The Japanese lilac tree--scientifically known as Syringa reticulata--is a large ornamental multi-stemmed shrub that features a vase-shaped crown, spreading branches, oval, dark green leaves, and profuse, showy flowers of creamy white, borne on large panicles. The Japanese lilac tree--which can grow to 25 feet tall and almost as wide--makes an impressive specimen planting or accent tree. By using the proper pruning techniques, you can encourage shapely and vigorous growth, promote flowering, and make your Japanese lilac tree more resistant to pests and diseases.
Prune the flowers from your Japanese lilac tree as soon as they are spent. According to the About Lilacs website, this will help prevent the tree from creating seeds, and instead focus its energy on buds for next year's bloom. Prune your lilac by hand with a pruning shears, rather than electric trimmers; those make the top of the shrub flat and thereby sacrifice the attractive rounded shape.
Use a pruning saw to remove any branches on your Japanese lilac tree that are dead, diseased, or damaged to reduce susceptibility to pests and diseases, disinfecting the pruning saw after every cut with a solution of one part bleach to nine parts water to prevent spreading pathogens.
Remove branches from the center of your Japanese lilac tree, cutting them back to the trunk. The goal is to open up the interior of the tree to more light and increased air circulation--particularly important for Japanese lilac trees, which are subject to fungal diseases.
Perform a rejuvenation pruning on mature Japanese lilac trees every few years by cutting one-third of the large, old stems all the way back to the ground in late winter. The following year, again in late winter, remove half of the old stems. The third year, remove the remaining old wood, thereby rejuvenating the entire shrub. Since lilac branches need to be three years old before they produce blooms, this method allows you to do a rejuvenation pruning and still enjoy yearly flowers.
Prune some of the suckers--small new shoots growing out from the trunk at ground level--after blooming has ended. Allow some of the suckers to remain unsnipped; they will turn into mature trunks that will replace the ones you cut during rejuvenation prunings.