Answer: Lilacs may bloom less if they are in need of more sunlight or if they have not been pruned regularly over the years. There are two ways to rejuvenate an old overgrown lilac bush that doesn't bloom much as a result. One is to do it slowly over three years, the other is to do it all at once.
The one third rule takes requires several years to take full effect, but in three years you will have removed all of the original older taller wood by cutting it at the base. Remove the oldest one third of the shoots/trunks/stems at the base in spring each year, then remove anything smaller than a pencil also at the base each year. Do the pruning only in the spring right after blooming time. This plant blooms on old wood and later pruning may remove the next year's flowers.
If you are truly impatient, you can also cut an old overgrown lilac bush entirely off at the base. This will stimulate a forest of shoots which will need to be thinned that summer and the following spring, but it can work wonders for the impatient among us.
Finally, lilacs also seem to appreciate an annual topdressing of well rotted manure and/or compost as well as an early spring application of a balanced fertilizer such as 5-10-5 according to the label instructions.
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