Why does my lilac not flower, and when should I prune it?

A lilac is one of the easiest shrubs to accidentally silence. It sets its flower buds through the summer for the following spring, so the pruning window opens when the flowers fade and closes within a few weeks. Cut it in autumn or winter — which is when most people tidy shrubs — and it produces leaves and nothing else.

What you'll need

• Secateurs
• Loppers
• Pruning saw
• Gloves
• Spade for suckers (optional)

Step by step

1. Deadhead as soon as the flowers brown, and treat that as the prune.Cut each spent flower head off just above the pair of buds or shoots immediately below it. Those shoots carry next spring's flowers. Doing this promptly is most of what a healthy lilac needs.
2. Do all other pruning in the same window.Within two or three weeks of flowering finishing. Remove dead, damaged and crossing branches, thin out congested growth in the middle, and shorten anything going where you do not want it.
3. Check for suckers and remove them.Many lilacs are grafted, and vigorous shoots from below the graft union are rootstock, not the variety. They flower poorly or not at all and steal vigour. Trace each one back to its origin and pull or cut it off at the root rather than at ground level.
4. Assess the light.Lilacs want full sun. In the shade of a building or a larger tree they grow tall and thin reaching for light and flower sparsely at the top. This is a common cause in a maturing garden where something else has grown up around it.
5. Check what it is being fed.A lilac near a lawn getting nitrogen-rich feed makes vigorous leafy growth and few flowers. It is not a hungry shrub; on reasonable soil it needs little more than a mulch.
6. Check the soil is not too acidic.Lilacs prefer neutral to slightly alkaline soil. On a distinctly acid soil they can be reluctant, and a dressing of garden lime in autumn sometimes improves flowering. Test before applying anything.
7. Renovate a leggy old lilac over three years.Cut about a third of the oldest stems right down to 300 to 450mm in late winter, and repeat for the next two years. You lose flowers on what you cut but keep the rest of the plant flowering, and by year three the whole shrub is renewed at a usable height.
8. Or cut it all at once, and accept the wait.A lilac cut entirely to about 450mm in late winter will regenerate strongly, but it will not flower for two or three years while it rebuilds. Choose this only if the plant is beyond staged renovation.
9. Thin the regrowth after renovation.A hard-cut lilac throws up a thicket of shoots. Select the best three to five per stump the following summer and remove the rest, or you recreate the congested plant you started with.

Tips

• A lilac that flowered well for years and then stopped after being tidied in autumn has almost certainly been pruned at the wrong time. It recovers fully once the timing is corrected.
• Newly planted and recently moved lilacs often skip a year or two. This resolves itself.
• Lilac suckers pulled off with a bit of root attached can be potted up and grown on if the plant is on its own roots. If it is grafted, the sucker is the rootstock and not worth keeping.
• The same after-flowering rule applies to forsythia, philadelphus, weigela, deutzia, kerria and ribes — all spring-flowering shrubs that set their buds over the summer.

Common mistakes

• Pruning it in autumn or winter with everything else — The buds for next spring formed during the summer and are sitting on the plant. A winter cut removes them and costs the entire display.
• Leaving rootstock suckers to grow — They are vigorous, they flower poorly, and given a few years they can dominate and effectively replace the variety you planted.
• Cutting the whole plant back hard and expecting flowers next year — A renovated lilac takes two or three years to flower again. Staged renovation over three years keeps some flower throughout.

If it doesn't work

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