Why Are My Leyland Cypress Trees Turning Yellow?

The Leyland cypress tree is a fast-growing evergreen tree that is popular among landscapers and homeowners. The Leyland typically has hearty annual growth, but a variety of disease, pest and maintenance issues can cause stunted growth and yellowing of the needles. Growth also depends on adequate watering and nitrogen levels.

The Leyland is a fast-growing, low-maintenance ornamental tree that can reach heights of more than 50 feet with annual growth of 2 to 3 feet, states the University of Georgia (UGA). The tree is favorable for homeowners, landscapers and cemeteries because of the fast growth. The Leyland also provides privacy and wind resistance because of its height. According to the UGA, the Leyland prefers sunny areas with well-drained soils. Inadequate soil conditions can lead to yellowing of needles and stunted growth.

Nitrogen helps plants with cell division and leafy-green growth. In the Leyland cypress, nitrogen helps promote green growth of the evergreens' needles. According to the Soil Science Education website powered by NASA, plants with a nitrogen deficiency have thin, spindly stems with stunted growth, and yellowish-green leaves. Nitrogen deficiency is common among Leyland cypress trees because the fast-growing tree needs a substantial portion of nutrients, which does not occur naturally in unfertilized soil in most areas. Nitrogen deficiency and root rot disease have similar symptoms; therefore, it is important to have a university-based or private lab determine the cause of the yellowing cypress tree needles.

Common in developing Leyland cypress trees, Phytophthora root rot is caused by the soil-born fungi, Phytophthora cinnamomi. According to the NCSU College of Agriculture and Life Sciences website, Phytophthora root rot is most common where soil drainage is poor. Infected trees exhibit root damage and yellowing of foliage and tip dieback. NCSU recommends laboratory analysis of affected roots to determine if Phytophthora root rot is the cause of the yellowing foliage.

Annosus root rot is caused by the fungi Heterobasidion annosum, and is common in developed Leyland cypress trees. Common symptoms of Annosus root rot are yellowing and slow decline, followed by death of the tree, states the NCSU. Annosus root rot spreads from freshly cut stumps of neighboring trees, commonly the pine tree. Trees infected with the root rot need to be removed from the growing area, as there is no cure for the disease and it spreads rapidly through the tree's root system.

Underwatering or improper watering of the Leyland cypress tree can result in yellowing of the tree's needles. According to New Mexico State University (NMSU), mature cypress tree roots extend several feet from the initial plant site. Watering around the trunk of the tree does not allow roots to absorb water, which leads to yellowing of the needles. NMSU recommends watering a mature cypress tree at least several feet in all directions to ensure adequate water absorption from the roots.