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Leyland Cypress

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Leyland cypress is a hybrid of two trees native to the Pacific Coast – Monterey cypress (Cupressus macropcarpa) and Alaska cedar (Chamaecyparis nootkatensis). The offspring of that cross produced what we know as the Leyland cypress (x Cupressocyparis leylandii). 

Leyland cypress is commonly planted in Union County in a row with tight spacing to create a ‘green fence’ (see my post on vegetative screening). It is found at nearly every store and garden center, it is fairly inexpensive, it establishes very easily with little care, and it grows very fast to create a vegetative screen quickly.

Unfortunately, Leyland cypress regularly succumbs to disease and I receive a large amount of phone calls asking for help with this tree. We’ve come to call it the ‘heartbreak tree’ because just when it has really filled out and starts providing an excellent screen, the beautiful foliage starts to brown out and the tree starts dying very quickly.

The cause of Leyland cypress decline is generally attributed to a few different fungal diseases: Botryosphaeria Canker, Seridium Canker or phytophthora root rot. I believe Seridium canker to be most common here in Union County. Regardless, there is no effective chemical cure for any of these diseases once they show up. These diseases are exacerbated by the tight spacing utilized when homeowners and landscapers plant Leyland cypress as a green fence. This tree is actually very large, up to 20 feet wide at maturity and 70 feet tall, but when planted as a screen I see spacing as tight as 3’ between trees. This means the foliage will be crowded very quickly. Sure, this will create a screen in as little as two years! But over-crowded foliage doesn’t get sunlight or airflow to dry the needles, which leaves the tree susceptible to fungal diseases. But planting Leyland cypress with 20’ between trees doesn’t create an effective screen. So what does all of this mean? This means that vegetative screening is an inappropriate use of Leyland cypress, unless you can accept that the trees will probably die after 15 to 20 years and then you have to start over. 

Planning and Planting – see my post on vegetative screening when it is time to replace your Leyland cypress. The best vegetative screen is a mix of different species, planted properly and spaced properly.

How to Manage an Existing Screen of Leyland Cypress – Managing existing Leyland cypress screens can be broken into two categories, cultural treatments to keep the tree healthy and sanitation protocols to remove diseased plant material from the site. The best expectation from enhanced management of Leyland Cypress utilizing these suggestions is slowing the progression of Seridium and Bot Canker. The ultimate goal is to extend the service life of your screen as long as possible, however there is no cure. 

Cultural Treatments

  • Remove grass from around the trees. Provide a mulch bed 2-3 inches thick of hardwood woodchips. Do not pile mulch against the trunks. Apply ½” thick layer of organic compost once per year. This will help to build general soil health, increasing the baseline vigor of the trees and improve their ability to naturally resist disease, thereby extending their service life. 
  • Facilitate watering during dry periods – There is not much that can be done for established plants when we receive too much rain, but the wild wet/dry fluctuations of natural rainfall predispose Leyland Cypress to disease. Try to manage water to 1 inch per week (natural rainfall plus supplemental watering).
  • Avoid over-fertilization of plants – This often produces lush growth that is more susceptible to disease.
  • Soil decompaction and sub-surface nutrient injections – These two services help maintain the health of existing Leyland cypress trees or help slow the spread of Seiridium and Bot Canker in trees that are diseased. De-compaction aids water penetration during dry periods and drainage during wetter times. Nutrients enhance plant health by improving the living soil web as opposed to adding a high level of growth-stimulating fertilizers.

Sanitation Protocols

  • Prune dead branches (either those shaded or those diseased) and sterilize pruning tools between cuts. Here are three ways to do this:
    • A 10% chlorine bleach solution (1 cup bleach to 9 cups water). Note that chlorine bleach can corrode pruning tools.
    • Full-strength Lysol – This seems to be the preferred method for maximum sterilization with minimum corrosion.
    • Alcohol wipes – No drying required and may be a bit less cumbersome in the field.
  • In situations where the height of the screening plants is valued, it may be acceptable for the plants in a screening row to be more open at the base by pruning up the lower branches. Anything that can be done to promote light penetration and air movement within the canopy can prolong the life and health of the trees.
  • Other diseases such as Cercospora needle blight also adversely affect the health and appearance of the trees. The photo showing Leyland Cypress Trees with browning foliage at the base of the plants is representative of Cercospora. While fungicides are effective against some of these other pathogens, repeated applications are often needed, and failure to apply the other management practices often results in a recurrence of the disease.

The best advice in the case of Leyland cypress is to only use them as single, large specimen trees. Using this tree as a vegetative screen will only end in heartbreak.