All established hedges require trimming to keep them dense and compact and formal ones require more frequent trimming than informal hedges. However always make sure when you’re working on hedges that you check there are no birds nesting, as it is an offence under the Wildlife & Countryside Act 1981 to damage or destroy the nest of any wild bird while it is in use or being built.
When to trim
For established hedges trimming is generally done in the growing season when hedges put on a lot of growth so that’s spring and summer. New hedges need formative pruning for the first couple of years after planting and that is usually carried out in winter or spring.
With conifer hedges, make sure you do not trim them after August, as this can encourage bare patches to develop in the hedge. Yew can be safely pruned into early autumn (September).
What to Use
Hand held shears are good for smaller hedges but larger ones are far easier with electric or petrol trimmers.
There is no need for the width of even vigorous hedges to exceed 60cm (2ft) if they are regularly trimmed. Formal hedges should be slightly tapered on both sides so that the base is wider than the top and light can reach the bottom of the hedge
Pruning an informal hedge is much like pruning normal shrubs. In general, when pruning informal hedges, remove misplaced shoots and cut back the hedge to its required size. Use secateurs or loppers where practical, especially if the hedge has large evergreen leaves, to avoid unsightly leaf damage.
Prune hedges where flowers are desired only once at the correct time of year (usually just after flowering) to encourage flowering the following year. Pruning at the wrong time of year could remove the growth that will flower next year.
Prune those plants that flower on the current season's growth once in spring, as they will still be able to produce flowers that year.
Reduce the current season's growth by half in summer for plants that flower on one-year-old growth
In the case of shrubs that produce berries like Pyracantha and Cotoneaster delay trimming until the berries have all gone.
Keeping hedges tidy and under control is far easier with regular attention so make sure you give it a snip when it needs it. Neglect your hedge and it will lose its shape and become unruly so frequent light snips are better than infrequent heavy pruning sessions.