Lilacs remain one of the most popular landscape plants in the North, and one of the easiest to grow. Other than trying to figure out which new ones to buy, the main question seems to be about pruning and renovating older plants.
Unless plants have unsightly or broken branches, you can usually not bother with pruning until your plants are ten years old or more. If your plants are six to ten feet high, maybe the same across, with older wood (thick branches, distinct bark like a tree), or perhaps branches that cross and rub on each other, you may think about pruning. Perhaps some branches are just in the way of walks or other plants. Some of the shorter lilacs such as Meyer, the popular Miss Kim, Tinkerbelle, and some species, have a nice rounded shape and probably wont ever need pruning if spaced properly.
You can remove spent blossoms (ďdeadheadingĒ) within two weeks after bloom, in order to improve the plantís appearance. If you prune much later you may be cutting off next yearís flowers, which start forming in branches the summer before. I like to leave the flowers for the finches and other birds to eat the subsequent seeds in the fall. Of my dozens of cultivars (cultivated varieties), I havenít noticed any effect on bloom the following year by leaving old flowers.
Just as you deadhead shortly after bloom, then is a good time to prune as well. I like to prune then as I can see the current seasonís growth, and branches that are in the way. Many like to prune in early spring (March or April), at which time you can clearly see branches without their leaves. But, if you prune then, you may have to come back after bloom and prune some new growth as well. Many of my plants look fine in early spring, but by early summer definitely need some pruning! Pruning in early spring will also cut off some of that seasonís flowers. Broken branches of course should be pruned off whenever seen.
Just be sure if pruning after bloom to not prune past mid-summer. This will allow time for the wounds to heal before winter, and any new growth that might be stimulated should harden properly before winter.
Make sure your pruning tool blades are sharp, to avoid tearing stem tissue which wont heal properly. I like to use three types of tools: pruners or secateurs, either the scissor or anvil type; larger loppers for the branches over one-quarter inch across (I like the ones with telescoping handles for increased leverage); and of course a folding pruning saw for the branches an inch or more across.
To prune, you can be extreme and cut most lilacs back to near the ground. Over several years they will send up new shoots, but in the meantime they are not attractive. I like to prune back about one third of older branches each year. This way you do not remove too much in any one year. Look for those older branches, or branches that are rubbing the bark off of others, or branches that are in the way. Prune them out as low as you can get, or as near the main stems as possible.
Some lilacs produce many new shoots from the base (watershoots or suckers). Remove all but a few of these each year, leaving ones to make new branches where you want, and that donít rub on others.
Another approach is to prune most branches back uniformly by about one third each year. Donít shear them as you would a hedge. Lilacs are not meant to have this sheared look, and donít have the branch structure leant to this. Rather, prune each major branch back to a side shoot. Remember, as with pruning any shrubs or trees, the shoot or bud just below where you prune will grow out in the direction it is pointing. If you donít want a shoot there, perhaps blocking a walk or path, prune elsewhere.
One problem lilacs may get in certain locales early in the season, especially those with late morning dew or a rainy Spring, is bacterial blight. Shoots affected wilt almost overnight, and in a few days whole stems turn brown. Donít be too hasty to prune, as a healthy plant will often send out new buds from these same branches later in the season, even in late June!
If by mid-summer no new buds appear the branch is probably dead, but still may contain this blight. Since this disease can be spread easily, special precautions are needed. Take a cloth or container of rubbing alcohol with you to prune and, after each pruning cut, wipe or dip the pruner blades in it to keep them free of this disease. Make sure to prune below the affected stem, to inner tissue that is not brown and discolored.
Of course lilacs will survive and bloom some on their own for decades,
as is often seen around old homesteads, even around foundations where the
home is no longer standing! Yet pruning is needed on older plants
for the best bloom and healthiest plants.
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