Lilac (Syringa vulgaris), Rose of Sharon (Hibiscus syriacus) and forsythia (Forsythia) are deciduous flowering shrubs that grow in U.S. Department of Agriculture plant hardiness zones 3 through 8, 5 through 9, and 5 through 8, respectively, based on cultivar. Lilac boasts fragrant white or purple flowers and is a spring-bloomer like the forsythia, with its yellow flowers. Rose of Sharon is a summer-blooming woody shrub that provides many different flower colors, depending on the cultivar. All benefit from annual renewal pruning, while lilacs and forsythia can also be cut back severely to rejuvenate older plants.
Cut back overgrown lilac or forsythia bushes to ground level or 1 to 4 inches above ground level to give older, sparsely flowering shrubs fresh life. Do that rejuvenation pruning early in the spring, before new growth begins.
Prune out one third of the older stems of three shrubs annually after flowering has stopped; that will be at the spring to get lilacs and forsythia and late summer or fall for Rose of Sharon. Cut the person stems back to the base of the plant.
Go back lilac bushes at the same time as the renewal pruning. Cut stems that are taller than the remainder of the tree or that stick out. Make the cuts just above a bud, leaf node or lateral branch at the desired height on the stem. Cut at a 45 degree angle sloping downward in the point about one-quarter-inch over the bud.
Remove dead, damaged or diseased limbs at any time during the year. Cut dead stems back to the base of the plant. Cut damaged or diseased stems back several inches into healthy wood, making the cut just outside a grass.