Pruning lilac: when & how

Grown for their attractive and scented flowers, lilacs need pruning to keep them looking their best. Find out all you need to know about pruning lilacs.

With their showy panicles of flowers and heart-shaped foliage, lilacs (Syringa) are a great addition to any garden. However, to prevent them from becoming leggy or unruly, regularly pruning lilacs is a necessary task. Read on to find out how to prune your lilac for the best floral display and when to do so.

Pruning lilac: when to prune

With lilac types ranging in colour and size, there are many to choose from. That being said, all lilacs flower on the old wood and should be pruned annually immediately after flowering to prevent losing the following year’s flowers. Here in the UK, Syringa bloom from April until June and should be pruned by the end of June at the latest.

Over time, old and neglected lilacs can grow tall and leggy and only produce flowers at the top of the shrub or tree. In this instance, you can cut back hard or do a rejuvenation pruning of your lilacs when they are dormant in winter to reduce their size and encourage new growth lower down.

Tip: when to cut back lilacs depends on the desired outcome and pruning method.

How to prune lilac

Pruning lilac trees and shrubs can be laborious. However, annually pruning Syringa will not only help to encourage the best flowering display the following year, but will also ensure good air circulation and help prevent disease. Here is a closer look at the different methods for pruning lilacs.

Maintenance pruning

Once the lilac flowers have faded, you can start your annual lilac prune by removing any dead, damaged or rubbing branches using either a pair of clean and sharp secateurs or loppers. Once completed, remove the spent flowers, cutting just above the top pair of leaves. You can also shorten any weak and thin or overly long branches, especially if non-flowering back to a pair of leaves or buds.

Over time, lilacs can become congested, which can lead to poor ventilation and fungal diseases. To prevent congestion, you can also remove the odd woody stem thicker than 5cm in diameter at the base using loppers or a pruning saw.

Lilac trees can sometimes produce suckers along the lower trunk that can spoil its appearance. You can remove these suckers, along with any growing at the base of the tree, using secateurs. Just be careful not to damage the bark or roots.

Rejuvenation pruning

As lilacs mature, they can become leggy and only flower up high where you are unable to appreciate them and their scent. In this case and with neglected or inherited overgrown lilacs, more radical pruning may be required to bring them under control and encourage them to flower lower down.

One option is to prune an overgrown lilac back hard all over to around 30cm above the soil. This type of pruning is best performed in late winter. You may need a pruning saw for the thicker stems, which should be cut just above a pair of buds if they are visible. Lilacs generally respond well to hard pruning. That said, hard pruning comes at the expense of flowers for a season or two and is not completely without risk.

Another option is to gradually rejuvenate the lilac over 3 years by pruning away one-third of the oldest stems back to 30cm each winter. By pruning only one-third of the old wood each year, some of the current growth is left behind and will continue to bloom and remain attractive. As a less radical option, this gradual method is often preferred.

Hard-pruning lilacs may initiate root suckers to develop, which divert the energy away from the shrub or tree. If they do appear, simply remove the suckers and dispose of them or use them to propagate new lilac plants from.

Fertilising lilacs is not always strictly necessary, especially when grown on fertile soil. However, after hard or rejuvenation pruning of a lilac, you can apply a fertiliser to help support the new growth. Our Plantura Flower Food is ideal, as it will encourage new flower formation and, being made from 100% natural ingredients, is safe to use around children and pets.

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In order for lilacs to thrive and flower best, they should be grown in full sun and on a free-draining soil. You can learn more about planting lilacs here in our separate article.