Hello,

There are several possibilities why this happened.  Drought  stress--- either from not getting well watered before the ground froze last winter or competing plants on that side of the landscape 'won' the water war. Salt damage or reflected heat damage from a nearby paved area. Wind-sun damage on one side, over the winter especially, killed the buds on that side. Other root zone issues- restricted amount of soil on that side of the plant. Lilac borer insects. Rotting or drowning roots from wet soils or standing water can be a problem especially if soil is clay.  And the Severe winters in 2014-2015 could have weakened part of the plant, which finally died this winter.

To test if branches are dead lightly scratch the surface of a twig. It should be greenish below bark, and white and moist in middle. You can prune off all truly dead branches. This will help light and air reach that side, encouraging new shoots to grow. It will take several seasons for new shoots on that side to grow and catch up with the rest.

Of course you want to correct the original problem if it wasn't due to those severe winters. I would have a certified arborist assess the tree and give you a plant health care program. Do this before you prune the dead branches so the arborist can see the whole problem. He/she can check the root zone, and check for pests. Find certified arborists by searching the following site under 'Find an Arborist'.  www.treesaregood.com

Thank you for using our service.

Hello again,

One other important consideration- if the root zone on that side was disturbed by digging, repaving or transplanting new material on that side-- this could have cut or disrupted roots on that side enough to kill the branches.

Thank you.