Hedge cutting seasons and the frequency of cutting depends on the species, but as a rule of thumb, formal evergreens like box or privet should be trimmed around two to three times a year during the growing season (around May to September); and some fast-growing conifers like leylandii may need trimming more frequently to keep them in control. Do not cut conifers after the end of August. Stocky deciduous hedges like beech or hornbeam should be cut in late August, and if major renovation is required, do that in late February whilst the plant is still dormant.
Always remember, it is important not to cut too early in the year so as to avoid disturbing nesting birds.