I have a lilac on a standard in a flower bed… likely 10- 15 years old and I prune it back every year to about the same size.
Problem is several of the main branches from the graft on one side have died and left one side of the top with a big empty area. How can I encourage growth from the bottom of the remaining heathy stems so that it will fill in the big gap.
Btw, I am in SW Ontario near Sarnia
Thank you for contacting the Toronto Master Gardeners with your question.
I notice that you already take care of your lilac and give it regular pruning. As well as including information on pruning in general, the links below offer some insight that should help you with your question. They both include some useful diagrams too. The importance of soil health is also emphasized. Once you have cut your lilac back, be sure to add some organic amendment such as compost to the soil around it.
https://www.thespruce.com/pruning-lilacs-how-and-when-to-prune-lilacs-1403004
https://www.finegardening.com/article/pruning-lilacs