Japanese tree lilac (Syringa reticulata) has a natural grace. When planted in full sun and moist, well-drained soil, it rarely needs pruning. Preserve its graceful, informal habit by limiting pruning cuts to those that are necessary to maintain size and create a healthy branching structure. Remove dead and diseased wood and any crossing or rubbing branches. If you must prune the plant, do so right after it flowers in early summer. If you prefer, you can trim off the developing seedpods immediately after bloom to maintain a neater appearance.