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An established lilac is close to drought-proof. Its root system is wide and reasonably deep, and in normal seasons it needs no watering at all once past its first two years. Routine watering does more harm than good, because constantly damp soil around the crown encourages root rot and keeps the plant producing soft growth instead of hardening off.
The exceptions are worth knowing. Water deeply during a prolonged drought, particularly in the weeks just after flowering when next year's buds are forming, because a shrub under real water stress at that moment will set fewer buds. Water new plantings weekly through their first season. Water any lilac in a container far more often, since a pot dries out in a day of summer heat.
When you do water, put on the equivalent of an inch (25 mm) at once and then leave it alone until the top four inches (10 cm) of soil have dried. Frequent shallow watering builds a shallow root system that then suffers in the next dry spell, which is the opposite of what you want in a plant meant to stand for thirty years.
Feed lightly, and watch the nitrogen
Lilacs flower best on soil that is not especially rich. A shrub sitting in a lawn that gets regular high-nitrogen fertilizer will grow vigorously, produce dark green leaves, and flower badly, because nitrogen drives shoot growth at the expense of flower buds. If your lilac is huge and green and gives you three trusses a year, look at what is feeding the grass around it before you look for anything else.
One feed a year is plenty. Apply a balanced granular fertilizer over the root zone in early spring as the buds swell, scratch it in and water it. Skip it entirely if the plant is growing strongly. A shrub putting on more than about eighteen inches (45 cm) of new shoot a year does not need feeding.
Where soil is acid, the more useful annual application is lime rather than fertilizer. A handful of garden lime spread over the root zone every second or third year keeps the pH near neutral. Test rather than guess, because over-liming an already alkaline soil locks up iron and turns the leaves yellow between the veins.
| Symptom | Likely cause | Correction |
|---|---|---|
| Big leafy plant, very few flowers | Excess nitrogen, often from lawn feed | Stop feeding, keep lawn fertilizer off the root zone |
| Weak growth, pale leaves overall | Acid or exhausted soil | Test pH, lime if low, single spring feed |
| Yellow between green veins | pH too high, iron locked up | Stop liming, mulch with compost, use a chelated iron feed |
| Healthy plant, no flowers at all | Too much shade, or pruned at the wrong time | Check hours of sun, review pruning date |
| Flowers only at the very top | Crowded, self-shaded shrub | Thin the oldest stems out over several years |
| Buds formed then dropped | Late frost after a warm spell | Nothing to do, the shrub recovers next season |
Lilacs need a real winter
This is the reason lilacs disappoint people in mild climates. The flower buds require a sustained period of cold to break dormancy properly, and without it they either fail to open or open thinly and unevenly. As a working figure, a common lilac wants something on the order of a thousand or more hours below 45F (7C) through the winter, which is a genuine cold season, not a few frosty nights.
In a climate with warm winters the symptoms are consistent: strong healthy foliage, plenty of growth, and either no flowers or a scatter of small malformed trusses. Nothing you feed, prune or water will fix this, because the limitation is climatic. If this describes your garden, the honest answer is that a full-size common lilac is the wrong shrub for the site, and the low-chill kinds bred for mild regions are the only route to flowers.
There is a mirror-image problem in very cold gardens. A hard late frost after an unseasonably warm spell can kill flower buds that had already begun to swell, giving you one flowerless spring on an otherwise perfectly healthy shrub. That is bad luck rather than bad management, and the plant flowers normally the following year.
Winter chill is also why a lilac that flowered magnificently for years in one garden may barely flower in another only a few hundred miles away. Before assuming you have done something wrong, check whether your winters are actually cold enough.
Suckers: keep the rootstock in its place
Lilacs sucker naturally, sending up shoots from the base and from roots running out into the surrounding ground. On a plant growing on its own roots those shoots are simply more of the same plant, and you cut them off to keep the shrub tidy and stop it colonising the border. On a grafted plant, they are a much more serious matter.
Shoots that arise below the graft union belong to the rootstock, not to the variety you bought. The rootstock is chosen for vigor, so those shoots grow faster than the top of the plant. Left alone for a few seasons they overtake it, steal the light, and eventually you are left with a shrub whose flowers are a different and usually plainer thing than the one you planted. People describe this as their lilac 'reverting', but nothing has reverted; the rootstock has simply won.
The habit to build is to walk round the base two or three times a season and look at where each shoot originates. Anything coming from below the graft union, or from the ground out beyond the crown, gets removed. Anything above the union is legitimate new wood and is part of the plant's renewal.
- Find the graft union once and remember it Clear the mulch from the base and identify the swollen joint or bark change near soil level. Mark it mentally against a fixed reference such as the top of a stake, so you can judge shoot origin quickly on later visits.
- Remove rootstock shoots at their point of origin Trace the shoot down to where it joins the root or trunk and cut it flush there. Cutting it off at ground level leaves a stub with dormant buds that sends up two shoots next time.
- Tear rather than cut where you can On soft young suckers, pulling them off with a sharp downward tug removes the basal buds with them and slows regrowth far more than a clean cut does.
- Deal with spreading root suckers separately Shoots appearing a few feet out in the lawn or border come from running roots. Dig down, cut them off the root, and check that spot again the following season.
- Keep a few basal shoots on own-root plants Where the plant is not grafted, retain two or three strong basal shoots each year as replacements for old stems you plan to remove. That is how the shrub renews itself.
The two things worth recording
Note the date the first flowers open each year and roughly how heavy the display was, on any simple scale you will use consistently. Lilacs alternate a little, and a run of several years of notes tells you whether a light year is normal variation or the start of a real decline from shade, crowding or bad pruning timing.
Also record where you removed suckers and whether they came from above or below the graft. On a grafted plant this becomes a map of where the rootstock is pushing, and after two seasons you will know exactly which side of the shrub to check first, which turns a hunt into a two-minute job.