By Judy Sedbrook, Master Gardener and Carl Wilson, Extension Agent with Colorado State University Cooperative Extension, Denver County
"Lilacs will bloom for a few weeks in May but their scent will linger for a lifetime." - Bill Orshefsky.
Though lilacs respond well to the landscape demands placed on them, with time and in some locations, their flowering may decrease.
Before assuming that you need to take drastic action to renew lilacs, consider factors unique to the year. A late freeze may knock out the blooms for a year, but plants may flower profusely with the next year's favorable spring weather.
What might have changed in the landscape to affect bloom? Construction may have cut roots or changed the amount of light or air circulation around plants. In that case, the only solution may be to move the plant or choose to grow it as a non-flowering shrub.
More commonly, the gradual shade cast by maturing trees compromises the plant's ability to produce enough energy from sunlight to make flowers. It may help to thin overhead trees so more light can reach the lilac. Plants need at least five hours of sunshine daily to bloom well.
Pest buildup over time is another likely culprit. Two pests, oyster shell scale and borers, are common. Scale insects are easy to see; they appear as lime-gray bumps on the stems. Borer damage is more hidden and generally identifiable only when stems are cut open.
Fertilizer rarely is the answer to a plant that once bloomed well but is now bloomless. While lilacs may derive some benefit from fertilizing once a year with a general purpose fertilizer, Colorado soils generally contain sufficient natural fertility to produce good results.
The good news is that renewal pruning of older lilacs helps solve both major causes for bloom decline -- shading and pests. The process consists of removing the oldest, thickest canes at the base. These are the ones most likely to be badly infested with scale and borers so insect problems are literally pruned away. Removal also eliminates much of the self-shading from overgrown foliage.
Young, thin canes are left to grow and produce the desired blooms. When pruning allows light to reach these vigorous shoots, they respond with healthy growth and flowers.
Consider removing some older canes this year, a few the following one, and some more the next. This process is the recommended way to maintain not only lilac but also many shrubs, and it will renew bloom. It results in a better looking shrub than tip pruning alone.
Tip or top pruning does have its place when a shrub grows out of shape because it is receiving more light on one side or another. To maintain the shrub well, it should be combined with renewal thinning of whole stems that are removed from the base.
What about an older, overgrown lilac that literally is two stories tall? In this case, drastic rejuvenation pruning is in order. Rejuvenation pruning is a kind way of saying cut every stem off 4 inches above the ground. While this "beheading" may seem drastic enough to kill a plant, resilient lilacs send up many vigorous stems the first growing season after pruning and form a 2 to 3 foot shrub.
After the first growing season, thin the many stems to a few healthy ones. This will eliminate competition among them. From then on, yearly renewal pruning will keep your lilac constantly young and vigorous.
Whether your lilacs are in need of a partial renewal or drastic rejuvenation, the time to prune is after May bloom but before flower bud set in early July. If bloom isn't a concern, dormant season (winter) pruning is fine as well as minor pruning at other times of year. While you will forfeit whatever spotty blooms your lilac may have produced, you will be amply rewarded with a renewed display of bloom in future years.
Photographs courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010