Lilacs are a versatile and hardy shrub, well-loved for their beauty and fragrance. Some lilacs can actually live for centuries, but for maximum health and blooming, they require some maintenance.
When neglected, they can reach 15 to 18 feet high with leaves and blossoms only at the very top of the branches; they become leggy, with limited foliage and few blossoms. They are also susceptible to certain pests and diseases, including borers, scales and powdery mildew. These problems can often be controlled through regular maintenance and the use of appropriate pesticides, when necessary.
The process for rejuvenating leggy, sparsely leafed bushes generally requires several years of appropriately timed pruning and regular care before the bushes are likely to regain dense foliage and substantial blooms.
In determining whether the investment of time and effort needed to restore lilacs is worthwhile, several things should be considered:
- Lilacs require full sun to flower well. If the bushes are in a shady spot, you may succeed in restoring the plants’ foliage, but without full sun, substantial flowering is unlikely.
- Lilacs prefer soil with a ph of 7.0 to slightly alkaline, and good drainage. The soil should be tested to assess the need for amendments. Excess nitrogen will encourage leafage, but inhibit flowering.
- Inspect for pests. Holes of one-sixteenth to one-eighth inch diameter in stems and branches is evidence of borers, and a professional arborist should be consulted. Scale insects can be managed using a hard spray from a garden hose and the use of dormant oil and summer oil.
- Powdery mildew should not have a significant impact on the plant, but pruning to increase air circulation will help to manage or prevent it.
The next step is pruning. Remove all pest-infested branches and stems that cannot be sufficiently managed through other means. Then, prune about one-third of the oldest branches (canes). Some experts recommend cutting these right to the ground to produce the greatest amount of new growth, but others caution that it may be overly stressful for plants already suffering the consequences of poor maintenance. Consider pruning to eye-level, ideally, just above a set of leaves (if any are present at that level).
Pruning should take place right after flowering, and before seed formation. Any spent blossoms should also be removed at that time. Permitting lilacs to form seeds may inhibit flowering the following year. Also, lilacs set their flower buds the summer before the year of bloom, so winter pruning should be avoided.
Vigorous growth should begin to appear at the base of the bush. Repeat the pruning process described above in each of the two subsequent years. Also, remove excess suckers and stems to prevent the bush from becoming overgrown. Within three to four years, the lilac should be fully rejuvenated with strong flowering. Starting in year four, continue an annual pruning schedule, pruning one-eighth to one-sixth of the oldest canes. At this point, the bushes should be healthy enough to prune the canes right down to the ground.
Of course, keep an eye out for pests as part of regular maintenance and trim other plants and trees as needed to ensure the lilacs continue to enjoy full sun. With proper care, you should be able to enjoy your lilacs for years to come.