Cut back young hedge plants and topiary by up to one-third in May to encourage bushy growth. Further trimming can be carried out between May and August, as required.
Trim mature hedges and topiary in August. It may be worth noting that although box hedging and topiary can be pruned towards the end of May, pruning at this time may leave the new flush of soft growth vulnerable to weather damage such as leaf scorch, the result of late frosts, drying winds or unseasonably hot sun and, additionally, diseases such as box blight. Pruning later, during August when the new growth has hardened off and slowed down should help minimise leaf damage due to weather conditions or disease and the hedge should remain neat through the winter months.
Old, neglected plants usually respond well to hard pruning in late spring (May) and can be cut back to within 15-30cm (6in-1ft) of the ground.