“Cypress foliage is very flammable.  It is sticky and aromatic, two signs that is contains volatile waxes, oils and other substances that burn hot.  Flammability may not be an issue in a situation where the trees are isolated from other plants and flammable material (like fences), but consider that in recent years Leyland cypress plantings have contributed to damaging fires in southern Oregon.”

Siskiyou Woodlander
Leyland cypress: requiem for a hybrid?  Posted on March 25, 2014 by bennemax

Read more here:
http://blogs.oregonstate.edu/sworwoods/2014/03/25/leyland-cypress-requiem-hybrid/