Time to get drastic with the pruning and renovate those old and lazy lilacs. After they bloom this year use a saw to cut out the oldest and thickest trunks. This will encourage new sprouts but may delay blooming. If you cannot take such drastic measure just remove one third of the old, thick trunks each year for 3 years. After pruning use a slow release plant food like Osmocote and add a mulch of compost around the base. Lilacs love an alkaline soil so adding dolomite lime or wood ashes will help to make the soil less acid. You might also consider one of the new better behaved lilacs. Miss Kim is a tidy dwarf and Bloomerang is a reblooming lilac that is said to flower all summer. If you do go new, let us know how these new lilacs work for you. Keep growing, Marianne Binetti