I asked Jerry Goodspeed, Director of the Utah State University Botanical Center & Ogden Botanical Gardens, to share a common spring gardening question and answer with Horticulture readers. His response:
Every spring we’re asked, “When and how can I prune my lilacs?”
Lilacs are a multi stemmed shrub, meaning there are many stems, or canes. Each year, the plant should produce at least one new stem from below the soil level. This category also includes red-twig and tatarian dogwoods (Cornus sericea and C. alba), ninebarks (Physocarpus), forsythia and viburnums.
We advise waiting until after the plant blooms, so that you can enjoy the most flowers. Then, the best method is to simply cut the larger, taller and older canes to the ground. Once the shrub reaches the desired height, most years you’ll need to remove just three to five canes. This approach opens up the canopy, allowing more light through, which keeps the plant healthy and stimulates new cane growth. It also keeps the shrub at the height you want and looking natural.