Landscapers value Leyland cypress trees (Cupressocyparis leylandii) for their fast growth and toughness. Although they do not require pruning, some homeowners prefer to control the evergreen's height or to shape a row of cypress into a thick hedge.
Prune Leyland cypress in March, suggests the North Carolina Extension. This shapes the evergreen's growth for the coming season. Leyland cypresses are fast-growing trees; trim them again in June or early July to keep the desired size. Do not prune in late summer or fall; this triggers new, frost-tender growth.
The University of Washington Botanic Gardens warns against topping Leyland cypress. Although this evergreen handles topping better than many other shrubs and trees, topping stresses the cypress, destroys the natural form and may cause disease or death. If you must top Leyland cypress, choose a dry day in midsummer.
Prune diseased limbs immediately and sanitise the saw or loppers between cuts. Prune Leyland cypress on dry days. Rain or high humidity contributes to canker, a fungal disease. Destroy the cuttings or place them in the trash; never compost diseased cuttings.