How can I get transplanted lilacs (ordinary ones) to grow into a privacy bush? Should I wait a year or two and then prune heavily? Except for the powdery mildew, they seem to be growing well. I want to hide my near neighbors air conditioning
Wait until next spring after bloom time, if you have any. This gets you in the habit of pruning lilacs annually which they need. By nature, lilacs sucker a lot so you should see new stems emerge next spring whether you prune or not. In fact, routine spring pruning includes removing excessive numbers of suckers. They crowd the interior, reduce airflow (making powdery mildew worse) and create so much shade that some interior branches lose their leaves. In order to preserve as many leaves so the slip can grow, perform only a heading cut on the long, slender branches. Consider removing one of the bushes; they are too close together for the large shrub.