Albeit belatedly, this spring we pruned back the Arctic Willow that had deprived our lilac of light. Given the lilac’s current shape (photo attached) I’m unsure how, or whether, pruning might restore some lower branches to grow. How severely can one prune a lilac?
Based on the picture, this lilac is definitely reaching for the sun. Lilacs can live to be a hundred years old plus. They are hardy plants that can be pruned back dramatically so don’t give up. The best time for rejuvenation (major) pruning is late winter or early spring before the leaves appear. This allows you to see the shape of the tree and to better decide how much to prune. In the Ask a Master Gardener portion of our website there are clear pruning instructions in Lilac Tree Near the End of Life, posted July 1, 2012. I am also attaching a link to another website that addresses rejuvenation pruning: