Quick answer
Ivory Silk lilac needs little pruning. Remove dead, damaged and crossing branches and any suckers, and shape lightly. Many gardeners prune soon after flowering so as not to cut off next year’s flower buds, though some sources recommend late winter while the tree is dormant. If bloom matters most to you, the after-flowering window is the safer choice.
About the tree
Ivory Silk is a cultivar of the Japanese tree lilac (Syringa reticulata). It is a small tree, with published sizes varying from roughly 15 to 30 feet tall and 10 to 20 feet wide, and large clusters of creamy white flowers in early summer. It is generally listed as hardy from about USDA zone 3 or 4 into the warmer zones (sources differ, so check the tag), prefers full sun and well-drained soil, and is often chosen for urban lots and under utility lines because of its modest size.
Do you need to prune it?
Not much. Unlike common lilac shrubs, a tree lilac has a single trunk or a few main stems and an upright, oval to rounded crown. The aim of pruning is mostly to keep that form clean, so ask whether each cut has a reason: dead, broken or diseased wood, rubbing branches, weak or narrow forks, or branches in the way of a path.
When to prune
There are two commonly given answers:
- Just after flowering (early to mid summer). Lilacs generally set next year’s flower buds on the current season’s growth, shortly after blooming. Pruning soon after the flowers fade keeps those buds. This is the usual advice for flowering shrubs and trees like this.
- Late winter, while dormant. Some references suggest this because the branch structure is easy to see. The risk is that any live branch you cut then carries flower buds, so you lose some of that summer’s bloom.
Avoid pruning late in the season. New growth encouraged by late cuts may not harden before frost. Dead wood and broken branches can come out whenever you notice them.
Tools and cuts
Use clean, sharp bypass pruners for twigs, loppers for branches up to about an inch or so and a pruning saw for larger ones. Cut back to a side branch, bud or the branch collar, never leaving a long stub or cutting flush to the trunk. For anything big enough to tear bark, make an undercut first, then cut from the top a little further out, then take off the stub. For more on the cuts that cause trouble, see will cutting branches kill a tree.
Step by step
- Stand back and look. Identify the main trunk and the best branches.
- Take out dead, broken and diseased wood first.
- Remove crossing or rubbing branches, keeping the stronger one.
- Cut off suckers growing from the base or roots.
- Thin lightly if the canopy is crowded, using the same approach described in crown thinning of trees. Aim to remove only a small portion of the crown.
- Check balance and stop before the tree looks over-trimmed.
How far back can you prune?
If a tree has become overgrown or flowers poorly, some gardeners rejuvenate it. A cautious approach is to remove up to about a third of the oldest, thickest stems in a year and spread bigger changes over several years. This advice is mostly given for multi-stemmed forms. A single-trunk tree lilac does not respond like a shrub, so heavy cutting of a mature one is risky. Do not top it. For an old tree, consult an arborist.
Suckers
Suckers are thin shoots that come from the base or roots. On grafted trees, they may belong to the rootstock and not the named cultivar, so they should be removed when you see them. Cut them close to their origin, ideally in early summer, rather than leaving stubs.
What if you do not prune it?
Nothing dramatic. The tree will still grow and bloom. Over time, though, you may see more crossing branches, a crowded interior, dead twigs and fewer or smaller flower clusters. Light, regular care generally prevents the need for harsher cuts later.
Possible drawbacks of the tree
Common complaints include a modest fragrance compared with common lilac, sensitivity to hot, humid climates, occasional pests and leaf diseases such as powdery mildew, and the need for sunny, well-drained sites. Flowers are also short-lived. Check local conditions and your zone before planting. For other small trees, compare low-maintenance small trees for your backyard.
Aftercare
Water in dry spells, particularly for young trees for the first couple of seasons. Mulch around the base without piling it against the trunk. A light application of slow-release fertilizer in spring is sometimes suggested for ornamentals, but a soil test is a better guide than a routine feeding. Pollarding is not a typical approach for this tree; see pollarding trees for the species that tolerate it.