When and how much do I trim an unruly lilac bush?

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I just bought a house and there is a gigantic lilac bush that only have a few blooms on it. When to trim, how much? I literally know nothing about lilacs!

  5 answers
  • Sunny Sunny on May 01, 2019

    You could trim old stalks off (usually 1 third of them in the spring each year for 3 years, after blossoming) or cut the whole thing down to almost the ground and within 3 years you should have VERY many lovely blossoms.

  • Mogie Mogie on May 01, 2019

    Lilacs grow best in full sun and well-drained soil. Lilacs grown in partial sun or shade will not flower well. The shrubs may take three to four years to establish themselves in a new site, but once established they can live for centuries. Soil pH (alkalinity or acidity of the soil) may affect the plant’s growth. Lilacs do well in a slightly acid to alkaline soil. New England soils are often very acidic and may require some modification for best lilac growth.

    To ensure abundant flowering, cut off all spent blossoms each year and prune the flowering stem back to a set of leaves, thus preventing seed formation. If this is not done, good flowering years may be followed by bad years. Since flower buds are formed the summer before they bloom, winter pruning will remove them.

    Remove dead, damaged, or diseased branches. Tall, leggy, poorly flowering plants require renewal pruning; remove about one-third of the oldest stems at ground level each year for three years. This encourages growth of vigorous new stems from the base. By the end of three years the plant should be fully rejuvenated with blossoms once more at nose level.

    Tough as lilacs are, they do need ample water. Aim for an inch of water a week, provided either by you or by nature.

    In our area the most serious lilac problems are powdery mildew fungus (Microsphaera alni), lilac borer (Podosesia syringae) and scale (oyster-shell scale, Lepidosaphes ulmi and prunicola scale, Pseudaulacaspis prunicola). Powdery mildew looks like whitish patches dusting the leaves. It is unattractive but in our climate is rarely serious. Borers leave 1/8-inch holes in stems and larger branches, often one to two feet above ground level. A minor infestation might be ignored, but more than a few borers should be diagnosed and treated by a professional. Oyster-shell scale is aptly named, as the pests look like 1/8-inch oyster shells on the stems, while prunicola scale covers bark with a dusty white mass. Control adult scale by pruning heavily infested branches; control tiny young “crawlers” with a hard spray of water from a garden hose (use a hand lens to see scale). Dormant oil and summer oil are also effective.

  • Amanda Amanda on May 01, 2019

    Hi Dee. I had one that was huge and very woody. I cut it down to about 3 feet. I would do this after it has finished blooming. Then next year it should fill out and not be woody. Good Luck!

  • Vimarhonor Vimarhonor on May 01, 2019

    Hello. Knowing when to trim lilac bushes is important. Most lilacs don’t require pruning until they reach about six to eight feet tall. The best time for pruning lilac bushes is right after their flowering has ceased

    Read more at Gardening Know How: Pruning Lilac Bushes: When To Trim Lilac Bushes

    https://www.gardeningknowhow.com/ornamental/shrubs/lilac/pruning-lilac-bushes-when-to-trim-lilac-bushes.htm

    https://extension.unl.edu/statewide/douglas-sarpy/pdfs/ce/resources/ce-g1860-lilacs.pdf

  • Michelle Leslie Michelle Leslie on Dec 17, 2020

    Hi Dee, here's a great article and video that will show you how and when to prune your lilac bush  https://www.thespruce.com/pruning-lilacs-how-and-when-to-prune-lilacs-1403004