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Lilacs grow very tall and develop thick trunks after 8 years, or sooner. New shoots that come up from the roots are not given much chance by the older wood, and it is necessary to amputate the old wood at or near ground level. Do not do this all at once. Instead gradually remove the oldest trunks over 3 to 4 years to keep the shrubs producing flowers as the new canes rise to replace the removed ones.