A common problem with lilacs,
especially the common
is that over time, they flower less and less. This
is usually due to the fact that most of the canes
have become too old to bear flowers properly. This
is the time to do some rejuvenation pruning.
Common lilac normally consists of
a number of stems (canes) coming out of the
rootstock. Some of the stems grow taller than the
rest and shade them, thus stunting their growth in
the process. Old wood on any plant tends to produce
fewer flowers than newer wood. So, the way to get
more flowers is to get rid of the oldest wood to
allow the newer canes to grow and flourish.
To rejuvenate this type of old
lilac, just cut out the thickest, oldest stems after
the plant stops flowering in the spring. Take out
about 15-20% of the canes each year so that about
every 5 years the entire shrub will be rejuvenated.
This will leave the younger, most floriferous canes
to produce more flowers. They will also be healthier
and a bit shorter than the very old canes you have