Trimming. All shrubs like Lilacs or Mock oranges or large Spirea should be pruned in such a way as to create new growth from lower branches or from the ground around the main trunk. Rule of thumb, is once the plant is at least 6-7 feet high and has at least 4-5 stems of different ages and sizes, you should cut out one of the oldest / largest stems at ground level or about 6 inches from the ground. Then after this has been done, trim all the tips of the remaining branches by removing about 6- 10 inches of growth at the TOP of those stems. This will encourage renewal of growth with fresh material every year so as to have clean, disease free and vigorous growth that will bloom profusely every year thereafter.