Yellowing Leyland cypress

I have 150 Leyland Cypress that I planted 6 years ago. They were doing great until this year. Not too long ago (about a month) I noticed some yellowing. I thought it was due to lack of watering, and I initiated a regular watering schedule. In spite of this, the yellowing has continued. I now have about 20 trees in what I believe is serious distress or probably very close to death. Please help me save my trees! What can I do? What do they need?

To preserve the pine tree portions that you wish to use as a plant stand, there are several penetrating wood preservatives that can be used. You will need to coat all cut surfaces and the bark with the preservative. If you want to use the stump portion, you can't paint the roots that remain in the ground, but you can treat all above-ground portions. The roots will eventually rot, but that will take a few years because of the resin in the pine tree. As to which preservative is best to use, your local paint or hardware store can give a much better recommendation than I can.

There are several things that could explain the yellowing of your Leyland cypress trees. Insufficient or too much water can cause yellowing. You say you are watering regularly. Is that once a week? Once every two weeks? Once a month? Once every two weeks would be the proper watering frequency.

How much water are you applying with each irrigation? The soil should be moistened to a depth of at least two-to-three feet with each irrigation. The amount of water to moisten the soil to that depth depends on the type of soil. Clay soil requires much more water than sand because it holds more water, but clay is hard to moisten. It must be watered very slowly so that the water penetrates the soil instead of running off. Sand holds less water but more readily accepts the water. The sand will dry faster, but once every two weeks watering should be sufficient.

Where are you watering the trees? As a tree grows, the roots near the trunk enlarge and cease absorbing water. They become pipes to carry water in from the roots farther out. The absorbing roots grow farther and farther from the tree as it grows. When you first plant trees, these absorbing roots are very close to the trunk, but after a few years they have moved out to and beyond the ends of the trees branches. When you irrigate these 6-year-old trees, don't water the soil next to the trunk. You should water the soil under the ends of the branches and outward for several feet. This places the water where the tree can use it.

Our southwestern soil conditions can also cause yellowing. If the tree is planted in soil with a very high pH, iron in the soil cannot be absorbed by the tree and iron chlorosis (yellowing) will develop. Application of iron and acidifier will help. If you choose to use an iron chelate to supply the iron, choose one that is adapted for very alkaline soil (FeEDDHA).

Finally, a potential cause that may have no cure: There are cypress bark beetles attacking cypress and juniper trees in the southwest. These insects feed on the layer of tissues just under the bark (the cambium and phloem). This stops the flow of nutrients from the leaves to the roots, which eventually starve and the tree suddenly dies. There are no chemical controls labeled for this pest other than "good ole water". A well-watered (properly watered) tree has a much greater chance of resisting the attack of these insects by pushing them out with pitch as they try to attack the tree. You will notice a lot of pitch on the trunk of trees even if they were not able to fend off the bark beetles, and you will see small, pencil-lead size holes in the bark. The insect often attacks the tree first at the ends of twigs, causing these twig ends to turn brown and break but still hang on the tree. This gives you some warning and a chance to try to irrigate to save the trees. The insects later invade the trunk, killing the tree. If you think this is the problem, contact your local Cooperative Extension Service office or a reputable tree care professional for help in confirming the diagnosis. If the trees are infested, remove the trees so that the insects can't spread to other trees.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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