Will a lop off the top make leylandii die?

My neighbour wants me to lop 3m off the top of my leylandii trees. Will that damage the trees? – Martin, London DAVID: Leylandii are fast growers, and easily get too large for the average garden. Topping them is unlikely to kill the trees, but you could get some dieback from the cuts. New branches should develop and grow upwards. Best do it now, July-august, because winter pruning increases the risk of dieback. Do not cut more than 30 per cent of the entire tree in any one year to be sure it recovers.