Q. My daughter has just moved into a house that has old growth lilacs and we need to know how to prune them without causing too much stress on the plants. Do you have any ideas?
A. By the way, Persian lilacs grow and bloom better here than the common lilac and are very showy. However, the lilac variety “Lavender Lady” requires less chilling and blooms very well here. These are not plants for rock landscapes but should be in the high water use zones and the soil should be amended well at planting and covered with organic mulch.
For good-looking lilacs always start pruning at the bottom of the shrub. At the bottom, identify the two or three largest stems coming from the base. Remove them with clean cuts close to the ground. I can't see the shrub but several smaller stems should remain that supports flowering for next year.
What you are trying to accomplish with many woody shrubs is to renew the shrub with new growth on a constant basis. You do this by removal of the largest stems close to the ground. This should cause smaller and newer growth to originate from the base keeping the shrub green, juvenile and full of flowers and leaves from top to bottom.
Every couple of years, repeat this type of pruning; remove the largest stems at the base. If done correctly, this will keep the shrubs renewed and looking good. This is all you need to do unless you have some crossed or broken branches at the top that you need to remove.
Don't forget to fertilize the with a good quality fertilizer made for woody plants. Fertilize plants whenever you take anything from them (pruning) or they give you something (flowering). So when you’re pruning or they bloom for you, you need to give back to them lightly in the form of a fertilizer. You can use fertilizer stakes. Put fertilizers close to the emitters or their source of water. Do this in late January through March and make light applications right after they finish blooming. I hope this helps.