Another Leylandii removal today. If you've got large Leylandii in your garden its definitely worth getting a proffesional out to have a look at it. Its the number 1 emergency call out we get.
This is partially due to the shallow root systems it puts down....in clay soils it can be as shallow as 2 ft. Leylandii also doesn't typically produce "tap roots".
The other reason they tend to fail in high winds is because the canopy is immensely dense and acts like a sail.
All in all, a great species to have if they're being maintained in a hedge, but let them get out of control and you could have serious problems on your hands.