One thing you have to be very carefull with mountain ash is that they are extremely susceptible to a bacterial disease called fire blight. This disease is very easily transmitted from tree to tree, or from other trees in the apple family, by bees when they pollinate the flowers in spring and it completely whiped out all the mountain ashes that were planted in our city a few years ago.
Here is a link that describes well the disease :
http://www.colostate.edu/Depts/Coop...Fruits/fire.htm