Lilacs are forgiving in one respect and stubborn in another. Give them sun and drainage and they will shrug off a British winter, flower for decades and scent an entire corner of the garden in May. Get the planting wrong, though, and you can spend years looking at a healthy green shrub that refuses to produce a single bloom. Most disappointments come down to planting day: too deep, too wet, too shaded. Here is how to get it right, whether you are digging into heavy clay or thin chalk.

What lilacs actually want from the soil

Common lilac, Syringa vulgaris, comes from the limestone hillsides of the Balkans, where the ground is stony, shallow and quick to drain. Two things follow from that. Lilacs tolerate lime far better than most flowering shrubs, and they simply will not forgive roots that sit in water.

What they need is straightforward: full sun for at least six hours in summer, a soil that is neutral to slightly alkaline — roughly pH 6.5 to 8 — and free drainage. Fertility matters much less than you would think. A lilac planted in rich, heavily manured ground tends to throw out long grey-green shoots and very few flowers. The same plant in modest soil, given sun and moving air, flowers hard.

On acid ground you can still grow lilacs. Apply garden lime at planting and again each spring, following the rate on the packet, and avoid spots shaded by mature trees.

Choosing a lilac that suits your ground

On clay, the variety matters less than the planting technique below. On shallow chalk, though, it pays to choose something compact and willing to flower young. These are reliable, widely available choices:

  • Syringa vulgaris 'Charles Joly' — deep magenta doubles on a shrub of around 3m. Strong scent, does well on chalk.
  • 'Katherine Havemeyer' — lavender-pink doubles, similar size, a classic French lilac.
  • 'Sensation' — purple flowers edged in white, slightly smaller and slower growing.
  • 'Madame Lemoine' — white, creamy buds, excellent as a specimen.
  • Syringa meyeri 'Palibin' — a dwarf reaching about 1.2m, flowering while still young. The best choice for a pot, a small border or thin chalk.
  • 'Miss Kim' — later to flower, neat and compact, and noticeably resistant to mildew.
  • Persian lilac (Syringa x persica) — finer leaves, paler flowers, good in a warm sheltered border.

Avoid the biggest French hybrids if space is tight. They are handsome, but a mature lilac casts a fair amount of shade and the roots run close to the surface, so little will grow directly beneath it.

Planting on clay: build a mound

Clay holds water in winter and sets like brick in summer, and lilac roots hate both. The answer is not to dig in bucketloads of grit across the whole border — it is to give the plant a raised root zone it can drain into.

Choose the highest, sunniest part of the garden, well away from the low corner where water collects. Dig a hole about 60cm across and 30cm deep, then fork the base to break the pan. Into the hole, mix two parts garden soil, one part composted bark or well-rotted manure and one part horticultural grit. Build this into a slight mound that sits a few centimetres proud of the surrounding ground, so the crown of the plant is never in a puddle. Water drains sideways and down into the clay beneath rather than sitting around the roots.

If your clay is truly heavy — grey, sticky and slow to warm in spring — plant in late April or early May rather than autumn. The soil is warmer, drainage is better and the lilac establishes faster than it would in cold wet ground.

Planting on chalk and limestone

Chalk presents the opposite problem. It is alkaline, which lilacs like, but it drains fast, dries out in summer and is often only a spade's depth above solid rock. Push a cane into the ground before you buy anything. If you can drive it 30cm without hitting rock, you have enough soil. If not, plant into a raised bed or a large container instead.

Improve a generous area rather than just the planting hole — a square metre of ground, forked over with plenty of compost or well-rotted manure, gives roots somewhere to run. Mulch every spring with a 5–8cm layer of organic matter and be prepared to water through the first two summers, because chalk will not hold moisture for you.

Planting day, step by step

  1. Water the rootball thoroughly an hour before planting, whether it came in a pot or bare-root.
  2. Dig a hole two to three times the width of the rootball but no deeper than the plant was growing before.
  3. Fork the base and add a handful of grit on clay, or compost on chalk.
  4. Set the plant so the graft union — the knobbly join low on the stem — sits level with the soil surface, or no more than 3cm below it. Planted deep, the stem can rot and flowering stops.
  5. Backfill with the improved soil, firming gently in layers with your heel. Do not stamp.
  6. Water in with about 10 litres, even if rain is forecast. This settles soil around the roots.
  7. Mulch 5–8cm deep, keeping it clear of the stem by a hand's width.
  8. Do not feed. A lilac in its first year needs nothing but water and light.

Spacing and position

As a rule of thumb, allow for the mature spread of the cultivar plus half a metre of breathing room. For the big French hybrids that means about 3m from other shrubs and at least 4m from a house wall. 'Palibin' needs only 1.5m, which is why it suits a small front garden so well.

For a loose flowering screen, plant common lilac 1.5–2m apart and let it grow naturally. Clipping lilacs into a tight formal hedge costs you most of the flowers, since they bloom on the previous year's wood. Keep them away from greedy surface-rooted trees such as birch and willow, and remember that the roots are shallow — hoe lightly or not at all near the trunk.

The first two summers, and a little patience

Water newly planted lilacs in dry spells — a full watering can a week is plenty — for two growing seasons. After that they are largely on their own. Deadhead faded flower clusters if you like, cutting back to a pair of sideshoots, but leave any structural pruning until after flowering next May. Remove suckers that appear from below the graft; those belong to the rootstock, not to your chosen variety.

Expect little in the first year. A young lilac often sits still, then puts on modest growth, and only starts flowering properly in its third or fourth season. Once settled, it will do the same thing every May for decades with almost no help from you. That slow start is worth waiting for.

Photo: 8926 / Pixabay