Pruning Shrubs

How to Rejuvenate an Old Lilac?

For many, the blooming of lilacs is the quintessential heralding of spring. This perfume that travels. These bouquets we enter the house! However, as time goes by, lilacs become larger and larger and the flowers become less and less accessible.

Ah! The beautiful lilacs! How to do without them? Image: RitaE en Pixabay

We end up with huge bare trunks. Our lilacs are losing their charm. Since common lilacs (Syringa spp.) are plants that sucker profusely, it is quite simple to carry out a makeover in order to make old lilacs look youthful again.  

Let’s Talk About Tools

Rejuvenating lilacs requires fairly severe pruning. To carry out this pruning, you will need to have a good pruning saw and long-handled pruning shears. Well-sharpened and clean tools are essential. The small hand pruner will also be practical.  

Let’s Talk About Lilac Flowers!

The pruning period for lilacs aims to compromise flowering as little as possible. That said, there will inevitably be a slight drop in flowering. But to limit the damage as much as possible, prune the lilacs immediately after they bloom. Yes, the pruning is done in summer!

The production of lilac flower buds begins in summer. As soon as a flowering is finished, the plant is already preparing for its next show! Flower buds develop in pairs at the ends of the stems. Already, in the fall, you can clearly distinguish the two large chubby buds which will bloom the following spring.

Thus, by pruning early in the summer, you allow the plant to develop new shoots and possibly new flower buds. If you prune in the fall or early in the spring, flowering is inevitably compromised.

That said, it is not sacrilege to perform lilac rejuvenation pruning in spring or fall. You just have to remember that you risk cutting flower buds.  

Proceed Step by Step

Often, the old lilac has several very old trunks surrounded by small regrowth. What you are looking to obtain, ideally, is a good mixture of very old trunks, middle-aged trunks and young regrowth to ensure succession.

By suddenly pruning all the old trunks to ground level, we often end up with a big hole in the landscape!

This is why the rejuvenation of lilacs occurs gradually. You can choose to make it a two- or three-year project, depending on the size of the shrub.  

The Radical Cut

The first step consists of choosing one, two or three trunks, among the oldest. These trunks will be sawed 3 inches (10 cm) from the ground! Yes, cut everything! To ensure good cuts, first prune the trunk to a comfortable height. Then make the final cut close to the ground, giving it a slightly inclined angle. The first cut is intended to remove weight, making it easier to make a nice cut close to the ground. This is the one that counts. The inclined angle will drain rainwater and prevent rot and disease entry.

We will do the same for the other trunk or trunks, if necessary.  

The oldest trunks are cut down to the ground, giving a slightly inclined angle to the cut. Image: Julie Boudreau

Choose the Next Generation

Once the large trunks have been removed, proceed to the selection of the trunks of the future! Choose young trunks with a good diameter and which are well positioned. For each old trunk removed, one or two young trunks are chosen. When the selection is made, cut at ground level all the stems that are within a 3 inches (10 cm) radius around the lilacs that we want to keep.

This trunk is of a good caliber to ensure the future. It will be kept. Image: Julie Boudreau

Subsequently, select young, well-positioned regrowths which can become the future succession of our young trunks. And this is how we put in place a whole rotation program where the youngest are already in place to replace the old ones!

Everything else, all those suckers that come out everywhere are pruned to the ground, just to concentrate the energy where it’s important: in the flowers!  

Any suckers growing where they shouldn’t are cut to the ground. Image: Julie Boudreau

Repeat the Operation

The following year, still after flowering, the last old trunk(s) are cut to the ground. Or extend the pleasure over another year! But already, the large hole left by the removal of the old trunks will quickly be filled. In just two or three years, the lilac regains its generous flowering and, above all, its youthful appearance.  

Delay Aging

Often, the problem with lilacs that are too old is that the flowers become inaccessible, because the plant is too tall. This phenomenon can be delayed or prevented by pruning, again, immediately after flowering. As soon as a branch reaches more than 6 feet (two meters) in height, it is pruned by around a third. This limits height growth and forces the plant to branch. And who says more branches… says more flowers!  

Rejuvenating an old lilac can unfortunately create a hole, with the disappearance of large branches. Luckily, lilac is a vigorous plant and you should take advantage of this character trait.

Julie Boudreau is a horticulturist who trained at the Institut de technologie agroalimentaire in Saint-Hyacinthe, Québec. She’s been working with plants for more than 25 years. She has published many gardening books and hosted various radio and television shows. She now teaches horticulture at the Centre de formation horticole of Laval. A great gardening enthusiast, she’s devoted to promoting gardening, garden design, botany and ecology in every form. Born a fan of organic gardening, she’s curious and cultivates a passion for all that can be eaten. Julie Boudreau is “epicurious” and also fascinated by Latin names.

1 comment on “How to Rejuvenate an Old Lilac?

  1. It is better to prune them annually, rather than only as a project that lasts for two or three years before letting them get overgrown again. Although I prefer to prune mine while they are dormant through winter, I know that it is better to prune them after bloom, and after new foliage has matured a bit, during summer. I leave stems that grew the previous year, as well as those that grew two years earlier and have already bloomed, but remove stems that are three years old. It is probably better to leave more stems that are four, five or six years old, but remove only those that are old enough to slow down. They will be branching more, and not producing long vigorous stems. We knew the technique as ‘alternating canes’. It is useful for elderberries, weigelas and forsythias also. I cut the old canes to the ground without concern for the angle of the cut because I do not want them to regenerate. They should die and rot as they are replaced by new basal growth.

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