The lilac tree (Syringa) grows as a tree or shrub and belongs to the family of Oleaceae, or olive. It normally attains a height of 15 feet with a 12-foot spread but a few varieties and cultivars grow larger or smaller ones. In early summer, from May to June, the tree produces abundant fragrant blossoms in shades of purple, lavender and white for approximately two weeks. Popularly grown as a landscape specimen, the lilac tree also makes an ideal hedge or privacy screen. It grows well in USDA hardiness zones three to seven.
The lilac tree enjoys full sunlight for maximum blossom production but also grows well in partial shade. It will withstand a wide range of well-draining soils. The soil should either be slightly acidic or alkaline in nature. Loam soil is the preferred lilac planting medium but the tree will grow in numerous soil types with a pH of 6.5 to 8.5. Maintain a planting distance of 10 to 15 feet between lilac bushes. Plant hedge lilac trees 6 feet apart.
Lilacs require pruning to maintain their overall appearance. Prune lilacs after blossoming takes place. Remove all spent flower heads. Prune the stems and branches to keep the tree's appearance neat and tidy. Never remove more then one-third of the tree. Drastic pruning will affect the following years flower production and sometimes delay blooming for up to four years. Maintain at least 10 stems of varying ages to ensure adequate flowering the following year.
Flower panicles appear in large clusters. The blossoms make excellent cut flowers due to their heavy fragrance. Most lilac tree varieties bloom from May to June but a few hybrid varieties such as the S. x prestoniae and S. x josiflexa lilacs offer later season blossoms according to the University of New Hampshire.
Pull up any suckers growing at the base of the tree to prevent it from becoming multi-stemmed. Hand pulling the suckers helps delay their return longer then clipping. Numerous lilac cultivars are grafted onto rootstock so suckers will not grow true to the parent tree's type. On non-grafted lilac trees, the suckers can be dug up and transplanted to new locations.
Maintain moist soil around the newly planted lilac tree until fully established. Applying 2 to 3 inches of mulch helps keep the soil moist and weed growth reduced. Water established lilacs once a week. Apply at least 1 inch of water per week to the lilac tree according to the Harvard University Arnold Arboretum. The lilac tree is drought tolerant once established but prefers ample water to thrive.
Pests and Disease
When planted in the shade, the lilac tree will often suffer from powdery mildew that is easily controlled with a fungicide. The tree has few pests. The lilac borer larvae can tunnel into stems if the lilac tree is weak. Maintain the tree's health with regular watering and fertilizing. The lilac leaf miner is easily treated by simple hand removal. Scales appear on the stems of the lilac tree but spraying with a horticultural oil will control the pest.