Common lilac care and pruning: your complete guide

Lilac care–Growing location for lilacs: fertile, well-drained soil

Lilacs thrive best in fresh, fertile, humus-rich, and well-drained soil, though they can tolerate less ideal conditions. They bloom most profusely in full sun, but partial shade can prolong flowering and help retain soil moisture.

How to plant a potted lilac sapling

Spring is the ideal time to plant a lilac. Dig a hole at least twice as deep and wide as the pot containing the sapling. Loosen the soil and fill the hole so that you can plant the sapling at the same depth as it was in the pot. Mix in about one-third compost or well-rotted horse manure with the soil. Water the sapling thoroughly before planting.

During the first summer, make sure the lilac receives adequate water. Applying compost or other mulch helps retain moisture and suppress weeds.

Apply compost or well-rotted manure to your lilacs each spring.

Pruning lilacs—only when necessary

Lilac flower buds form at the tips of old shoots, so avoid unnecessary pruning. Age alone isn’t a reason to prune, since even old bushes often bloom magnificently.

In poor soil, shrubs may become scraggly and their flowering diminishes. In such cases, rejuvenate the lilac by sawing off two or three of the poorest branches at the base early in spring.

New branches will grow to replace them. Repeat this procedure the following spring. It’s not recommended to cut back the entire shrub, as this will result in numerous new shoots but delayed flowering for several years. After pruning, it’s beneficial to spread ash and compost at the base of the shrub.

Shaping a lilac into a tree

You can grow a lilac sapling into a small tree. Remove all shoots except the strongest, most upright one, which will become the main shoot and the tree’s trunk. You can support this shoot with a stake. Cut off the lower branches from the main shoot, leaving only a few top branches. If the shrub is very short, allow the trunk to gain height first. In the following years, you can remove the lower branches as the trunk grows taller and produces new top branches. It’s advisable to prune the top branches to encourage branching—remember that a branch will develop near each pruning point. Shape the crown in spring before the leaves emerge. Pruning in spring promotes growth, while summer pruning inhibits it.

Transplanting lilacs

You can transplant a lilac shrub in spring once the ground has thawed, or in autumn after the leaves have fallen. Before digging up the shrub, prepare the new planting hole. Try to get as many roots as possible from a wide area. In the spring and summer following the transplantation, the lilac will need ample watering to establish roots in its new location. Remove any grass or weeds growing at the base of the shrub, as they can hinder rooting.

Propagation of lilacs

A quick way to get a flowering shrub is to detach root suckers from an old common lilac early in spring and plant them in a new location. Use a sharp planting trowel to sever the root cluster of the root sucker from the mother plant. Prune the seedlings so that only a few pairs of buds remain. Plant the lilac sapling immediately in its new place, and remember to water it throughout the growing season.

Lilacs can also be propagated by layering. Bend a branch down to the ground, secure it, and cover it with soil, leaving the tip exposed. After a couple of years, it will have rooted and can be separated from the parent plant.

Common lilacs can also be propagated from softwood cuttings taken when the mother plant is in vigorous growth. Protect the cuttings from direct sun by using a plastic tunnel, for example. Cloudy weather enhances rooting.

Seed propagation requires a three-month cold treatment. Sow the seeds in autumn directly into the ground or a container, and cover them with sand.

Lilac pests

Aphids

If you notice ants climbing on the trunk of your lilac, it’s a sign of aphids on the leaves, which the ants keep as their ‘dairy cows.’ Aphids excrete sweet honeydew, which is a delicacy for ants. Feeding damage from aphids can cause the lilac’s leaves to turn brown.

You can control aphids with their natural predators, such as ladybugs and birds. Alternatively, you can use a strong water spray or crush them by hand. If necessary, you can use approved pesticides, but keep in mind that these can also harm beneficial insects.

Bugs (Heteroptera)

Lilacs have other pests as well, such as various bugs (Heteroptera) that suck sap from their leaves. Bugs are small, agile insects with hundreds of species in Finland. They are active in the mornings and evenings, and hide during the day in litter and sheltered places away from predatory insects and birds. Their saliva damages plant cells, and bug damage often appears as browning and distinct spots on leaves. Young or weakened lilacs are more susceptible to bug damage. Therefore, it’s crucial to maintain the overall health of the lilac by providing adequate nutrients and water.

Leafhoppers

In recent years, Igutettix oculatus, a species of leafhoppers, has become more common on lilacs in Finland. This leafhopper overwinters in the egg stage within the lilac’s leaf buds. The larvae are pale and translucent, while adult leafhoppers are multicolored, often with a brownish base color. They live on the undersides of the leaves. Leafhopper damage appears as small pale spots or streaks, leading to a general paleness of the leaves in late summer. You can combat leafhoppers by applying a spray in spring (paraffin or rapeseed oil-based products) before the buds begin to swell at the end of April.

In general, large woody plants can withstand quite severe damage. The exceptions are young, newly planted seedlings or otherwise weakened individuals, to which pest-induced stress can cause serious symptoms.

Controlling lilac pests

An effective method for controlling harmful insects is to install a few nest boxes for tits in your yard. Tits collect thousands of insects to feed their young and provide a song concert as a bonus. Other insect-eating birds can also significantly reduce pest populations.

Lilac diseases

Wilt disease

If your lilac’s leaves turn pale and gradually brown, the reason may be wilt disease, or Verticillium wilt, caused by the Verticillium spp. fungi. As the name suggests, it stunts the lilac’s growth. The leaves lose color, wilt, and drop. Wilt disease affects other ornamental plants as well. The only control method is to remove the affected shrubs and growing medium, and to replace the soil before planting new seedlings.

Honey fungus

The common decomposer of old trees and shrubs, honey fungus (Armillaria mellea) can also cause the lilac to wilt and eventually die.

Frost damage

If lilac buds turn brown and shrivel, or if the flowers wither before opening, frost damage may be the cause. If temperatures drop below freezing when the lilac is in bud, the frost can damage the buds.