I have heard that Leyland Cypress tends to outgrow its root supply and die at about 20 years of age. I really don’t want to plant something that is going to die that soon. Does anyone know if this is true. Google was of no help in this.
I think they are a little longer-lived than that; I have seen some specimens in a local arboretum that must be thirty to fifty years old or more (they are enormous and majestic forest trees though).
As a domestic garden hedge, they may only last twenty years until they simply outgrow the situation to the extent that you are forced to trim them back too severely for effective regrowth.
Yew makes a much neater hedge anyway and should last for centuries - it is slower to grow in though and you have to get all male plants unless you want to cope with the sticky berry-like fruits.
From my experience, Mangetout has it right. People tend to plant them not realizing that they get BIG. They then prune them in an attempt to make them fit their assigned place - the trees become stressed and die.