Over grown lilac bushes - Ask Extension
I have white lilacs bushes the have not flowered much in the last couple of years. I have read that I can cut them back to about 6 inches in early spr...
Knowledgebase
Over grown lilac bushes #871336
Asked June 03, 2024, 2:47 PM EDT
I have white lilacs bushes the have not flowered much in the last couple of years. I have read that I can cut them back to about 6 inches in early spring. I just want to know if this is the right approach to rejuvenate them?
Wanita hatt
Genesee County Michigan
Expert Response
According to the Chicago Botanic Garden, 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" 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. (See our recommendations for this below.)
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.
If you choose to use rejuvenation pruning, we recommend you do it over a 3-year period. In late winter hard prune 1/3 of the tallest limbs to within 12" of the ground. Hard prune another 1/3 of the shrub the second winter, and the final 1/3 the third winter. The bush may look a little off balance during this period of time, but by using this process there is less damage or stress to the plant.
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" 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. (See our recommendations for this below.)
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.
If you choose to use rejuvenation pruning, we recommend you do it over a 3-year period. In late winter hard prune 1/3 of the tallest limbs to within 12" of the ground. Hard prune another 1/3 of the shrub the second winter, and the final 1/3 the third winter. The bush may look a little off balance during this period of time, but by using this process there is less damage or stress to the plant.
