Answer: To renew your plants, you could use a three year approach. Right after it blooms, trim out about one third of the trunks/main branches by cutting them off at ground level. Also remove any new shoots from the ground that are smaller than a pencil in diameter. The next year, trim out half of the remaining old stems and any new growth that is smaller than a pencil. The third year, trim out the remaining old growth. From then on, continue to trim out small new growth and periodically remove some of the oldest stems. This will reinvigorate the plant and encourage thick new growth. Keep in mind too that lilacs need to be in full sun to grow and bloom their best.
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