My leylandii hedge has grown too tall and wide. Can I trim it back?
You can always reduce the height but you cannot cut back into old wood to make it much narrower as leylandii will not re-shoot from old wood (wood with no green shoots on it). As long as you leave green shoots, then the plants will shoot back. Leylandii should be trimmed regularly from a young age, if this is done, they can be kept any height and any width. Unfortunately, if you have inherited a huge hedge or let one go, then, in many cases, it is better to cut it down and start again.
