Lilac

Light and exposure

Lilac wants full sun, that is the number one condition for a good bloom. Give it at least six hours of direct sun a day. In shade or even part shade the shrub survives just fine but barely flowers, with thin, sparse clusters.

In our gardens it is often planted on its own in the middle of a lawn, at the back of a border, or along a well exposed south or south west facing fence. It does not mind wind or open spots, a real advantage in our windswept countryside.

Watering

A young lilac, planted less than two years ago, needs regular watering while it roots in, especially during dry summers. A generous but occasional watering works better than a daily trickle, it pushes the roots to grow deep.

Once established, it is a remarkably self sufficient plant. A mature lilac here practically manages without watering, barring an exceptionally long drought. What it really cannot stand is too much water. Waterlogged soil in winter rots the roots far more reliably than a dry summer ever weakens it.

Soil and repotting

Lilac is not fussy about soil type as long as it drains well. It tolerates lime very well, unlike many shrubs, and actually prefers neutral to slightly alkaline soil over acidic ground.

What it cannot cope with is heavy clay that stays waterlogged. If your ground holds moisture, plant it on a slight mound to raise the base above the wet. A little well rotted compost at planting time is enough, there is no need to feed it further afterwards.

Temperature and humidity

This is one of the hardiest shrubs in our gardens. It shrugs off temperatures down to minus 25 or minus 30 degrees without any trouble, more than enough to see out any winter in the Herve region.

Ironically, lilac actually needs that sharp winter cold to flower well the following spring. It is this cold dormant period that triggers the clusters to form. In regions with mild winters, flowering often stays poor and patchy, a problem we simply do not have in zone 8a.

Pruning and upkeep

The key thing to remember first. Lilac flowers on wood formed the previous year, and next spring’s flower buds are already being set during the summer right after this year’s bloom. Pruning in winter or early spring means cutting off this year’s flowers, a very common mistake.

The right move is to remove the spent flower clusters just after flowering, cutting the stem just below the cluster, above the first pair of young shoots. This stops the plant wasting energy on setting seed and encourages a generous bloom the following year. Every three or four years you can also cut one or two of the oldest stems back to the base to rejuvenate the clump, lilac suckers back from the base very readily.

Pests and diseases

Powdery mildew is the most common issue, especially in late summer. A greyish white felting then covers the leaves, often on plants that are crowded or dry at the roots. It looks bad but is not really serious, the shrub will flower normally again the following year. Giving the planting room to breathe and not crowding several bushes together already keeps it well in check.

More of a concern is lilac bacterial blight, caused by Pseudomonas syringae, which blackens and wilts young shoots in spring, especially after cool damp weather. Cut well back below the affected part and destroy the wood you remove. Lilac leafminer tunnels into the leaves and rolls them up, damage that is mostly cosmetic. Scale insects sometimes settle on old, weakened plants, brushing them off is usually enough to deal with them.

Propagation

The simplest and most common method here is to lift suckers, the shoots that come up from the roots around the parent plant. Just dig one up with a bit of root attached in autumn or winter and replant it elsewhere.

One thing to watch for though. Most lilacs sold in nurseries are grafted onto a different rootstock, often a seedling common lilac. Suckers from a grafted plant will then give you the rootstock, not the variety you bought. This trick only reliably works on an own root lilac, often the case with old plants passed down in families. Layering in autumn, by pegging down a low branch and burying a node under a few centimetres of soil, also works well and guarantees you get the right variety back.

Month-by-month care calendar

Green Hub tips

• Never prune lilac in winter or early spring. This year’s flower buds are already formed on last year’s wood, you would be cutting off the coming bloom.
• Cut off the spent flower clusters right after flowering, in May. It is the one pruning job that really matters, and it encourages a good bloom the following year.
• Plant it in full sun, never in shade. A shaded lilac survives just fine but barely flowers, by far the most common mistake.
• Pull out suckers as soon as they appear at the base, or they will take over the whole bed. On a grafted plant they will not even give you the variety you bought.