QuestionsHow to growTrees and ShrubsPruninglilac. 0 Vote Up Vote Down noreen asked 16 years ago Dear Gerry ,could you please tell me if and when I can prune a lilac bush , it is 6foot high by 4foot wide and is too big for the shrub border.Thank you , Noreen 1 Answers 0 Vote Up Vote Down Gerry Daly Staff answered 6 years ago At that size it is still small in fact. Any pruning will lose flowers so it is perhaps best to replace it.