Whether light lavender, dark purple or even white, I love the color and scent of common lilac, Syringa vulgaris. I often see bushes densely covered in blossoms, yet my lilac bushes are sparse with few blooms. Why are some bushes so thick and fragrant, and mine almost bare?
When people asked me this question when I was at Cooperative Extension, I would enquire “When did you prune last.” Lilacs bloom on year-old wood, so they should be pruned right after they bloom. Too often people wait until fall to prune, not realizing they are cutting off all the flowering wood. On the other hand, others ritually prune in the spring, no matter what the plant is. They too cut off most of the blossoms. While I usually prune at the appropriate time of year, I didn’t prune the lilacs last year, because there were no blossoms due to a late freeze.
Newly planted shrubs, two to three years old don’t need to be pruned. Shrubs that are old, such as the 40 year-plus plants in our yard, may be at the end of their lives. Mature shrubs of middle years should have 1/3 of the growth, 1½ inches in diameter or larger removed at ground level every few years. This will encourage vigorous young growth, which is more likely to bloom.
Lilacs require six hours of sunlight per day for flower development. Less sun means fewer to no blooms. None of my lilacs are in full sun, and, with the shade of all our trees, they probably aren’t getting six hours of sun. Crowded plants grow spindly and bloom less. One row of my lilacs is quite crowded in with sumacs, currants and cotoneasters. Another factor that may have played into reduced blooming at our house is the amount of water we had in 2016-17. Lilacs do better in well-drained soil. In a normal year, our gravelly soil is almost overly well-drained. But, that winter, the soils were saturated with minor flooding and perhaps roots were damaged affecting flower production.
I think my lack of fertilization is an additional factor. Yes, lilacs need little care, but with sandy soil and the huge leaching effect of the rain these last two winters, I should have fertilized last year in the spring and again in the fall. I haven’t fertilized yet this year either.
If I plant new plants in more sun and fertilizer more regularly, I might get the lush lilacs I so envy!
JoAnne Skelly is Associate Professor & Extension Educator, Emerita, University of Nevada Cooperative Extension. She can be reached at email@example.com.