‘Tis a frequently asked question; ‘Why didn’t my lilacs bloom?’. The same question is posed about other spring blooming shrubs like azaleas and forsythias, and the answer is found by asking more questions.
Pruning
When was the last time you pruned the lilac? Lilacs set their flower buds for next year in summer. If you pruned the shrub anytime from mid-summer through the following spring you cut off all the potential blooms and that is why your lilac did not bloom this year. If you need to prune a spring flowering shrub (lilacs, azaleas, forsythias, wisteria, spirea, bridal wreath, etc.) do so immediately after the plant has flowered and all (or most) of the blooms have faded.
Winter Weather
If you have not come near your lilacs with a set of pruning shears in years, then stop and consider how the winter weather was the previous winter. If the winter in your region was exceptionally cold, lilacs (along with other spring blooming plants) will refuse to present you with their glorious spring flowers. There’s no remedy for this act of nature, but you can counteract the wintry affects by planting cold-hardy plant varieties more suited to your region.
Southern gardeners have the opposite problems, sometimes the winter weather does not get cold enough to enable the plants to set buds. Lilacs and other spring blooming plants need a cold period in order to set their buds. A warm winter means no spring flowers. The only remedy for this is to plant lilac varieties that set blooms in warm climates.
Late Spring Cold Snaps
March weather is unpredictable, and in the south, we can have winter, spring and summer all in the same week during the month of March and the varying temperatures effect spring plants. A late spring cold snap will prevent lilacs from blooming. The frost or below freezing temperatures will kill the burgeoning flower buds.
Immaturity
A lilac shrub must reach maturity before blooming, usually 4-5 years. Most shrubs bloom the second year after planting, but lilacs and wisteria are notorious for making us wait for their huge blooms. Purchase an older plant from the nursery if gardening patience is not one of your virtues.
Over-Feeding
Over-feeding a lilac (or any other plant) with a high nitrogen fertilize will lead to a shrub with dense foliage, but no blooms. Have your soil tested, amend as recommend and only feed lilacs once in the early spring or late winter with a balanced 10-10-10 fertilize.
Sunlight
Lack of direct sunlight may also be the culprit behind your lilac not blooming. These shrubs need at least 6 hours of direct sunlight per day in order to bloom. Prune away overhanging tree branches to allow more sunlight to get through or move lilac to a sunnier location.
Source:
The Old Farmer’s Almanac
