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Pruning lilacs

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Lilac (Syringa 'Red Pixie')
Photo: Jardin botanique de Montréal (Robert Mineau)  

Four types of pruning

Formative pruning

This type of pruning begins when the shrub is first planted and continues for about ten years. You must first select and keep a few main branches to form the basic structure. For those lilacs that are prone to suckering, like S. vulgaris and S. x hyacinthiflora, keep 8 to 12 well-angled branches and regularly remove all suckers. Switch to maintenance pruning once the shrub has grown to about 3 m.

Maintenance pruning

First of all, maintenance pruning involves removing all dead or broken wood, branches that are too low (less than 30 cm from the ground), those that are too curved or too close together and any branches that are rubbing or crossing.

Maintenance pruning is also a way to rejuvenate a shrub, by removing some of the weaker, old and less vigorous branches each year. Suckers that form at the base of the tree every year can be selected to replace the main branches. Choose a few of the strongest suckers and get rid of the rest. Removing branches gradually won’t radically alter the shrub’s appearance and, since it may take the new stems up to five years to bloom, your lilac will continue to flower each year. On grafted plants, it is important to keep only those branches growing above the graft union.

Some lilacs, including S. vulgaris, are more susceptible to attack by lilac borers, insect pests that infest branches over 2.5 cm in diameter. A good way of reducing the risk of damage by these insects is to regularly remove larger branches and keep only smaller ones. On those species that produce few suckers and are less susceptible to lilac borers (e.g. S. x prestoniae), however, do not remove the larger branches unless they show signs of weakness.

Maintenance pruning is also a way of preventing fast-growing shrubs from becoming too wide and opens up the centre of a shrub to let in light. You can remove up to 25% of the framework of a healthy plant in a given year.

Maintenance pruning is the best way to keep a lilac vigorous. Otherwise, your plant will weaken after about twenty years.  

Deadheading

After flowering, it is best to remove the spent flower panicles before any fruit form, especially on lilacs with large panicles, such as S. vulgaris, S. x hyacinthiflora and S. x reticulata. Flowers that have gone to seed are not very attractive and drain nutrients from the plant. For lilacs with small inflorescences, however, like S. x prestoniae and S. meyeri, this is very labour intensive and probably unnecessary. In general, it is a good opportunity to remove damaged, diseased or poorly angled branches.

Lilacs form flower buds at the tips of branches during the previous growing season. This means that it is important to prune just under the panicles a week or two after the plant has finished blooming. If you prune the branch tips heavily or after the flower buds have formed, you will have fewer blooms next year.

Rejuvenation pruning

Some old, neglected and damaged lilacs can be cut back hard. Such drastic rejuvenation or renewal pruning is used mainly for those species that are especially prone to suckering (S. vulgaris and S. x hyacinthiflora). It involves cutting the shrub back to about 15 to 25 cm above the ground. For grafted lilacs, keep only the stems growing above the graft union. In subsequent years, continue with formative and maintenance pruning as described above. It will take a few years for a lilac to rebloom after drastic rejuvenation pruning.

For species that produce few suckers and form water sprouts on the main branches (e.g. S. x prestoniae), any necessary rejuvenation pruning should be done over a period of two to three years. This will allow you to cut back the main branches gradually.

When to prune?

Formative pruning

The early years:

  • In early spring, before the buds open (April), or
  • After the leaves have fallen in autumn (September, October)

Subsequently, just after blooming.

Maintenance pruning

Just after blooming, so as not to interfere with the next year’s blooms.

Deadheading

Once flowering is finished, before fruit forms.

Rejuvenation pruning

Iin early spring (April) or fall (September, October), but never during the growing period.

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