Lilacs are generous plants. Give them sun, decent drainage and a cold winter, and they will reward you with dense cones of scent for weeks in May. But they are also quick to show their displeasure, and the same three problems come up again and again in British gardens: leaves that turn yellow, a grey-white dusting over the foliage, and a thicket of suckers crowding the base of the shrub.

None of these is a disaster. Two are usually cultural rather than infectious, and one is simply the plant doing what lilacs do. What matters is telling them apart, because the fix for each is quite different.

Yellow leaves: work out which yellow you have

"My lilac's leaves are going yellow" covers at least four separate situations, and the treatment depends entirely on which one you are looking at. Before you reach for a feed or a fungicide, note three things: where the yellowing starts on the plant, whether the veins stay green, and when it happens.

  • Older leaves at the base, yellowing evenly, in late summer or autumn. This is normal senescence. The shrub is withdrawing nutrients before leaf fall. Nothing to do beyond clearing the fallen leaves.
  • Yellow between green veins, on newer growth, from spring onwards. Classic iron or manganese deficiency, and almost always a pH problem rather than a shortage in the soil. Lilacs prefer neutral to slightly alkaline ground; on very acid or waterlogged soils the iron is present but locked away.
  • Pale, washed-out foliage all over, with weak growth and few flowers. Usually simple hunger, or a plant that has been shaded out by a neighbour. Lilacs flower best in full sun and resent crowding.
  • Sudden yellowing and wilting on one side, often in midsummer. Suspect root trouble: waterlogging, drought stress on a young plant, or honey fungus in the area. Check the base of the stem for white fungal sheets under the bark.

Interveinal yellowing is the one worth acting on. A soil test kit from a garden centre will tell you the pH in minutes — anything below about 6.5 is worth correcting for lilacs. Spread garden lime at the rate on the packet in autumn or early spring, work it lightly into the surface and water it in. Do not expect an instant fix; it takes a season or two to shift pH in the root zone. As a stopgap, a liquid feed containing chelated iron applied to the leaves in early summer will green the plant up within a fortnight, but it is a cosmetic repair, not a cure. Fix the soil and you fix the plant.

If the yellowing came on suddenly after a wet winter, the answer may just be drainage. Lilacs hate sitting in soggy ground. If your soil is heavy clay, plant with plenty of grit and organic matter mixed into the planting hole, and keep the crown slightly proud of the surrounding soil.

Powdery mildew: the grey dusting on lilac foliage

Powdery mildew is the most common disease of lilacs in Britain, and it is easy to recognise: a floury white or grey coating on the upper leaf surface, often starting on shaded lower leaves in late summer. Affected leaves can twist, drop early or look dusty and tired by August.

It is worth knowing that powdery mildew is not usually fatal. It is a stress symptom as much as an infection. The fungus thrives when the plant is dry at the roots but humid around the leaves, when air cannot move through the shrub, and when the lilac is growing in shade. That combination — dry soil, still air — is exactly what happens to a lilac crammed against a fence.

What to do about it

  1. Improve air movement. Thin out congested growth in late winter, removing the oldest stems at the base and any branches that rub or cross in the centre.
  2. Water the roots, not the leaves. A good soak once a week in dry spells does more good than a daily sprinkle, and it keeps the foliage dry.
  3. Mulch in spring with well-rotted organic matter to even out moisture levels through summer.
  4. Clear fallen leaves in autumn and bin them rather than composting, as the fungus overwinters on debris.
  5. If the problem is severe every year, a garden fungicide containing sulphur or potassium bicarbonate can be applied at the first sign of the coating. Follow the label exactly, spray in the evening, and never spray in full sun or when bees are active on the flowers.

Resistant varieties exist, and if you are planting new stock it pays to ask at the nursery. Old favourites such as Syringa vulgaris cultivars vary in susceptibility, while many of the Syringa x persica and Syringa microphylla types cope far better with British humidity. Note that S. microphylla 'Superba' flowers later and often repeats in autumn, which spreads your season as well.

Do not feed a mildewed lilac with a high-nitrogen fertiliser. Soft, sappy growth is exactly what the fungus wants. A light dressing of a balanced feed in spring is plenty.

Suckers: the thicket at the base

Most garden lilacs are grafted onto a rootstock, and that rootstock sends up shoots from below the graft union. Suckers are vigorous, straight, often with slightly different leaves from the top growth, and they will eventually smother the flowering cultivar if you ignore them. Even own-root lilacs sucker to some degree, spreading outwards into a clump.

There is a right and a wrong way to deal with them. Cutting suckers off at ground level with secateurs simply prompts more. The reliable method is to dig down to where the sucker joins the root, twist or tear it away cleanly, and firm the soil back. Do this in spring when the soil is workable and the shoots are still soft enough to pull by hand.

If the mass of suckers has already become the main event, take the whole clump in hand over two winters:

  • Year one: remove the weakest third of the stems, including all obvious suckers, and clear the ground around the base so you can see what you are doing.
  • Year two: remove another third, keeping the strongest flowering stems.
  • Ongoing: check the base every spring and pull any new sucker while it is small. Two minutes now saves an hour later.

Do not confuse suckers with the natural spreading habit of a healthy lilac, or with Syringa species grown on their own roots, where a colony is part of the charm. If your plant was bought as a named cultivar and the base shoots are flowering differently from the top, that is the giveaway that you are looking at rootstock.

A calm, practical routine

Most lilac troubles come back to the same handful of causes: not enough sun, poor drainage, crowded growth and a plant left to its own devices for a decade. Set aside half an hour in late winter each year. Cut out dead, damaged and crossing wood. Pull any sucker you can reach. Spread a mulch, water deeply in dry spells and keep the ground around the base clear.

Then leave it alone to flower. Lilacs are long-lived shrubs when they are given reasonable conditions, and a well-grown specimen can look good for decades. If yellowing persists despite correcting the soil, or a plant suddenly collapses in midsummer, it is worth asking a knowledgeable nursery or your local garden society for a second opinion before spending money on treatments.

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