, in clusters. Some cultivars have red pomes, others have yellow, orange or pink pomes. Attractive to birds. Edible, sometimes made into syrups, juices, preserves or teas. Weight of fruit clusters often bends branches.
Flowers Small, white, in 3-4″ diameter clusters, in May after leafing.
Bark Attractive, golden-amber to gray-brown, with prominent lenticels.
Insects and diseases Fireblight, leaf rusts, Cytospora canker, sunscald. Aphids, pearslugs, scales; borers are likely under stressful conditions.
Cultivars Several, based on growth habit and fruit color. Commonly-sold ones include ‘Cardinal Royal’ and ‘Black Hawk.’
Landscape value High, a “4-season” tree, with flowers in spring, attractive summer foliage, fruit display and leaf color change in fall; bark color in winter.
Best advice ‘Cardinal Royal’ has performed well in CSU trials. Its growth habit is upright-oval, with red pomes. European Mountain-ash does best planted on East or North exposures in large areas that are mulched; avoid South or West exposures. Best out of lawn areas, due to grass competition and the high lawn fertility provided may promote fireblight.
Michael Dirr, Manual of Woody Landscape Plants (University of Georgia, 1990)