Overall, lilacs are easy to care for and problem-free shrubs. Occasionally, problems arise like failure to bloom, flowers opening out of season, powdery mildew, and other disease or insect issues.
More information about common lilac problems and how to manage them is presented below.
Cultivars of the common or French hybrid lilac (Syringa vulgaris) often do not bloom for several (five or more) years after planting. The shrubs must grow and mature before they are capable of flowering.
Plants should receive at least six hours of direct sun each day. Lilacs planted in partial shade will not bloom well.
It is generally not necessary to fertilize lilacs. However, lilacs can be lightly fertilized in early spring. Heavy fertilization may promote excessive vegetative growth and discourage flowering.
Pruning can also affect flowering. Lilacs bloom on the previous year’s growth. The best time to prune lilacs is immediately after flowering in spring. Pruning lilacs in late summer, fall, or winter may remove many of their flower buds.
While the common lilac usually doesn’t bloom for several years after planting, several other lilacs bloom when quite small. The dwarf Korean lilac (Syringa meyeri), ‘Miss Kim’ lilac (Syringa patula ‘Miss Kim’) and Preston lilacs (Syringa x prestoniae) often flower within one or two years of planting.
Occasionally lilac will bloom out of season during the fall months. This out-of-season bloom is usually triggered by stressful environmental conditions during the summer growing season such as heat, drought, severe defoliation from disease or pest, and/or heavy pruning.
These spring blooming plants set their flower buds for the following year in early summer shortly after the previous spring blooms fade. Stressful conditions put the plant in a dormant-like state and when the cooler temperatures of fall arrive, some of the those flower buds are triggered to bloom.
This untimely bloom will not harm the plant, although there will be fewer flowers on the plant the following spring. Once fall blooms are observed, there is nothing that can be done to fix the issue. In future years, good care when there are stressful growing conditions will help reduce the chance of the out-of-season bloom.
Some spring blooming species have varieties or cultivars that have been selected because they will bloom in fall - although that late season bloom is not typically as floriferous as the spring bloom. Bloomerang® lilac (Syringa 'Penda') is one example. These plants are intended to bloom in the fall and are often marketed and sold as special cultivars that regularly have blooms in both spring and fall. Fall blooms on these varieties are normal.
The white substance is likely powdery mildew. Powdery mildew is a fungal disease. It appears as a white, dusty growth on plant foliage. Amongst lilacs, the common lilac (Syringa vulgaris) is highly susceptible to powdery mildew, while the Preston lilacs (Syringa x prestoniae) and dwarf Korean lilac (Syringa meyeri) are resistant.
Powdery mildew is favored by high humidity, cool nights and warm days. Plants growing in partial to heavy shade are most susceptible to powdery mildew.
Powdery mildew does not cause serious harm to lilacs. The damage is mainly aesthetic. Spraying with a fungicide is not warranted. When planting lilacs, select a site that receives at least six hours of direct sun each day. Powdery mildew will not be a serious problem in sunny areas. Judicious pruning of nearby trees (to increase the amount of sunlight) should help reduce the severity of powdery mildew on lilacs growing in shady locations. Another option would be to transplant the lilacs to a sunny site.
Overall lilacs are easy to grow and relatively pest and disease free. Powdery mildew is a common issue with some species of lilac, in particular the popular common lilac (Syringa vulgaris). When evidence of disease or insect damage are observed, there are a few issues that can cause these problems.
- Pseudocercospora Leaf Spot
- Phytophthora Shoot Blight
- Lilac Bacterial Blight
- Lilac Ash Borer
- Oystershell Scale
- Verticillium Wilt
Over time lilacs can get large, leggy and unattractive. Often this is paired with poor bloom. Pruning these large, overgrown shrubs has special considerations outlined in this article.