Nothing lives forever. Whether plant or animal, living things seem to have typical life spans: some measured in centuries and some in days.
Leyland cypress is enormously popular in Southern landscapes. It’s been planted by the millions. Even so, it is not particularly suited for heat and drought. Both lead to disease, from which a Leyland has a hard time recovering.
In my observation, Leyland cypress trees that were planted properly so they grow a wide root system, last 25 – 50 years. In contrast, trees that were planted hastily in clay soil have roots that just can’t keep up with the need for water in summer. They might last only 10 – 15 years.
If you have important Leyland cypresses in your landscape, it is imperative that they get water in summer. Depending on their size, 10 to 100 gallons of water might be needed per week. Your goal is to keep a ring of soil 10′ out from the drip line moist (but not soggy) whenever temperatures are above 90 degrees.
If this is not done, you risk losing your trees.