Mountain Ash

Posted by shane - January 22, 2016 - A-Z Plant Maintenance, Trees - No Comments


Winter berries on American mountain ash. S. LePage photo.

American Mountain Ash (Sorbus americana)

European Mountain Ash & cvs. (Sorbus aucuparia cvs.)

Showy Mountain Ash (Sorbus decora)

Mountain ash is one of the most commonly planted ornamental shade trees in Alberta.  It is also one of the last species to survive its original planting in older neighbourhoods, commonly found nearby elm, green ash, manitoba maple, and spruce of the same age.  It is a true, four-season tree, with nice white flowers in spring, red or orange berries in summer, excellent fall colour, berries that persist throughout winter, attracting birds.  Most homeowners with mountain ash trees tell stories of drunken waxwings banging against their living-room windows after ingesting fermented berries.

Properly planted with the correct exposure, mountain ash makes an for an excellent tree in most yards, provided you don’t mind cleaning up what’s left of the berries in early spring.  Mountain ash has few pests and disease.  Stressed and older specimens most commonly catch fireblight, a potentially devastating bacterial disease that is bad some years, and almost non-existent in others.  Mountain ash is a preferred species for eriophyid leaf mites, which cause unattractive protuberances on the leaves, but do not seem to affect the health or vigour of the tree.  I have also seen light infestatations of oystershell scale on mountain ash in Calgary.  This can be a more serious problem, and heavy infestations can be effectively treated with properly-timed insecticides or dormant oil sprays.

The main killer of mountain ash trees is poor branch stucture, which can easily be avoided with proper sructural pruning of young trees.  Left to their own devices, mountain ash will eventually self-destruct, as poorly attached limbs fail from the tree.


Filtered sunlight is ideal, to preserve the olive-coloured colour of the outer bark.  Unprotected trees in full southern exposure will develop scorched, orange-coloured trunks, that can later lead to ‘cat-faces’ or dead areas of trunk tissue.  If young mountain ash must be planted in full-sun as young trees, consider using a bio-degradabe trunk wrap to prevent sunscald.  Older trees manage well in a southern exposure as their canopies shade the trunks.

Pruning Time

Dormant season, preferably after leaf-drop, from late-October to April, before the buds break.  This will prevent the spread of fireblight, which is especially important in older specimens.


Prune these trees in the same manner as other medium-sized, flowering, ornamental trees, such as the chokecherries (‘Schubert’, mayday, and amur cherries).   Train as a central leader standard, with well-spaced scaffold limbs.  Do not allow multiple trunks to form from the base or from other areas on the main trunk.  While this is the most common scenario in the landscape, and perhaps the only form you will see in older trees, it ultimately is the reason for their early demise.  I suspect that properly trained mountain ash trees will outlive their predecessors by one to two decades or more.

Remove dead, damaged, and diseased limbs on an annual basis for young trees.  Older trees can be pruned on a 2-3 year rotation, depending on their appearance.  If fireblight develops, be sure to remove it during the dormant season of the year it is first noticed.

Crown Reduction

Generally inappropriate.  As with most tree species, mountain ash does not tolerate topping.  It is best to plant this tree allowing for its mature size and shape.


Normally unnecessary after the first year or two after planting if the rootzone is covered in a natural, moisture-conserving mulch, such as fine wood chips or pine mulch.  Mountain ash do not like ‘wet feet’, so do not plant at the bottom of a hill or in an area that is subject to standing water.

(c) 2016 Shane LePage, Wild Rose Garden & Tree Service Inc., Red Deer, AB.