Pruning shrubs

/Pruning shrubs
Pruning shrubs 2013-11-19T10:18:54+00:00

The more a plant enhances the garden, the more care it needs.  Shrubs with exuberant flowers need regular attention to continually give of their best, and an important element in their care is pruning.   There are few rules about what to prune when and there always exceptions (and exceptions to the exceptions). One good 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, Buddleia davidii, should be cut right down to their base, leaving a woody base. 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 to do the cutting, 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 would result in a fine example of one of those 3m tall shrubs with a solitary flower at the tip that colonise railway embankments.  The Russian Sage, Perovskia atriplicifolia, should be treated in the same way, but waiting until late spring to do it to will lessen the possibility of frost damage to new shoots.

The majority of shrubs that flower on new growth are not as vigorous as Buddleia or Perovskia, and cutting back all the stems can leave a shrub looking thin and weedy.  The best strategy is 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 then err on the side of caution, cutting back fewer stems. Hydrangea paniculata, for example, can end up as a an ugly stump if all of it is continually cut down. The best way to deal with it is to cut any dead, overcrowded and thin stems back to the base but cut back the flowering stems back to the next bud.

Not all plants in a genus should be pruned the same way.  Buddleia 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-year’s growth.  These are shrubs that will continue flowering and maintain a neat shape without any pruning.  However, vigorous new shoots produce 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.  Simply cut the flowered stems off just above a bud about 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 prunning. The witch hazels and viburnums are best left alone.  Every three of four years it may be necessary remove some branches in order to prevent the shrub from becoming overcrowded.  Do this after flowering and cut back the branches either to the base or to the joint of another branch.  Remove any dead branches at the same time.

Sharpening secateurs

Don’t go anywhere near a shrub with blunt secateurs. The cuts you make must be crisp and clean, and this requires a sharp blade.  If you carry a small sharpening stone in your pocket you can sharpen you secateurs regularly.  Little and often are the watchwords.  Draw the cutting edge of the blade firmly and slowly five or six times over the stone at an angle of about 30°, with the bevelled  edge of the blade downwards.  Then pass over the other side of the blade just once to remove any fragments of metal that have resulted from the sharpening.  That is it.  Get into the habit of doing it often and your secateurs will stay sharp.

Shrubs that flower 

on new growth

Buddleia davidii

Clerodendrum bungei

Diervilla

Hydrangea paniculata

Philadelphus

Spirea x bumalda, S. japonica

Shrubs that flower 

on previous-season growth

Buddleia alternifolia

Buddleia globosa

Buddleia colvilei

Deutzia

Exhorda

Forsythia

Ribes

Spirea x arguta, Spirea thunbergii, S. veitchii

Weigela

Flowering shrubs that require minimal pruning

Amelanchia

Calicarpa

Hamemalis

Magnolia

Syringa

Viburnum