- Written by Kevin R. Lee Kevin R. Lee
- Published: 06 April 2018 06 April 2018
The genus Sorbus, the Mountain Ash, is one of our favorites. About the only minus score on these Trees would be a rather unpleasant flower aroma, but that's about it. Hortus Third speaks of 85 species in the northern hemisphere. I will mention six or seven varieties. Deep in European cultural roots are the Rowan Tree and the Whitebeam.
Calgary does not have a native Mountain Ash, the closest being the western Mountain Ash, S. scopulina. The next closest is the Sitka Mountain Ash, S. sitchensis, from the west coast and rarely in the Alberta mountains. The most common Mountain Ash in Calgary is the American, S. americana, the European S. acuparia, followed by its cultivars, the Rossica, and the Black Hawk. Next would be the Showy, another eastern Canadian Tree. The last one we will discuss is the Oak Leaf Mountain Ash, a hybrid, S. hybrida. This Tree has a single leaf rather than the classic Mountain Ash pattern.
The American Mountain Ash is native through the southern half of Ontario and Quebec and throughout the Maritimes. This is the Tree commonly planted in the west because of its overall hardiness. After decades they can attain a great size and there are fifty-footers out there. More commonly they are thirty-plus feet high and multi-stemmed. A classic form, easy to climb, it attracts children and arborists alike.
The European Mountain Ash is similar in size and form to the two native Americans, the main visual differences being the orange berries and a lighter colored bark. Whereas the bark of the American is a dark greenish brown, the European is more of a tan color. The European has been the choice of plant breeders and there are two good cultivars. The first is the Rossica, a good Tree with a much narrower growth form, close to being a Fastigiata. If you love Mountain Ash but do not have a lot of space, this might be your choice.
The Black Hawk is another good choice for space limitations, sold as a smaller Tree. This cultivar is the newest and therefore somewhat unproven.
The Showy is very similar in form to the American. It derives its name from having the most robust of Mountain Ash flowers. It is not easy to distinguish from the American.
Last to mention is the Oak Leaf Mountain Ash, again a relative newcomer. The leaf is very attractive. This Tree is designed to be more columnar.
Bugs and diseases
Good, naturally strong Trees, the Mountain Ash is mostly trouble free, especially if regularly watered. That said, they are all pome fruit bearing, and subsequently can and do get fire blight. Every species has slightly different looking symptoms. Look for a set of dry brown crinkled-up leaves. As the larger branch starts to die, leaves turning yellow will be present. Further symptoms are cankers and large dead branches, I sure hope you catch it before that. Treatment is a thorough pruning.
Mountain Ash are also susceptible to a fungal disease called Silver Leaf. The fruiting bodies are white and resemble small sea shells. They exude a toxin. When I find these I scrape them off down to the wood. This seems to help. The wood they are present on may be dead.
Europhytic mites prey on Mountain Ash. They do no significant damage. Sometimes called Blister mites, their inner leaf activity causes small pockets to form on the leaf surface. Undersides of leaves will present small tan felty spots. You can spray with dormant oil before bud break to help control them.