Why Are My Leyland Cypress Trees Dying?

As a consulting arborist I get this question a lot. And it’s no wonder when you think about the dramatic decline Leyland cypress have been undergoing. Throughout the region we can see the dark green foliage transition to a reddish brown then an ugly dead grey.

There are a handful of pests and diseases that commonly attack Leyland cypress but the primary culprit here is winter damage. The past two winters (one with a late freeze and the other with a deep freeze) were simply too extreme for many of these trees. As a hybrid of two Pacific Northwest species Leyland cypress are simply not designed for the winter extremes of our zone.

With some plants winter damage is obvious but it is trickier with waxy-leaved evergreens like the Leyland cypress. First of all the desiccation and dieback take a while to manifest, causing us to notice the impacts well after the fact and over an extended period of time, similar to what we see with the spread of disease. Second, the stress caused by winter damage makes the tree more susceptible to its common enemies, especially canker diseases, and it is possible that the final death blow is actually delivered by one of these secondary impacts. Visit this link to learn from West Virginia University’s extension office about canker and its role in Leyland cypress decline: https://shar.es/1q15JE

I see three basic management options:

  1. Do nothing. Let the damage run its course but run the risk of dead wood decay working its way into healthy tissue and causing further damage. You also run the risk of canker disease developing, which is even more likely spread to other parts of the tree or to other Leyland cypress trees.
  2. Prune out dead sections and/or remove dead trees. What is dead is dead and will not ‘fill in’ over time. Cutting out these sections minimizes further decay and reduces the likelihood of disease. Use sharp tools and be sure to sanitize them with alcohol or bleach solution when you move from one section of the tree to another or from one tree to another. If you suspect a disease like canker consider spraying the tree with a fungicide after pruning (and possibly during the following growing season). I recommend neem oil as an option that is biologically derived and has minimal impact on beneficial insects.
  3. Replace with hardier diverse species, keeping an eye toward natives. If you avoid monocultures you avoid the likelihood of a single pest or disease – or frigid winter – taking out your entire population at once. And consider wildlife habitat by planting native species such as eastern red cedar (a prime habitat tree), eastern arborvitae (a.k.a. eastern/northern white cedar) and eastern white pine. Native shrubs will also fill in a space very quickly and increase your butterfly and songbird visitors.

Finally, a word about watering. No matter their age, any stressed tree will benefit from a slow, deep irrigation (e.g. irrigation bags, soaker hoses, buckets with tiny holes) during hot dry periods. In fact, some suggest that newly planted or stressed Leyland cypress should be watered well into October or November to ensure that they have the vigor to withstand the stress of a harsh winter.

Shawn Walker is a consulting arborist and owner of Trees 101 (www.trees101.net) based in Shepherdstown, West Virginia.