Established hedges require trimming to keep them dense and compact.

Formal hedges require more frequent trimming than informal hedges.

When to trim Hedges

New hedges require formative pruning for their first couple of years after planting. Formative pruning is usually carried out in winter or spring.
After this, maintenance trimming is carried out, usually once a year for informal hedges and twice a year for formal hedges. Some formal hedges may need three cuts a year. Maintenance trimming is generally carried out between spring and summer.

Trimming Techniques

Hand-held hedge shears are fine for smaller hedges, but for large hedges you'll probably find it easier to use an electric or petrol hedge trimmer. No matter what you use always make sure the equipment is sharp and well lubricated.
Always think of your safety when using a powered hedge trimmer. Wear safety goggles and sturdy gloves. Before starting, remove any obstacles on the ground. Avoid using powered tools above shoulder height and use sturdy step ladders or platforms, ensuring they are stable. Electric hedge trimmers are ideally used with a residual current device (RCD) and should not be used in damp conditions. Place the cable over your shoulder to prevent it being accidentally cut.

Formal Hedges

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. This is known as cutting the hedge to a batter.

Follow these tips to ensure an even, symmetrical hedge:

Cutting straight, crisp edges by eye can be difficult. Use a taut horizontal string tied between two stout canes to act as a guide to cut the top of the hedge level. Canes or stakes pushed into the ground help with vertical lines.
To shape the top of the hedge (e.g. to an arch), cut a template of the shape required from cardboard or plywood. Place the template on the hedge and cut following the line of the template, moving it along as you proceed.
When using shears, ensure that the top of the hedge is cut level and flat by keeping the blades of the shears parallel to the line of the hedge.
When using a hedge trimmer, keep the blade parallel to the hedge and use a wide, sweeping action working from the bottom of the hedge upwards, so that the cut foliage falls away.

Informal Hedges

Pruning an informal hedge is much like pruning normal shrubs. See the links below for more specific information on pruning 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.