With their flowers and strong scent, lilacs (Syringa spp.) have been common garden plants for generations. Sometimes, these easy, reliable shrubs fail to bloom. If you're having trouble getting your lilac shrub or tree to flower freely, there are a few cultural practices to check.
Lilacs take as much as 3 to 5 years to settle in and start blooming. In addition, shrubs that have been severely cut back for renewal take several years to resume blooming. If your lilac needs renewing, use the 3-year method. Cut back one-third of the old stems in the first year, one-half of the remaining old stems the second year and the rest the third year. This method forces the shrub to put out new growth but retains enough stems of blooming age to give you flowers every year.
For best blooming, prune lilacs only after flowering finishes. Lilacs bloom on old wood, so pruning before the plant flowers removes the current season's flower buds. The buds for the following year are set in the weeks immediately after flowering, so pruning after midsummer reduces the next season's bloom. Deadhead lilacs by snapping or cutting off the seedheads, so that the plant's energy goes into bud formation for the following year instead of making seed; otherwise, you will have heavy flowering only every other year.
Too Much Shade
The most common reason lilacs don't bloom well is not receiving enough sunlight. They require at least four to six hours of direct sunlight a day to flower. If your lilac is shaded by a tree, limbing the tree or thinning its branches may increase the sunlight enough for good blooming. Consider moving the shrub to a sunnier location in early spring while it's still dormant.
Too Much Fertilizer
Lilacs need only light feeding. Too much fertiliser, especially nitrogen, encourages stem and leaf growth at the expense of flower buds. If your lilac is surrounded by lawn, don't apply grass fertiliser over the lilac's root zone, which is roughly the same diameter as the shrub's top growth. Lawn fertiliser is high in nitrogen.