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Renewing shrubs
With time, and in some locations, lilac flowering may decrease. But
before you take drastic measures, consider all the potential reasons your
lilacs aren't flowering. For example, a late freeze may eliminate flowers
for one season. Shade from new buildings and maturing trees also limits
bloom. Plants need at least five hours of sunshine daily to bloom well, so
you may need to thin overhead trees to let in more light. Fertilizer is
rarely the answer for a plant that once bloomed well.
 Renewal
pruning solves the problem of decreased bloom, as well as problems caused
by oyster shell scale and borer insect buildup. Remove the oldest,
thickest stems at the base to prune away insect problems. The remaining
young stems will grow and produce the desired blooms. January to mid-March
is the best time to prune.
Consider removing one-third of the older stems this year, another third
the following year, and the remaining third the next year. This is the
best way to maintain lilacs and many other shrubs. This method produces a
better looking shrub than tip pruning alone, which should be done only to
shape the shrub.
Older, overgrown lilacs may require drastic rejuvenation pruning, which
means cutting off every stem four inches above the ground. Resilient
lilacs will grow two to three feet the first season.
Thinning stems so that only a few healthy ones remain eliminates
competition among them. From then on, yearly renewal pruning will keep
your lilac blooming.
For more information, see the following Colorado State Extension fact sheet(s).
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