Overgrown Lilac bushes
Thank you for the question. Cutting the lilacs off at 5 feet will give you many bare trunks which aren't very pleasing to look at. Your thoughts on pruning are good however, because older lilacs respond well to regenerative pruning. The best time to do it in our area is after they bloom in the spring. If you prune them this year, you will cut off the spring blooms.
We suggest taking about a third to half of the oldest branches down to the ground each year. By the third year you will have renewed the plant. The oldest canes have scaly, cracked bark and dry, brittle wood. The shrubs will respond by producing new shoots from the base of the plant.
Keep in mind that lilacs like and need a lot of sun to bloom well. It looks like trees have grown and shaded them so they will stretch toward available light and not bloom well. If lilac blooms are important to you, consider thinning/removing nearby tree canopy to increase light.
To keep your lilacs looking their best, they need to be pruned, fertilized and shaped almost every year, soon after they are done blooming in the late spring.
Here is our publications on pruning trees and shrubs: http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/pruning-trees-shrubs/
Thank you for contacting Extension.
Thank you for this detailed reply! I appreciate your knowledge and will take your advice.