September 2001
Plant of the Month
Showy Mountain Ash
(Sorbus decora)
zone: 3
by Tim Power

Plant of the Month
September 2001
Showy Mountain Ash
Sorbus decora

Over the last several years, tree growers have noticed a remarkable shift in demand from traditional shade trees toward trees with color, seasonal interest and with proportions more closely matching suburbia's lower, more sprawling houses. As we tried to adapt to this change, we broadened our selection of ornamental trees, not just with crabapples, but with Prunus, Cornus, Amelanchier, Viburnum. One of our old standby trees which is finally catching on as a result of this change toward ornamentals is Showy Mountain Ash, Sorbus decora.
Mountain ash has been a part of our offering for many years, but years ago we felt that the genus was hard to grow well, touchy in digging, and had a tendency to tip on its own roots in the nursery. In retrospect, much or our dissatisfaction really revolved around European Mountain Ash (Sorbus aucuparia). We had horrible infestations of fire blight and a fungal disease called black rot. Since we dropped European Mountain Ash and its varieties several years ago, we seldom see fire blight and we never encounter black rot anymore. The issue of tipping over on the nursery is now less prevalent because our liner suppliers are doing a better job o putting a vigorous, diffuse root system on our mountain ash liners.
Other mountain ash which hold promise include Oakleaf Mountain Ash (Sorbus x hybrida) and Korean Mountain Ash (Sorbus alnifolia).
Showy Mountain Ash is native to Minnesota and is particularly prevalent along the North Shore of Lake Superior. Its native range extends east to Labrador and south to New York. Like most trees, it performs best in deep rich well-drained soils, but it is remarkably adaptable as long as it does not have wet feet. We have shipped it B&B into North Dakota, Montana, Colorado and Wyoming without major problems, though we recommend against using it on very high pH soils. We have shipped it both bare-root and B&B to Alaska, one of only five or six plants we ship that far north.
Insect problems in mountain ash include round headed apple tree borers, aphids and mountain ash sawfly larvae, though we seldom see infestations which justify spraying on the nursery. Most insect infestations are the result of stress and can be mitigated by site improvement or, better yet, picking the right site in the first place.
Bailey's catalog shows Showy Mountain Ash as reaching 20-25 feet high, and 20 feet wide, about the size of a large crab. Under less than ideal condition it may remain quite a bit smaller. Grown in the open, it develops a full, rounded form. It will be taller and lankier in the woods and does not do well in full shade. Leaves are a rich dark green above and are slightly larger and more rounded than those of European Mountain Ash. Fruit is abundant, attractive and bright red, and is the feature for which the plant receives its name (decora = handsome or showy). Rather than being messy in the fall, ripe fruit is stripped by the birds long before it would otherwise drop. Fall leaf color fades from green to yellow, then often to burgundy red before dropping.
Though it is listed as a slow grower, we have found that its growth rate on the nursery can be accelerated by removing flower heads once they form. This procedure not only speeds growth, but it keeps plants growing upward with a central leader, since there is not heavy fruit to deform the branches. Plants then put their energy into vegetative growth instead of fruiting. We can produce 1-3/4 inch caliper trees in four years from 3 or 4 foot-whips. Starting with a larger branched liner often results in a coarser, less attractive plant at the sizes which we sell (1-1/2 to 2-1/2 inch caliper), though they eventually make up just fine.
By Tim Power, Law's Nursery, Inc.

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