Extension Educator, Horticulture
Ok. I have tried to work with you. I have talked nice to you and gently pruned you. Then I talked not-so-nice to you and gave you a severe haircut. But you refuse to stay in your place. Miss Kim you are lovely, but you have outgrown your space and it's time to go.
I'm sure I'm not the only one that has conversations with their plants (or at least I hope I'm not the only one). When I planted 'Miss Kim' lilac over ten years ago the label said she would get 3 feet tall by 3 feet wide - the perfect size for the location. She also offered sweet fragrant lilac flowers just as my old fashioned lilacs were fading and disease resistant leaves so none of the pesky powdery mildew of her old fashioned relatives. She appeared to be the perfect plant.
The first few years 'Miss Kim' was delightful with her fragrant flowers and low maintenance. However, she quickly surpassed her 3 foot description. With past experience I knew our rich soils can easily produce plants of porcine proportions; however 'Miss Kim' showed no signs of stopping. I notice 'Miss Kim' is now labeled at a more realistic 4-6 feet. Perhaps if I had started pruning earlier in her life I could have kept her in bounds.
Regular pruning can correct many problems. Pruning can maintain plant size, but also encourage flowering, remove diseased or dead limbs and help control insect and disease problems.
However, timing is important, especially when pruning spring blooming shrubs such as lilacs. Early blooming shrubs develop their flower buds during the summer and fall of the previous year. This is often called "blooming on old wood". Therefore as a general rule, shrubs that flower before June 15 should be pruned soon after flowering. Pruning these shrubs in late summer, fall or early spring will remove the flower buds and therefore the flowers.
Spring flowering shrubs can be lightly pruned by removing old flower stems or pruned by the renewal method. With the renewal method each spring after flowering, the largest stems are removed to the ground with pruning shears or loppers to stimulate new growth from the crown. Remaining stems are shortened for a pleasant shape. Reserve hedge shears for formal hedges.
Shrubs that could be pruned this spring soon after flowering include old fashioned lilac, deutzia, kerria, mockorange, weigela, forsythia, viburnum, St. johnswort, and redtwig and yellowtwig dogwood.
Shrubs that bloom after June 15 can be pruned in early spring. Summer and fall flowering shrubs bloom on new wood or stems that were produced in the same season as flowering. Many of these shrubs can be pruned by the scary rejuvenation method. Rejuvenation is the complete cutting of all stems down to 4 to 6 inch stubs, generally in February and March.
The following respond well to rejuvenation pruning: Anthony waterer spirea, abelia, honeysuckle, beautybush, snowberry, slender deutzia, and privet. On spring flowering shrubs you may decide to prune in March and sacrifice flowers for that season or prune right after flowering.
I experimented with my 'Miss Kim' lilac with renewal pruning one year then complete rejuvenation in later years. The new growth was sparse and disappointing. At least in my case 'Miss Kim' hated the severe pruning that can be successful on old fashioned lilacs. Light pruning every year after bloom would keep 'Miss Kim' in bounds and looking good.
For more info contact your local UI Extension for the circular U5040 Pruning and Care of Trees and Shrubs or PH: 800-345-6087 https://pubsplus.uiuc.edu/