Leyland cypress, Cupressocyparis Leylandii, is a fast-growing evergreen planted to provide privacy and for windscreens. They are slender trees with needles in the shape of flattened sprays. With skillful, consistent pruning, they can be turned into a hedge. There are, however, problems with growing these trees and in recent years, they have been plagued by disease.
Lack of Sun
To grow well, a Leyland cypress needs full sun. If you plant it in the shade, it will thin rapidly and shed its lower branches.
Soil Too Wet
Plant Leyland cypress in soil that is well drained. If the soil is to wet or you water the trees too much, they can develop fungal root rots. If you dig a hole 1 foot deep and 1 foot wide and fill it with water, it should drain in less than 3 hours.
Growing Too Tall
If you don't prune your Leyland cypress to prevent vertical growth, it can become 60 to 70 feet tall, which can be downright dangerous if there are utility lines overhead.
Plants Too Crowded
Do not plant Leyland cypress trees too closely; the mature trees can become overcrowded, causing poor air circulation and blocking sun to the lower branches. As a general rule, do not plant Leyland cypress closer than 8 feet from one another. When the limbs touch, remove every other tree. Limbs rubbing together can become wounded, inviting disease.
Leyland cypress of all sizes are affected by the Seiridium canker, a fungus that grows on branches and stems. On smaller plants, this can cause stem dieback. The cankers are sunken, purplish or dark brown patches on the bark often with resin flow. The fungus, appearing on the bark as tiny black dots, turns branches a bright, reddish brown.
Seiridium canker is spread by the rain and irrigation splash and can be spread by dirty pruning tools. There are no known chemicals to kill Seiridium canker. Remove infected branches and sterilize pruning tools with a mixture of one part chlorine bleach to nine parts water or with rubbing alcohol.
The Botryosphaeria canker, Botryosphaeria dothidea, looks like Seiridium canker, producing scattered bright, reddish-brown dead branches and twigs. Botryosphaeria can produce narrow cankers more than a foot long on the trunk. The cankers can kill branches. Its spores are spread by the wind and no chemical is available to kill this fungus. Mulch plants yearly and keep them watered during dry periods. Avoid heavy use of fertilizers and severe pruning. Remove and destroy dead branches.
Cercospora needle blight is starting to appear on Leyland cypress. The needles turn brown next to the stem and infection usually comes with wet weather. The spores, spread by the wind, are present throughout the spring and summer. Spray your trees with fungicides that contain copper at 10-day interviews from when the buds break until the new growth is matured.
Annosus root rot, a fungus, is spread through the root system and is a common disease of conifers and affects Leyland cypress. The roots decay, causing the foliage to turn reddish-brown and the plant to eventually die. There are no chemicals to control Annosus root rot. Remove infected trees, and treat stumps with dry granular Borax.