AMES, Iowa – Horticulture specialists with Iowa State University Extension and Outreach answer lilac questions that will bring more spring blooms to home gardens. To have additional gardening questions answered contact the specialists by calling or emailing the ISU Extension and Outreach horticulture hotline at 515-294-3108 or firstname.lastname@example.org.
Cultivars of the common or French hybrid lilac (Syringa vulgaris) often do not bloom for several (five or more) years after planting. The shrubs must grow and mature before they are capable of flowering. Selecting a favorable planting site and 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.
Good care during the first two or three years is also 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 by placing wire fencing around the shrubs in fall.
While good cultural practices will aid plant growth, some practices may actually inhibit flowering.
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. 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 ‘Miss Kim’) and Preston lilacs (Syringa x prestoniae) often flower within one or two years of planting.
The white substance is likely powdery mildew. Powdery mildew is a fungal disease. It appears as a white, dusty growth on plant foliage. In the home landscape, powdery mildew is commonly found on lilac, phlox, monarda, zinnia and turfgrass. Amongst lilacs, the common lilac (Syringa vulgaris) is highly susceptible to powdery mildew, while the Preston lilacs (Syringa x prestoniae) and dwarf Korean lilac (Syringa meyeri) are resistant.
Powdery mildew is favored by high humidity, cool nights and warm days. Plants growing in partial to heavy shade are most susceptible to powdery mildew.
Powdery mildew does not cause serious harm to lilacs. The damage is mainly aesthetic. Spraying with a 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.
The spent flowers on lilacs aren’t very attractive. Removing the spent flowers (deadheading) improves the appearance of the lilacs. Removing the spent flowers also prevents seed pods (capsules) from forming on the lilacs. Deadheading allows the lilacs to use much of their energy for flower bud development rather than seed pod formation. As a result, lilacs that are promptly deadheaded after flowering often flower 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.