Q: Our mountain ash tree seems to be dying off on the left side only. It's been doing that for about 3 years now.
The time to do something about this tree was when you first saw it was in trouble. There could be many reasons for the dieback. I recommend you get an onsite inspection from a certified arborist. To find a professional in your area go to www.tcia.org and enter your zip code. The dead parts of the tree cannot be brought back to life so it might be time to look for a new tree. Best And Happy Yardening, Nancy