Lilacs appear to come in two varieties – the natural shrub-like hedge forming type and the type that grows as a tree. Actually, these are both the same plant, the different shapes are simply from trimming in a particular way. If you prefer lilac trees to lilac bushes, the following tree trimming guide can help you achieve just that.
When to prune
For most shaping and trimming, pruning cuts are made in late spring or early summer, right after the trees stop flowering. The only exceptions are when there is damage or if you need to cut the tree back severely. You can trim out damage at any time, and major cutting back is best done in late winter before growth resumes. Keep in mind, late winter trimming will remove some of the flowering branches, though.
Start with the suckers
Suckers grow up around the base of the lilac, often beginning within the first year of planting. If you want a single-trunk tree, then you will need to cut these out every summer. Use a sharp knife to cut off suckers either flush to the trunk – if they emerge from the trunk – or just beneath the soil surface – for those coming up from the roots. This will be the main form of pruning for the first couple of years, since it is the vital method for developing the tree.
Keep the trunk clean
Your other late spring and early summer pruning task is to maintain a clean trunk so the lilac develops a full canopy. To do this, remove lateral branches from the lower half of the trunk, allowing foliage and branches to only develop on the top half. This is also a good time to create a balanced canopy. Cut off any excess branches so that the lateral branches are spaced evenly around the top of the trunk. Make all pruning cuts flush to the trunk.
Consider a do-over
One nice thing about lilacs is that you can usually start over if you have patience. If your lilac has developed a bad form or doesn't seem to be doing well, simply cut it back to about one foot from the ground in late winter. The tree will send up new shoots come spring. Allow these shoots to grow for the first year, and then pick the strongest one as your trunk the following spring. The rest are pruned away and then you follow the same pruning steps as outlined above to create the tree-form of your lilac. Just keep in mind it may be several years before you see flowers again if you choose this route.
Talk to a tree trimming service like Kansas City Tree Care, LLC in your area for more help.