Richard E. Bir
Too often I get the question "Why won't lilacs grow here?" or "Why don't my lilacs flower?" Since lilacs do grow well, at least in the mountains, in North Carolina and we are doing research to determine which ones do well in the piedmont and possibly the coastal plain, the answer is usually improper culture.
Lilacs perform best in full sun. At least six hours of sunshine per day is optimum in the mountains and elsewhere in Zone 6 and cooler where lilacs are most often grown. Powdery mildew is the primary disease problem on lilacs. It is much less of a problem when plants are grown in full sun, at least in the mountains (our experience is limited in the piedmont and coastal plain). Growing disease resistant cultivars can virtually eliminate problems with powdery mildew on sun grown plants.
Lilacs perform best on fertile, well-drained soils. Annual applications of fertilizer and liming at least once every three years will usually satisfy nutritional requirements. Lilacs are not fussy regarding which fertilizer is applied. 10-10-10 seems to work as well as compost. It has been suggested that excess fertilizer will lead to more problems with lilac borers but I have not seen any research to substantiate this claim.
Lilacs should be pruned. Pruning is necessary for continued healthy growth and flowering in North Carolina where we often have a much longer growing season than elsewhere where folks may have grown lilacs. Remove 1/4 to 1/3 of the largest stems each winter and do not allow more than 6 to 12 major stems to remain. Stems should be spaced so that they do not rub. In addition to removing the older, larger stems remove all but one or two replacement suckers an inch or so below the ground following flowering and remove all small twiggy growth annually (easiest in winter). Damaged and diseased wood should be removed as soon as it is noticed.
Lilacs will survive less than optimal conditions but will often produce shoots and foliage rather than flowers. Proper siting, fertilization and aggressive pruning are often necessary to keep lilacs thriving with an annual display of flowers.
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North Carolina State University