Answer: The one third rule takes several years to take full effect, because in three years you will have removed all of the original older taller wood by cutting it at the base. To encourage the plants, remove the oldest one third of the shoots/trunks at the base in spring each year, then remove anything smaller than a pencil also at the base each year. If it still seems overly dense, you can remove a few more shoots again working at the base. Do not prune except for immediately following bloom because this plant blooms on old wood and later pruning may remove the next year's flowers. The plants 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.
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, but it can work wonders for the impatient among us.
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