I have a lilac bush given to me as a gift 13 years ago. I
don’t know the variety but the leaves look slightly different from the
common lilacs I see. This bush has healthy looking leaves and while it
has slowly put on growth over the 13 years it has never bloomed. I have
tried adding ashes to the soil to make it more alkaline but nothing seems
to work. What is the problem and how can I get this bush to bloom?
There are several reasons lilacs may fail to flower. Here is an excerpt (no longer available online) from North Dakota State University Extension horticulturist Ron Smith in
answer to a question similar to yours:
Lilacs fail to flower because of insufficient sunlight, planted too
deeply, too much nitrogen, improper pruning or winterkill of the flower
buds. You said the lilacs get plenty of sunlight, but unless you used a
lawn fertilizer to provide nutrients, it isn’t likely too much nitrogen
is the problem. If you planted too deeply, pull some of the soil back so
the top of the roots are slightly exposed. If you pruned in July, then
doing so removed the flower buds for the next growing season. If winter
killed the flower buds, then hope for milder winters or purchase hardier
lilacs.
Colorado State University Extension’s article, “Renewing Lilacs,” (no longer available online) offers other suggestions, such as late freezes, decreasing sunlight, and pest problems.
Sunset’s Western Garden Book (2001 ed.) says that annual pruning is
needed for optimal flower production. Most lilacs bloom on wood formed
the previous year, so they should be pruned just after flowering. Remove
the spent blooms and cut back to a pair of leaves. There are a few lilacs
which bloom on new growth, so it might be useful to know exactly what
type of lilac you have. You could send photos or bring in samples for identification.