Hello Alan,

My view is that the Leylandii or any other evergreen will not become dormant in the sense that we think of it in relation to the deciduous shrubs and trees. If it is a matter of reducing the top and not cutting hard back into the face of the trees I would do this during late September or wait untill next April May. Really, the only thing to avoid is the penetation of hard frosts into the open wounds and who knows what will happen regarding the UK winters.

The conifer rarely re-clothes itself after being cut hard back to bare wood.