Mountain ash should be pruned in late winter while they are still dormant. Wait until next March to remove this branch.  It may not look very large but it is a major limb or your tree. The tree is still very young and so it is stressed from being transplanted.  It will require several years before it can regrow its original root system.  Continue to water the tree during very hot or dry weather. Some areas of the state are abnormally dry for early spring.

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