Answer: Lilacs may bloom less if they are in need of more sunlight, if they have been pruned at the wrong time of year, or if they have not been pruned regularly over the years. Routine pruning would be to remove the faded flowers and to remove any suckers smaller than a pencil each spring. After bloom, also remove some of the oldest stems by cutting them at the base, at the ground. This causes the plant to constantly renew itself and stay vigorous. It also helps it stay shorter and bushier.
There are two ways to rejuvenate an old overgrown lilac bush. One is to do it slowly over three years, the other is to do it all at once.
The "one third" method takes 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. In year one, remove the oldest one third of the shoots/trunks/stems at the base. In year two, remove half of the remaining old trunks and stems. In year three, remove the rest of the original trunks and stems. Also, each spring remove any new sprouts smaller than a pencil. Do the pruning only in the spring right after blooming time and cut as close to the ground as you can. Since this plant blooms on old wood, pruning later in the season may remove the next year's flowers.
If you are truly impatient, you can also cut an old overgrown lilac bush off entirely 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. This is best done in the spring right after bloom, or if you don't mind sacrificing a year of blooms, do it in very early spring.
Finally, lilacs 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. It is also worthwhile to note that some lilac varieties bloom better in alternate years, so the display is spectacular one year and lackluster the next. There is nothing you can do to change that.
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