Japanese Tree Lilac (Syringa reticulata).
From: http://bernheim.org/explore/arboretum/bernheim-select/japanese-tree-lilac/
“Japanese tree lilac is native to northern Japan. Plants are found growing on cliffs and in scrub, usually on limestone. The Olive or Oleaceae family also contains privet and forsythia. Syringa has about 30 species of trees and shrubs native to Europe and Asia. Japanese tree lilac is the only species that attains a tree-like form and size.”
Westmount Overlooked 