The Japanese tree lilac (Syringa reticulata) is a native of Japan. It is a very large shrub or small tree that produces very large, showy flowers in the summer. The trunk can be trained to grow straight until it is about 10 feet tall, at which point it branches into a stiff, upright, rounded head of foliage. Its flowers grow on 10- to 12-inch inflorescences and lend the tree its appeal.
Flowers of the Japanese tree lilac differ from the spring-blooming lilacs that most people are familiar with. Tree lilacs produce abundant clusters of creamy white flowers for two weeks in the summer. The fragrance of the tree's flowers is not as pleasant as spring-blooming lilacs, and some may find the scent musky and unpleasant. While the flowers may not smell very good, the sight of a 20-foot tree covered with clusters of white flowers certainly makes up for it.
A tree lilac grows 20 to 25 feet tall, and the crown has a spread of 15 to 25 feet. Its branches are stiff and spreading, and it develops an oval to rounded shape. The branches droop as the tree grows, and it will require regular pruning to maintain the shape and keep vehicle and pedestrian areas clear of hanging branches. It grows naturally with multiple trunks but can be trained to grow with only one and needs little pruning to develop a strong structure.
Tree lilacs are relatively pest free but not completely. Like all lilacs, they are susceptible to attack from lilac borers, which tunnel holes in the branches, causing the tree to wilt and the branches to break, especially in drought-stressed trees. Trees that are regularly irrigated during dry spells are less likely to suffer permanent damage from pests and are more likely to fend them off.
The cultivars 'Ivory Silk' and 'Summer Snow' are more consistent than the species and have more flowers. Ohio State University names 'Ivory Silk' the "cultivar of choice" because of its habit of flowering profusely once it reaches a height of 10 feet.
Tree lilacs are a great choice in many different landscaping schemes. They are becoming more common as street trees in urban areas because they tend not to interfere with overhead power lines and can tolerate poor soils. They also make good shade trees, are excellent specimen trees and grow well as a shrub border. They are often planted on highway medians for beautification purposes and function as farmstead windbreaks. They are of little value as a food source for wild animals but may be of some value for nesting songbirds.