How to Prune: Lilacs

  • Difficulty Rating: Beginner
How to Prune: Lilacs

Venerable lilac is one of the most recognizable flower forms and fragrances in the garden.

Their cone-shaped clusters of colorful flowers open in mid-spring, usually timed perfectly for Mother’s Day bouquets. Ranging in color from white to pink to varied shades of purple, the lilac easily integrates into any garden color palette. And, with literally thousands of species, varieties and cultivars on the market, there is a lilac sized to fit every gardener’s space. Plus, with a little consistent pruning care, these hardy plants will easily thrive for generations to come.

Most lilacs mature into multi-branched tree forms, but to maintain this handsome shape, they may need a bit of pruning assistance from you.

If you have a lilac in your garden already, you may need to begin by doing some renovation pruning on it. While lilacs have been known to recover from a complete, hard cut to the ground, if they survive, it will take the plant years to recover from such extreme pruning. Instead, consider removing no more than one third of each lilac’s living branches each year using the following step-by-step approach:

With every cut you make, be sure to remove the entire branch to the point where it meets another branch, making a “y” shape. Do your best to leave the little crest at this connection point – known as a branch bark ridge or collar -- intact on the tree. This collar enables the plant to close off the cut and protect itself going forward.

To begin, trim out all of the dead or broken branches from the interior of the plant using a pair of bypass shears and a hand saw. Because this material is dead; it does not count toward your 1/3 removal rule. And, cleaning out all of this dead material can be done at just about any time of year. When all this detritus is gone, step back from the plant and evaluate the remaining live material.

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Lilacs have a tendency to form suckers and watersprouts. Suckers are small shoots that emerge from the root system of the plant, creating a thicket around the main trunk. Watersprouts are small shoots that surface similarly, but off of main stems, taking a direct upward growth form. Both of these types of shoots should be removed from the plant. Removing these will encourage the plant to put energy into the enduring, sought-after tree-like form. Although this type of pruning can be done just about anytime of year, removing suckers and sprouts in late summer should reduce how much you need to do in following years. By late summer, unlike in spring, plants have less likelihood to put on copious new growth following a heavy pruning job.

Once the dead, broken and suckering sprouts have been removed, step back again, and assess the remaining plant. If you haven’t removed too much living material already, examine your lilac for any branches that are crossing inward rather than outward from the main stems and remove those to improve the overall plant form aesthetic. If any branches are rubbing together or growing onto a building, remove those. Removing productive but unwanted branches like these is great to do when the plant is in full bloom. Not only will you improve the plant’s overall look, but also you’ll harvest loads of bouquets.

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And, always take the time to step back from the plant to evaluate your work. If you accidentally cut off a desirable branch, you can’t put it back on! Reviewing as you go will help you avoid removing too much material or creating a lopsided tree. Generally, after removing dead material, suckers, watersprouts, and branches growing in strange directions, your remaining refurbished lilac should have only a few main trunks with branching at the top. If additional branches are awkward, remove them judiciously within a month or so of the plant finishing its bloom time.

Never shear your lilac with hedge clippers. And avoid trying to cut the top off to make it shorter; this will simply result in loads of new suckers shooting upward on a debilitated and deformed tree.

Keep in mind that heavily overgrown lilacs may take a few years to prune back into shape. Taking out one third of living growth per year leaves the plant with sufficient living growth to feed itself. Overdoing it in one year may do the plant in. Also, avoid pruning your lilac in autumn, when cuts may stimulate soft new growth susceptible to the coming cold. And, while removing dead material and suckering growth is fine in winter, hold off on pruning main branches until your lilac is blooming. Cutting out bare winter or early spring branches filled with dormant buds equals throwing away that many spectacular, sweet-smelling flowers just waiting for Mother’s Day to arrive.

(Lilac floral arrangement photo courtesy Blush Custom Floral.)