Lilac tree, or the Japanese tree lilac (Syringa reticulata), is an attractive deciduous tree with ornamental bark and particularly showy white flowers in very late spring. Native to Japan, this cold-hardy plant is tolerant of many soil conditions and is more resilient to pests and diseases common to other species of lilac. Cultivated varieties of this tree are preferred over the wild species for use in gardens.
Japanese tree lilac used in gardens is endemic to Japan. Two other natural variants of the tree have been found locally in Korea and extreme eastern Russia.
This lilac species is highly regarded for its tolerance to cold and ability to survive in many soil conditions. It is listed as an appropriate plant for culture in USDA Hardiness Zones 3 through 7. It is adaptable to a variety of soils, from those that are fertile and rich, to those that are compacted, waterlogged, stricken with seasonal drought and in urban areas where air pollution is common.
Japanese tree lilac becomes a large shrub or small tree with a singular trunk and oval to irregular but rounded canopy of foliage. It will reach a mature height of 20 to 30 feet with a spread of 15 to 25 feet. In very late spring or early summer, branch tips bear large clusters of tiny white to pale greenish-white flowers that have a mild malodorous fragrance. Its leaves are dark green and do not turn any color in autumn before dropping off for winter. The bark is a satiny smooth brown-gray with hints of mauve.
Plant lilac in full sun exposures, where it will receive at least 8 to 10 hours of direct sunlight daily. Although tolerant of many soil types, including both acidic and alkaline soils, the finest growth occurs in moist, well-draining, slightly acidic soils that are average to rich in organic matter. In regions with long, hot summers this tree is not at its finest.
Although the species form is attractive, three cultivated varieties or cultivars are preferred by gardeners for use because of uniform growing habits, increased flower production and better quality leaves. Cultivars Ivory Silk, Regent and Summer Snow are superior.
Japanese tree lilac is extremely resistant to common lilac pests and diseases such as powdery mildew, scale and borers. However, if the tree is stressed with infertile soils and drought, the resistance diminishes. Watering the tree during drought and fertilizing the tree annually with a slow-release granular fertilizer or replenishing organic mulch around the root zone keeps the plant healthy.