In colder areas box hedging should be clipped this month so that the new growth has time to harden off before autumn frosts arrive in October. Before you trim, take cuttings to increase your stock of plants. They will root easily in a cold frame or in a sheltered part of the garden. If necessary, improve the soil with some sand to keep it well-drained. When trimming any formal hedge, aim to maintain a slightly broader shape at the base than at the top. This allows sunlight to reach the lower leaves, preventing baldness at ground level.