Lilac earns its place in a British garden for about three weeks of the year, and then spends the rest of it sulking. That sulk is usually down to us rather than the plant. Almost every disappointing lilac has one of a handful of problems: it was pruned in the wrong month, planted in the wrong spot, or left to cope alone in soil that bakes dry in July and sits wet all winter.
The good news is that lilacs are forgiving once the basics are right. Most respond within a season, and an old neglected shrub can often be brought back with two or three years of patient pruning. Work through the checks below and you will usually find the culprit fairly quickly.
1. Pruning at the wrong time of year
Lilac flowers on wood it made the previous year. That one fact explains most cases of a healthy-looking shrub that never blooms. Cut it back in autumn, winter or early spring, and you are removing the very shoots that were going to carry this year's flowers. The window closes around midsummer: in most of the UK, aim to finish by mid-June and treat early July as the absolute limit.
Spent flower heads should come off as the blooms fade, before seed sets. Cut the truss back to a strong pair of leaves rather than into the bare old stem — a stub of leafless wood often dies back.
- Deadhead as each truss fades, working round the shrub every few days in late May and June.
- Take out dead, crossing and damaged stems at the same time.
- Thin the congested centre so light and air reach the middle of the plant.
- Every second or third year, remove one or two of the oldest stems right down at ground level.
- If the shrub has been cut hard by mistake, leave it alone and be patient — it may take two or three seasons to flower again.
If you inherit an overgrown lilac whose flowers now sit ten feet up, renovation is worth doing, but spread it over two or three winters and accept a gap in flowering while it recovers.
2. Planting depth and grafted roots
Many lilacs are grafted onto a seedling rootstock or privet, and the join matters. Plant so the point where the stems meet the roots sits at the same level it did in the pot — no deeper, and certainly not buried under a mound of soil or mulch. Bury the union and the plant struggles, flowers poorly and throws up rootstock suckers with a will.
Those suckers, if left, will eventually swamp the named variety you paid for. They are easy to spot: the leaves are usually smaller and differently shaped. Pull or tear them away from the main stem rather than snipping them at ground level, which only encourages more. Mulch thinly around a young lilac and keep compost clear of the bark.
3. Soil that is waterlogged or bone dry
Lilac is more accommodating about soil than its delicate looks suggest. It will happily grow on chalk and other alkaline ground, which makes a pleasant change from most flowering shrubs, so there is no need for ericaceous compost. What it will not tolerate is sitting wet. On heavy clay, plant on a slight mound with grit and garden compost worked into the planting hole, or grow a compact variety such as Syringa meyeri 'Palibin' in a large pot.
At the other extreme, a lilac in thin sandy soil that dries out every summer will flower thinly and lose its leaves to mildew early. A generous mulch of well-rotted organic matter each spring, kept a few centimetres clear of the trunk, evens out the moisture and does more for flowering than any feed.
4. Not enough sun
Lilacs want full sun — six hours or more a day, ideally including the middle of the day. In shade they grow tall and thin, reaching for the light, and flower only at the very top where the sun reaches. They are also far more prone to mildew and to the plain ugliness of a bare-legged shrub. If yours is in the wrong place, moving a young plant in autumn or early spring is far kinder than years of corrective pruning.
5. Feeding too enthusiastically
Lilacs are not hungry plants, and generosity backfires. A high-nitrogen feed, a rich pile of manure or fertiliser drifting across from the lawn produces soft, leafy growth at the expense of flowers — and soft growth is exactly what mildew and aphids fancy. A balanced general fertiliser in early spring, plus the annual mulch, is plenty. Stop feeding by midsummer so the new growth ripens before winter.
6. Powdery mildew and other leaf troubles
The grey-white dusting that appears on lilac leaves in late summer is powdery mildew, and it is a symptom rather than the disease itself. It thrives where roots are dry, air is still and the plant sits in shade. Improve those three things and it usually fades in importance.
- Water thoroughly in dry spells, at the base rather than over the leaves.
- Thin crowded shoots so breezes move through the shrub.
- Clear fallen leaves in autumn rather than leaving them to overwinter under the plant.
- Accept that a late-summer trace of mildew on an otherwise vigorous lilac is mostly cosmetic.
If it arrives in June every year, that is the site telling you something. Shoots that blacken and die back in spring are a different problem, usually a bacterial blight, and affected stems should be cut out well below the damage and the prunings binned rather than composted.
A lilac pruned in the wrong month will forgive you within a year or two. One grown in the wrong place rarely does.
Your lilac troubleshooting checklist
Run through this in order and you will usually find the answer without guesswork. Is it in full sun? Is the graft above ground? Does the soil drain? Was it pruned after midsummer last year? Has it been fed too richly? Are rootstock suckers competing with it? Is the centre congested enough to trap damp air?
Fix the first problem on that list that applies, then wait a full season before judging the results — lilacs are slow to answer, but they answer clearly. If you are dealing with an old, neglected shrub on rented or shared ground, it is worth checking with whoever owns the land before starting renovation work, and taking professional advice if a large tree needs moving.
Photo: Wyxina / Pixabay
