Trimming Hedges
Most hedges need to be trimmed after planting, and then trimmed twice annually: in spring and late summer. A formal hedge, however, needs to be clipped more regularly to maintain its shape.
How Much Should I Cut Back?
The majority of deciduous plants, especially those with a naturally bushy, low-branching habit should be cut back by one third on planting, as should the evergreens box and shrubby honeysuckle. The very vigorous and upright growers, such as hawthorn and privet, can be cut back to 15 cm (6 in). The following year, all these species should be trimmed lightly, and then cut back by one-third in their second winter. Once the hedge has reached the desired size, trim it back annually to within 6 mm (1/2 in) of the old wood.
Most flowering hedges flower on last year's wood, and so can be trimmed back after flowering. However, Japanese rose (Rosa rugosa) should be cut back hard in late winter or early spring. Fruiting hedges, such as pyracantha and cotoneaster, can be lightly trimmed either before or after flowering.
Leyland cypress and other vigorous conifers are used extensively as hedges. In most cases, trim only their side-shoots in the early years, leaving the leading shoots untouched. The most vigorous species may need trimming two or three times in the growing season. Once the leading (apical) shoots have reached the desired height, trim them level to create a flat-topped, wider-growing hedge.
Close-leaved evergreen hedges, which do not allow light and rainwater to penetrate easily, are best trimmed to a batter; that is, slightly narrower at the top than bottom. This means that light can reach the lower parts of the hedge, preventing browning and dropping of the leaves on lower branches.
Equipment
Most hedges are cut with shears or an electric trimmer, although informal hedges and those with large evergreen leaves should, where practical, be pruned with secateurs to avoid unsightly damage to the leaves. You should always wear heavy-duty gloves and goggles when trimming evergreens.
Mains-powered hedge-trimmers must be used with care; they can be very dangerous if mishandled. Always use a 'residual current device' at the socket to reduce the risk of electrocution if you cut the cable. Try to use a hedgetrimmer with a blade stopping time of no more than half a second, and a two-handed switch, where the machine will only work when both hands are on it. You can minimise the risk of a hand coming into contact with the blades by using blade extensions. If you have a tall and broad hedge you may need 60 cm (24 in) blades; otherwise, 40 cm (15 in) blades will be quite adequate.
It is very difficult to judge accurately by eye while you are trimming a hedge; it is only when you have finished that any mistakes become obvious. Strings attached to four posts or canes, one at each corner of the hedge and set at the height required, will provide a cutting line and help to maintain a straight edge.