How to Prune Lilacs Correctly

prune lilacs Pruning

When the lilacs have faded, you should reach for the pruning shears and cut out the withered inflorescences. Read here what else is important when pruning the flowering shrubs.

The lilac (Syringa vulgaris) is an old cottage garden plant and is still one of the most popular flowering shrubs. Its flower buds are usually in pairs at the ends of the branches formed the previous year and open from the end of April to mid-May, depending on the region. At the beginning of June, the fragrant splendour is usually over and the dried inflorescences are no longer particularly attractive. Then it is time to reach for the scissors and prune the lilac.

How to prune lilacs

Pruning lilacs: The most important things in a nutshell

  • In spring or autumn, young lilacs are pruned and old shrubs are pruned for regeneration. This involves cutting back part of the main branches or shoots. In the case of young plants, weak and bent shoots are also removed.
  • After flowering, you can carefully cut out the withered inflorescences to stimulate the formation of new shoots. Cut back every third flowering shoot a little more to prevent the shrub from becoming bald from the inside out.
  • Varieties of the sweet lily form unwanted root runners that should be removed regularly during the summer.

When should you prune lilacs?

In order to maintain your lilacs and stimulate the formation of new shoots, you should carry out the so-called maintenance pruning at the end of May at the earliest – when the flowering period is over. In addition, treat low lilac species to a thinning pruning directly after flowering. With a vigorous rejuvenation pruning, old, senile shrubs become vital and flowering again. The right time for this is in early spring or autumn. This is also the optimal time to give young lilacs a nursery pruning.

Maintenance pruning for lush blooms

If you are bothered by the wilted flower corollas, you can remove them with garden shears immediately after flowering. Cut them out without damaging the young, still soft shoots that have sprouted just below the flower heads – they are already bearing the flower buds for the next season.

Whether the removal of the old inflorescences actually leads to the plant putting more energy into the formation of new flower buds is disputed among experts. Observations show that even unpruned lilacs remain flowering well into old age. However, the older branches become senile over time and the lateral branches inside the crown gradually die. This leads to the shrubs becoming bald from the inside out over the years and to relatively strong branching in the outer crown area. To counteract this process, you should cut back every third flowering shoot a little more after flowering and either divert it to an existing side shoot or to an eye. Stronger pruning down to the two-year-old wood is also possible. Tip: Simply cut back a few bouquets for the vase at regular intervals during the flowering period – this will automatically prevent the crown from becoming senescent and calloused.

prune lilacs

Remove root runners

All varieties of the noble lilac (Syringa Vulgaris hybrids) form root runners. Especially many of the unwanted shoots form on the main roots of the grafted lilac varieties close to the surface. These offshoots, which are not “rootless”, are wildlings – they should therefore be removed again and again during the summer while they are still thin and only weakly woody. Rip the runners out of the soil with a strong jerk in the direction of the trunk. Because of the runner problem, most lilacs are nowadays propagated in the laboratory by meristem culture. They usually form only a few runners and these have the same flower colour as the mother plant – for these reasons they are less problematic.

Thinning out old lilacs in spring by pruning

The noble lilac also tolerates vigorous rejuvenation pruning, but you should spread this over a period of two to three years. In this way you can prevent the flowering from stopping altogether for a few years. In early spring, cut off one third to one half of the main branches at different heights – from about knee height to just above ground level. In the course of the season, they will sprout again with numerous new shoots, of which only two to three strong, well-distributed specimens will be left the next spring. These are shortened again so that they become stronger and branch out well.

Pruning lilacs

Pruning for young lilacs

If you have bought a new lilac, you should remove all bent and weak shoots when planting in spring or autumn and shorten the main shoots by about one third to one half. You will then have to do without flowers in the first year, but the young shrubs will build up nicely from below and become all the more splendid when they are older.

Pruning dwarf lilacs

Low lilac species such as the dwarf lilac (Syringa meyeri ‘Palibin’) or the Korean lilac (Syringa patula ‘Miss Kim’) differ considerably in growth from the noble lilac. They usually grow only 1.5 to 2 metres tall and form a very dense, bushy crown. These species do well when thinned out immediately after flowering. Cut off the oldest branches close to the ground every three years or so.

Jonathan Harvey
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