Most hedges have to be clipped after planting, and then cut twice yearly; in spring and late summer. A formal hedge, however, needs to be trimmed more frequently to maintain its shape.
The majority of deciduous plants, especially those with a naturally bushy, low-branching habit ought to be cut back by one third upon planting, as should the evergreens box and shrubby honeysuckle. The vigorous and vertical growers, such as hawthorn and privet, can be cut back to 15 cm (6 in). The following year, these varieties ought to be trimmed lightly, and then cut back by one-third in their 2nd winter. Once the hedge has reached the ideal dimensions, cut it back annually to within 6 mm (1/2 in) of the old wood.
Leyland cypress along with other vigorous conifers are employed broadly as hedges. Generally speaking, cut only their side-shoots in the early years, leaving the leading shoots untouched. The most vigorous species might need trimming 2 or 3 times in the growing season. Once the leading (apical) shoots have attained the desired height, trim them level to make a flat-topped, wider-growing hedge.
Many flowering hedges blossom on last year's wood, and so may be trimmed back after flowering. Nevertheless, Japanese rose (Rosa rugosa) should be cut back hard in late winter or early spring. Fruiting hedges, such as pyracantha and cotoneaster, can be gently trimmed either before or after flowering.
Close-leaved evergreen hedges, that do not allow light and rainwater to penetrate easily, are best cut to a batter; that is, slightly narrower at the top than bottom. This means that light can reach the lower parts of the hedge, stopping browning and dropping of the leaves on lower branches.
Equipment
It's very difficult to determine accurately by eye while you are trimming a hedge; it's only when you have finished that any blunders become evident. 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 manage a straight edge.
The majority of hedges are cut with shears or an electric trimmer, although informal hedges and those with large evergreen leaves ought to, where practical, be pruned with secateurs to avoid unsightly damage to the leaves. You should wear heavy-duty gloves and goggles while trimming evergreens.
Mains-powered hedge-trimmers should be used with care; they can be incredibly dangerous if mishandled. Always use a 'residual current device' at the socket to cut back the danger of electrocution if you cut the cable. Try to use a hedgetrimmer with a blade stopping time of a maximum of half a second, and a two-handed switch, where the machine is only going to work when both hands are on it. It is easy to minimise the risk of a hand coming into contact with the blades by using blade extensions. Should you have a tall and wide hedge you might need 60 cm (24 in) blades; in any other case, 40 cm (15 in) blades are going to be quite adequate.
