Yard and Garden: Incorporating lilacs into your landscape
The sweet-smelling lilac is synonymous with spring. These large, suckering shrubs grow well in Iowa and are widely planted across the state.
Iowa State University Extension and Outreach horticulturists answer common questions about growing lilacs in your landscape.
Question: A lilac that was planted three years ago has never bloomed. Can I do anything to encourage the lilac to flower?
Answer: Cultivars of the common or French hybrid lilac (Syringa vulgaris) often do not bloom for several years after planting. In many cases, the shrubs must grow for five or more years before they are mature and capable of flowering.
Selecting a favorable planting site and promoting good cultural practices during establishment encourage lilacs to flower as quickly as possible.
Lilacs perform best in well-drained soils in full sun. Plants should receive at least six hours of direct sun each day. Lilacs planted in partial shade will not bloom well. Because lilacs often live for many years in the landscape, they are sometimes shaded over time by surrounding trees and shrubs as they grow.
Good care during the first two or three years is important. Apply a 2- to 3-inch layer of mulch around each shrub to conserve soil moisture and control weeds. Water lilacs on a regular basis during dry weather.
Also, protect lilacs from browsing rabbits over winter by placing wire fencing around the shrubs in fall and removing it in spring.
It is generally not necessary to fertilize lilacs. However, lilacs can be lightly fertilized in early spring. Heavy fertilization may promote excessive vegetative growth and discourage flowering.
Pruning can also affect flowering. Because lilacs bloom on the previous year’s growth, the best time to prune lilacs is immediately after flowering in spring. Pruning lilacs in late summer, fall or winter may remove many of their flower buds.
While the common lilac usually doesn’t bloom for several years after planting, several other lilacs bloom when quite small. The dwarf Korean lilac (Syringa meyeri), "Miss Kim" lilac (Syringa patula) and Preston lilacs (Syringa × prestoniae) often flower within one or two years of planting.
Q: The leaves on my lilac are covered with a white substance and/or dark spots. Is this a serious problem?
A: The white substance is likely powdery mildew. Powdery mildew is a fungal disease. It appears as a white, dusty growth on plant foliage.
Among lilacs, the common lilac is highly susceptible to powdery mildew, while the Preston lilacs and dwarf Korean lilac are resistant.
Dark spots on leaves can be caused by several different species of disease-causing fungi, such as Pseudocercospora or Septoria. The leaf spots typically start small and enlarge over time. These spots can occur on most species of lilac but are very common on Japanese tree lilac (Syringa reticulata).
Powdery mildew and fungal leaf spots are favored by high humidity, cool nights and warm days. Plants growing in partial to heavy shade are most susceptible to powdery mildew while fungal leaf spots occur readily on plants in full sun.
Powdery mildew and fungal leaf spots do not cause serious harm to lilacs. The damage is mainly aesthetic. Spraying with fungicide is not warranted.
When planting lilacs, select a site that receives at least six hours of direct sun each day. Powdery mildew will not be a serious problem in sunny areas.
Judicious pruning of nearby trees (to increase the amount of sunlight) should help reduce the severity of powdery mildew on lilacs growing in shady locations. Another option would be to transplant the lilacs to a sunny site.
Removing and destroying leaf debris in the fall and pruning out dead branches is the best and easiest strategy in reducing the leaf spot diseases caused by Pseudocercospora or Septoria.
Q: After flowering, should I remove the spent flowers on lilacs?
A: The spent flowers on lilacs aren’t very attractive. Removing the spent flowers (deadheading) improves the appearance of the shrubs and prevents seed pods (capsules) from forming.
Deadheading allows the lilacs to use much of their energy for next year’s flower bud development rather than seed pod formation. As a result, lilacs that are promptly deadheaded after flowering often bloom more heavily the following season than those that are not deadheaded.
When deadheading lilacs, make the pruning cut at the base of the flower cluster and just above the uppermost leaves. While deadheading is beneficial, the removal of spent flowers may not be feasible for home gardeners with large numbers of lilacs. Shrubs will still bloom in subsequent years even when not deadheaded.
Q: When should I prune lilacs?
A: Lilacs set their flower buds for the following season in mid-summer, not long after the spring blooms fade. Pruning shrubs immediately after flowering is the best time to prune lilacs and will help the lilac produce the most flowers the following spring.
Lilacs respond best to the removal of select branches at their point of origin or entire branches from the base of the suckering shrub rather than with shearing or clipping. This strategic removal of branches preserves the shrub’s natural shape.
Removing the oldest and largest branches within 6 to 8 inches of the base every three to five years can also reduce the overall size of the shrub and promote new growth that blooms better than older stems. Large old branches can be removed immediately after flowering or in late winter (March or early April).
When pruned in late winter, some blooms will be sacrificed, but larger branches are often not as floriferous, so it is much easier to see and remove the branches when the plant is dormant and without leaves.