How to Prune Shrubs
The best time to prune deciduous climbers and shrubs depends on whether they bloom on growth produced in the same or previous years. Generally, those that flower after midsummer are pruned hard in spring. Those that bloom in winter, spring and early summer are pruned soon after flowering.
This group contains spring show-stoppers, such as forsythia and flowering currants, as well as early-summer bloomers, including mock orange, weigela, deutzia and spirea. These all flower on stems that were produced the previous year; prune them just after flowering so that new growth can ripen throughout the summer. Remove dead and diseased growth, and cut back old flowering stems, leaving new shoots to take over. Thin overcrowded growth, cutting a third to a fifth of the oldest stems to ground level. Apply an all-purpose granular fertilizer, water well and apply a deep organic mulch.
1. Encourage Flowering
Prune early-flowering shrubs, such as shrubby honeysuckle, in early summer after flowering. Cut a third of the oldest stems to 12 inches from the ground with a pruning saw (image 1), reducing the length gradually to prevent tearing.
2. Maintain Vigor
Trim back tall stems to stimulate new buds to shoot lower down the stems and make a bushier plant (image 2). This also gives new growth room to develop and mature and still leaves sufficient old wood to bloom the following year.
This group contains shrubs and climbers that flower in late summer and autumn, such as butterfly bush, shrubby mallow, panicled hydrangea, Russian sage and hardy fuchsia, as well as late-flowering clematis. Prune all the stems back hard in late winter or early spring to promote lots of new flowering shoots.
1. Shorten Stems
To prevent tall, fast-growing shrubs such as butterfly bush from being damaged by autumn storms, prune the tallest stems after flowering (image 1). Then carry out the main pruning in spring.
2. Cut Back Hard
In spring, remove long whippy stems and thin twiggy growth with clippers (image 2) to reveal the main framework of branches. Using a pruning saw, cut back to create a low structure of healthy stems.
3. Encourage Larger Blooms
Pruning encourages more flowers and healthier growth (image 3). It breathes new life into old shrubs, and can even increase the longevity of short-lived plants, such as shrubby mallow.
Many evergreens aren't as deciduous shrubs and grow more slowly. They are best pruned sparingly to reduce their size and keep them tidy after the frosts, between late spring and late summer. To avoid removing any blooms, prune summer-flowering evergreens, such as escallonia, when the flowers have finished. Never prune evergreens in autumn because any new growth won't have enough time to harden off before frost and could be damaged. If any stems are harmed by frost, leave them until the following spring before removing them.
DK - How to Grow Practically Everything, 2010 Dorling Kindersley Limited
Spring-flowering ceanothus grow very quickly, and without annual pruning they soon fill out their allocated sites and become untidy.
Trim Plants Lightly
Shrubs, such as California lilac, should be pruned lightly after flowering since they may not grow new wood if cut back too hard. Maintain a compact, flower-filled shrub by pruning long, straggly branches by 10–12 inches, but leave some of the shorter stems unpruned to help the plant maintain its strength. Cut above a leaf bud to prevent die-back and stimulate new growth below the cut.