Originally Posted by Iceblossom

Lilacs typically grow rather slowly and even if grafted onto root stock, it is generally a smoother leaved plant.

I tried a google image search and it suggested kratom -- while appearances are similar I very seriously doubt costco would be selling or that it would survive winter!

I'm wondering if whatever it is, it was mislabeled, or perhaps an opportunistic plant managed to get safely into growable and protected soil. Do you have any alder species near you? Or perhaps beech?? The leaves seem darker green than alder and more textured, I don't have experience with beech trees but some of the leaf pictures were close.