Most lilacs flowered well this season. A mild winter, no severe spring freezes and cool weather have allowed flowers to develop well and survive for a longer period. If lilacs failed to bloom, several factors need to be kept in mind to prevent it from happening again. One of these includes pruning.
If lilacs bloom, but perhaps not as well as the previous year, the reason may simply be that a year of heavy flowering often uses up food reserves so flowering will be lighter the next season. However, under good growing conditions and proper pruning this obvious reduction in flowering can generally be avoided.
Lilacs are not heavy feeders, but some fertilization is beneficial. This should be done about flowering time or immediately following it. A fertilizer with an analysis higher in phosphorous should be used, so those with a ratio of approximately 1:2:1 such as 5-10-5 might be chosen. Amount to apply will depend on size of bush, so follow directions of manufacturer.
If a plant seems to be growing too fast and vigorously, the cause may be over fertilization. High nitrogen is especially likely to cause the plant to put energy into shoots with little energy going into formation of flower buds. If this is a problem, fertilize less, or if the excess is coming from lawn fertilization, try to avoid lawn fertilizer close to the lilac. If not possible, add supplemental phosphorous around the plant. If all else fails, try root pruning by cutting the root system about 18 inches from the plant (or more for a very large plant). This is done by pushing a sharp spade into the soil around the plant to reduce some of the upper feeder root system.
Do not allow all the suckers that often develop at the base of lilac bushes to remain. Cut out a majority of them but allow enough to remain to produce younger shoots that will be ready to replace older shoots when they become too tall or perhaps no longer flower well. Some lilacs are budded or grafted on common lilac or privet roots. If the plants are known to be grafted, all shoots arising from the base below ground line must be removed. If they are not cut out,they will gradually weaken and kill the older grafted portion.
Lilacs may be attacked by borers. When pruning the bushes, remove any older stems that show holes along the trunks as well as weakened growth with dead twigs. Ants are often attracted to such old, infected shoots. Any stems that seem particularly attractive to ants should also be removed.
If a lilac plant seems to have difficulty flowering regularly, make an effort to remove dead flower clusters before they form seed pods. Some varieties tend to be heavy seed producers and abundant seed production will suppress flowering the following year. Do not wait too long to do this, however. Once the seeds have formed there will be little influence on improving flowering during the next season.
How to Prune Lilacs
Lilacs can be pruned with a standard set of pruning sheers, so long as you follow the proper directions.
Lilacs should be pruned as soon as possible after flowering. If they are pruned too late, flower buds that may already be in the process of being initiated will be eliminated. Also, new shoots that may develop from branches that are cut back will form too late in the season to have time to initiate flower buds.
Ideally, the reduction in height or overall size of old lilacs should be done gradually. Over about a three year period, remove about 1/3 of the oldest growth each year. In this way the entire bush will be renewed and the height lowered. If plants have not become overgrown, a yearly light pruning will maintain flowering and shape, and is better than infrequent severe pruning.
Excess shade from large trees or structures may also suppress flowering in lilacs. Sometimes pruning some tree limbs can allow enough light on the plants to restart flowering. In most cases, plants will have to be moved into a sunny location if abundant flowering is desired.