Pruning: Flowering shrubs

Flowering shrubs need regular attention to continually give of their best and an important part in their care is pruning. There are few rules about what to prune when, and there are always exceptions. One guide is to prune as long before the shrub flowers as possible.

Some shrubs flower on stems that have grown during the current year, others on stems that were formed during the previous season. The time of year a shrub flowers, and the age of the stem on which it flowers, will determine how it should be pruned. Regular pruning of flowering shrubs will keep the plant to an attractive shape and will encourage large, healthy flowers.

In general, pruning is only necessary for mature shrubs, so wait a year or two after planting before starting to prune. Shrubs that flower on the current season’s growth tend to flower in late summer or autumn. The ideal time to prune these is during the winter or early spring. The stems of the butterfly bush, Buddleja davidii, should be cut right down to the base, leaving a woody clump.

Start pruning

Cut just above the first shoot at the bottom of the stem. Even by the end of a single season the stems can be thick and woody, so you will need loppers or a pruning saw rather than a pair of secateurs. After cutting back all the stems you will be left with an unattractive stump, but not cutting it back could result in a shrub 3m tall with a solitary flower at the tip, similar to those found on railway embankments.

The Russian sage, Perovskia atriplicifolia, should be treated similarly, but waiting until late spring to do it to will lessen the chance of frost damage to shoots.

The majority of shrubs that flower on new growth are not as vigorous as Buddleja or Perovskia, and cutting back all the stems can leave a shrub looking thin and weedy. The best strategy is to remove about a third of the stems by cutting them down at the base.

If you are unsure how much of the shrub to cut down, err on the side of caution, cutting back fewer stems. Hydrangea paniculata, for example, can end up as an ugly stump if all of it is cut down continually. Cut any dead, overcrowded and thin stems back to the base, but cut back the flowered stems to the next bud.

Variations

Not all plants in a genus should be pruned the same way. Buddleja alternifolia, for example, flowers on stems that have grown the previous year, so removing these stems in the spring would leave you with no flowers. As soon as its flowers have died, follow down along the stem and cut just above a new shoot.

Cistus, the sun rose, and Convolvulus cneorum also flower on previous-season growth. These are shrubs that will continue flowering and maintain a neat shape without any pruning. However, vigorous new shoots produce the most flowers, while older branches produce few or none at all. Removing flowering shoots just after they have flowered will stimulate the plant to produce more flowers. Cut the flowered stems off just above a bud about a half to two-thirds down the stem. It won’t have a dramatic impact on the shape of the shrub, but you will have more flowers the following year.

Some flowering shrubs do not benefit from annual pruning. Witch hazels and viburnums are best left alone, although every three or four years you can remove some branches to prevent the shrub from becoming overcrowded. Do this after flowering and cut back the branches to the base or to the joint of another branch. Remove any dead branches too.

Renewing old shrubs

Shrubs that have been neglected and left unpruned will develop into a scraggy mass of branches with few flowers, or a tall lanky shrub with bare stems and miserable flowers out of sight at the top. If you’ve let a shrub get into such a state, or you’ve inherited one, don’t despair. You can bring it back to its former, happier self.

• Completely removing any dead branches.
• Take out branches that are growing across the shrub and any that are rubbing against each other. Aim for an open structure that the air and light can get into.
• Cut down the shrub by two-thirds. Drastic, but beneficial.

Points to consider

• In the year following such severe treatment the plant will put its energy into producing new shoots, so you won’t get many flowers.
• Pruning shrubs in this way can be done at any time of year. With deciduous shrubs it’s easier to do it in early winter, after the leaves have fallen, so that you can see clearly what you are doing.
• Not all shrubs respond to this treatment. Daphne, Genista and Cytisus (except C. battandieri) will not re-grow from mature, woody stems. In general, if you see dormant buds on old stems, the shrub will re-grow. Rhododendrons will respond to hard pruning but, remember, the buds will be hidden under the bark.

Plant notebook

Shrubs that flower on new growth:
Buddleja davidii, Clerodendrum bungei, Diervilla, Hydrangea paniculata, Philadelphus, Spiraea x bumalda, Spiraea japonica.

Shrubs that flower on previous-season growth:
Buddleja alternifolia, Buddleja globosa, Buddleja colvilei, Deutzia, Exochorda, Forsythia, Ribes, Spiraea ‘Arguta’, Spiraea thunbergii, Spiraea veitchii, Weigela.

Flowering shrubs that require minimal pruning:
Amelanchier, Callicarpa, Hamamelis, Magnolia, Syringa, Viburnum.

Words: John Hoyland is a plant expert and former nursery owner