How big and invasive are the roots of lilac bushes?
I want to landscape a small backyard and plant a lilac bush close to the fence. There is a concrete retaining wall. How far do I have to stay away from the concrete?
Lilac aren't considered invasive. There are many varieties; make sure you know the name of the cultivar you choose, and check the plant tag for size at maturity. If there's no plant tag, get the information from the garden center. Then plant far enough from the wall to allow the bush to mature without squashing up against the wall. Here are some tips for lilac growing: https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-care-growing-and-planting-lilac-bush-plants.htm