University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article
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MOUNTAIN ASH TREES
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
Mountain ashes are small trees for cool northern landscapes, particularly attractive in fall with their orange or red berries.  Some may be short-lived, due to disease or pests.  Some have been crossed with other trees to produce offspring with edible fruit.
           
There are several small trees that go by this name, the most common being the European mountain ash (Sorbus aucuparia) and its cultivars (cultivated varieties).  Most reach 25 to 30 feet tall, have leaves divided into many small leaflets, and are showy with white flower clusters in June leading to clusters of attractive orange or red berries (botanically these are “pomes”, similar to apples) in autumn.  Fruit are quite attractive to wildlife and birds.  Leaves turn a nice golden brown in autumn.
           
Quite cold hardy (USDA zones 3-6, or average winter minimum of -40 to -10 degrees, F), these, like their relatives, don’t grow well in hot climates.  Generally they don’t grow well if the soil is too dry or too wet, nor alkaline.  Best is a moist but well-drained, acidic soil.
           
Similar in characteristics to the European is the native American mountain ash (Sorbus americana), seldom found in nurseries.  You may see this one along cold swamps in mountain areas.  Another native is the Showy Mountain ash (Sorbus decora), having berries about twice the size of the European.  Of the three, the European is perhaps best in landscapes and better adapted to variable conditions.  None of the mountain ashes grow well with air pollution of urban areas.
           
The Korean mountain ash (Sorbus alnifolia) is slightly larger than its relatives, growing 25 to 40 feet high and 35 to 35 feet wide.  It has a rounded shape, while the other mountain ashes are upright oval.  The white flowers in late May (in the north) are followed by small, pink red fruit and golden-orange leaves in autumn.  Leaves aren’t divided into many small leaflets like its relatives, but rather look more like alder leaves—hence the species name relating to the alder genus (Alnus).
           
The good news is that mountain ashes aren’t attacked by the emerald ash borer, being in a different genus from other ash trees (Fraxinus) and even different family (rose family, while other ashes are in the olive family). The bad news is that, similar to pears and apples, they may get fireblight disease. This causes ends of branches to suddenly turn black and droop, as if burned. 
           
Canker disease on trunks, and sunscald (winter burning of trunks from sudden heating by sun on cold days), also may damage trees. Borers are an insect pest that is common on many, but often less a problem on the Korean mountain ash.  These main problems may occur more often on trees stressed by other pests such as aphids and scale insects, or diseases such as crown gall, scab and rust.
           
You may find cultivars of mountain ash that are improvements over the species.  Red Cascade is a showy, compact (30 feet high and 20 feet wide at maturity) cultivar of the American species with bright red fruits and orange-yellow fall color. Cardinal Royal is a vigorous cultivar of the European species, having bright red berries as indicated by the name, and yellow to red fall leaves.

Fruit of these ornamentals is not edible fresh, being quite bitter, except for some cultivars selected from its native Europe and Asia.  ’Rabina’ is a cultivar from native stands in Russia, selected for its tart but sweet, bright orange fruit.  ‘Rosina’ is a cultivar from the east of Germany, selected for its reddish-orange fruit that sweeten after frosted in fall. 

Mountain ash fruit are high in vitamins A and C, as well as niacin.  In addition to the sweet fruit of these cultivars being eaten fresh, from their pectin content they are good for jams.  “Rowan berries”, as the fruit of the species is called in Europe, make a slightly bitter jam or jelly good with venison.  They contain sorbic acid, named from the genus.  Europeans also use them to fortify wines and cordials.

This tree is interesting in that has been crossed with other fruits to make new ones.  The ‘Shipova’ (x Sorbopyrus auricularis) is one of those rare crosses between different genera, this between the mountain ash and a pear, resulting in a hardier fruit (zones 3-9).  The fruit resemble a small, rounded pear with a reddish tint.  It is sweet, the taste being similar to a pear.

Another bigeneric cross, this one between the mountain ash and chokeberry, resulted in ‘Ivan’s Beauty’.  It is a small tree, only reaching about 10 feet high, with dark green leaves larger than those of the mountain ash.  Its dark purple berries are good fresh, or used to make a juice.

Ivan Michurin was a famous Russian fruit breeder in the late 1800’s and early 1900’s, known for crossing fruits of quite different geographic regions and ending up with over 300 new species.  He came up with another bigeneric cross in addition to the above, this one between the mountain ash and the hawthorn and known as ‘Ivan’s Belle’.  This too has large, attractive dark green leaves on a small tree.  The sweet, yet tart, berries are wine red and the size of small cherries.  Use them fresh, in jams, and even for wine.  These less common mountain ash with edible fruits are usually found in specialty nurseries and from online sources.

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