It’s a good idea to begin pruning hedges from an early stage to establish a shape and prevent leggy, unhealthy growth. The amount you prune back will depend on the type of hedge. Start shaping the hedge as soon as practical – a formal hedge whether new or established, should be tapered so that it is wider at the bottom than the top, ensuring the lower part will get the sunlight it needs for healthy growth. Once the hedge is established, regular maintenance will ensure it keeps healthy and neat.
If a formal hedge has got out of shape, cutting it back severely is often the only remedy. But bear in mind that this method is not suitable for all species. Conifers, in particular, react badly if you cut into old wood. 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.
Top tip: Always remember, it is important not to cut too early in the year so as to avoid disturbing nesting birds. If you are unsure if birds are nesting in your hedge consult this RSPB article which tells you all you need to know so that you are working within the law: RSBP Hedge Law.