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With regular pruning, the common lilac (Syringa vulgaris) maintains an elegant natural form and produces an abundance of fragrant blossoms. Without regular pruning, it soon turns into a mass of gangly stems and produces only a few flowers toward the top of its tall branches. Lilacs are hearty plants. Even the most overgrown specimens can usually be rescued and restored to their previous vigor using some specific pruning techniques.
Lilac wood must be three or more years old to produce blossoms, so with hard pruning, expect to sacrifice flowers for at least three years. Start by cutting the entire plant back to within 6 to 8 inches of the ground in late winter or early spring (March or early April). Large shrubs like lilacs have extensive root systems.
A severe pruning stimulates the plant to send up a large number of shoots during the growing season to replace the growth lost to pruning. In late winter of the following year, select and retain several of the strongest, healthiest shoots to form the main framework of the shrub. Remove all others at ground level. Cut back the remaining "framework" shoots to just above a bud to encourage branching.
Rejuvenation pruning is best done over a 3-year period. This allows you to enjoy a certain amount of flowers every spring, while ensuring the entire bush gets renewed in three years time. All pruning is done in late winter.
After three years you should see new growth begin to appear at the base of the plant.
Once lilacs become established, they require very little care other than a little bit of annual pruning to keep them looking their best. This pruning should be done in the early summer as soon as the shrub is done flowering.