- Lilac bushes (Syringa L.) are hardy to temperatures as low as 60 degrees Fahrenheit below zero. This hardiness does not extend to high temperatures. Lilac bushes are not suited for warm climates. Prolonged mild temperatures prevent lilac bushes from entering a needed dormant period. Without this dormant season the bush will not flower in spring and over an extended amount of time the bush will die. High humidity and warm weather encourage pests and disease from which lilac bushes, planted in these adverse conditions, will not recover.
- Site your lilac bush is a well-draining soil patch with full sun. Most lilac shrubs do well when planted in the late fall before the ground freezes. In extremely cold climates plant the bush in spring.
The nursery line is the mark on a tree or shrub's main stem indicating its planting depth. Plant nursery-bought lilac shrubs 2 to 3 inches deeper than the nursery line. Planting at a depth lower than 2 to 3 inches can kill the bush. Once the ground settles, your lilac bush's soil should be even with the surrounding ground. Mulch around the bush's base. Mulch provides needed insulation. When ground freezes, heaving can occur -- such buckling damages lilac bush roots and can cause death. - Removing more than one-third of the bush's overall growth results in plant death.
Early spring is the best time to prune lilac bushes. Pruning at other times destroys next season's buds and the result will be a flowerless bush come spring. Pruning in summer exposes young branches to lilac borers, an insect that enters stems and branches where it breeds. Lilac borers cause shrubs to wilt and appear lackluster. Oozing sap brought on by borers will likely attract other insects like yellowjackets and aphids. Apply lindane spray to infected trees. Fortunately none of the other diseases and pests likely to attack a lilac bush are severe enough to cause plant death. Use common insecticides and horticultural soap and oils to control powdery mildew and other problems. - Choose a lilac bush in accordance with its landscape purpose. A dwarf lilac will not take on a tree form even when pruned into that shape. Allow little-leaf lilac bushes room to spread and large tree lilacs overhead room to reach their full 25- to 30-foot heights. Encourage healthy bushes by deadheading flowers as soon as they fade. This improves health and vigor.
Lilac bushes are sometimes killed by grazing deer and root-munching gophers. Contact your local university extension for assistance with animal control measures.
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