When is the best time to trim the tree?
The mountain ash is a tree commonly grown for its low maintenance and pleasing branching and overall shape. Because it is fast growing, it may need some care to encourage growth in the right direction. As with most deciduous trees, the mountain ash should be pruned in late winter to early spring, before foliage appears. Pruning later would risk the bacterial disease of fireblight in a mountain ash.
There are several good book references on pruning. Lee Reich’s The Pruning Book, suggests that a mountain ash would have some difficulty tolerating severe pruning. It is important not to neglect the task to avoid drastic action.
The Hillier Gardener’s Guide to Trees and Shrubs offers more specific information on pruning the mountain ash. Because it is susceptible to some pests and diseases, any dead branches should be removed immediately.
The following link provides good information on pruning.
Please note that after a severe drought season such as we have experienced in 2016, leaving the tree until late February next season will give the tree a chance to strengthen again and be able to tolerate pruning.