Question: I planted a whole row of Leyland cypress trees to act as a screen. They’re quite large now, but one by one, they are turning brown and dying. What is causing this and what can I do to save them?

Answer: I’m sorry to hear about your trees! There’s nothing so sad as seeing a whole row of evergreen trees start to die.

Leyland cypress (xCupressocyparis leylandii) are a common, fast-growing ornamental evergreen and often the “go to” landscape plant for evergreen screening and windbreaks in yards.

They can easily grow 3 feet per year and reach heights of 100 feet or more and almost half as wide. Originally created from a hybrid cross of the Alaskan cypress and Monterey cypress, the Leyland cypress’s wild popularity may ultimately lead to its downfall.

Because of factors arising from their being planted too close together, as in a screen, Leyland cypress tend to do very well for the first 10-15 years or so, but then start to decline as their shallow root systems compete for water and nutrients. Leyland cypress planted in rows become more susceptible to drying out and experiencing cold damage, exposed to windy conditions and extremes in temperature.

As the trees grow and mature, lack of sunlight can lead to dieback of the interior branches. Though they thrive best in moist, cool climates with moderate temperatures and are hardy to Zone 6, they do not tolerate sudden temperature fluctuations.

There is no treatment for winter cold injury on Leyland cypress. Should you notice dieback after a particularly cold spell, wait until spring to see if new growth should occur. If new growth does not occur, then you know the branch needs to be pruned above where green color is still visible.

They can tolerate heavy pruning, but if more than 1/3 of the tree is damaged, you will need to remove the tree.

Drought or cold winters can stress Leyland cypresses and make them susceptible to insect pests and diseases. The shear amount of Leyland cypresses in the landscape has caused major epidemics of cankers and needle blight to spread like wildfire, killing off many trees. Their close planting proximity only makes the problems worse.

Cankers and needle blight are both caused by different species of fungi. Cankers will kill large portions of a tree at once, causing the foliage to turn a bright red-brown, while needle blight attacks the needles of the tree from the trunk, working outward.

These fungi are normally kept at bay by hot, dry temperatures, but a wet and cool spring can cause them to thrive since rainwater will carry the fungal spores to other branches throughout the tree leading to death of the entire tree.

Symptoms of canker diseases are branches that start to turn yellow to reddish-brown and the presence of slightly sunken cankers with resin ooze exuding profusely several feet down on an infected branch, closer to the main trunk.

Pruning out the cankers 3-4 inches below the infected area. Destroying damaged foliage during the dry season may slow the spread of these fungi, but be careful to avoid spreading the disease to other trees. Always sterilize the pruning equipment with a bleach solution following each cut. If the canker appears on the trunk, it’s best to just remove the tree.

Keep your cypress trees well watered and mulched during lengthy drought periods, and try to avoid overhead watering. If you must overhead water, apply early in the morning so the foliage dries out during the heat of the day.

Fungicides are not effective for controlling these fungal diseases, so preventative care is crucial. Keep your trees dry and remove infected trees before they can spread the pathogens or pests.

Leyland cypress should be spaced a minimum of 12-15 feet if they are to be grown as a screen. Check with your local nursery to see what spacing is required for other species.

Since Leyland cypress trees are so susceptible to diseases, as well as to winter and weather damage, it may be wise to find alternative species to plant. Replace any diseased or dying trees with a disease-resistant alternative. There are many alternatives you could plant that would serve your purposes.

Green Giant arborvitaes, Arizona cypresses and Japanese cedars are all evergreen conifers that would do well in your landscape. Some of them are almost indistinguishable from the Leyland cypress, and are less susceptible to disease which will save you money and heartache in the long run.

Do you have a gardening question? Contact the Douglas County Master Gardeners via email at douglasmg@oregonstate.edu, by phone at 541-672-4461 or visit 1134 SE Douglas Ave., Roseburg. This month, the Plant Clinic will be open Mondays, Wednesdays and Fridays, 1-4 p.m.

Douglas County Master Gardeners are trained volunteers who help the OSU Extension Service serve the people of Douglas County.

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