Q: Why doesn’t my lilac bloom in the spring?
A: There are several possible reasons why your lilac fails to bloom. The most common causes are lack of adequate sunlight and improper pruning. Lilacs (Syringa) need to be planted in a location that receives at least six hours of strong, direct sun per day. They are very tolerant of different moisture conditions as long as they are planted in well-drained soil. Another reason why your lilac may not bloom is because it is being pruned at the wrong time of the year. Lilacs bloom on the previous season’s growth and develop next year’s buds shortly after blooming in the spring. Pruning needs to be done at the same time that the spent flowers are removed within a couple of weeks after the plant blooms so that next year’s buds are not removed. Young lilacs can be lightly pruned to keep them in shape, within bounds, and producing abundant flowers. Older plants may require a hard pruning almost to the ground to rejuvenate the plant and produce more flowers. Flower buds on early blooming varieties can be damaged by a late freeze.
As lilacs mature, they can grow leggy and overgrown, with little foliage towards the bottom of the shrub. When this happens it may be necessary for a complete rejuvenation by pruning them to within 12 inches of the ground. This should be done in late winter when the shrubs are dormant. Lilacs respond well to this renovation; however, the plant’s bloom cycle will be interrupted for one season, maybe more. To prevent the interruption in bloom cycles, lilacs can be given a rejuvenation pruning over a two-year period. Hard prune half of the shrub’s stems one year and the remaining stems the second year.
Lilacs are not heavy feeders. Excessive fertilization, especially nitrogen, can often encourage lush vegetative growth at the expense of reproductive growth, or flower development. Fertilizing will not increase the amount of blooms. If lilacs are planted near turf that is regularly fertilized, this could also be a cause of a lilac's failure to bloom.