It recently came to my attention that one of our clients (landscape company) is having issues with a 30 ft row of established Arborvitae along the side of their yard. Each year, this client loses one or two trees to a gradual decline, with the leaves slowly browning from the inside and working it's way through to the outer layer of growth. There are no signs of pest or disease, and the trees seem to take turns dying back, so that it's never in the same spot twice. The row is irrigated by a line of sprinklers set on a timer to control the amount of water they recieve, and they are carefully fertilized and mulched every spring. The trees are planted closely together to create a privacy screen, and adjacent plants seem unaffected by the dying trees. Based off our research, our best guess is that the soil beneath each tree is becoming compacted over time, keeping it from attaining enough air and water. However, none of our attempts to remedy this situation seem to make a difference. Would you happen to know of anything else that could/would cause this sort of die back?

I attempted to make contact via email prior to this post and was unable to connect to get more informantion. Arborvitae have not faired very well in recent years. Older established plants suffered in 2012 even having been watered. Established plants are going to use water and fertilizer differently than newly planted trees. It is best to monitor new plantings very carefully given their limited root system. We normally to not suggest fertilizing new plants so they do not generate more foliage/needles than the limited root system can support as the season gets hotter and dryer. Soil drainage is temporally changed whenever plants are installed. The soil structure is disturbed and it takes a few seasons to be reestablished. During this time, the soil may be holding more water than the surrounding undisturbed soil displacing needed soil oxygen. You mention irrigation as part of the landscape. If the soil profile is only being kept moist in the upper portion then that is where the majority of the roots will be and anytime the moisture level changes it will impact the plant. Mulch is going to actually hinder getting rid of excess if moisture in this situation, where normally much is very desirable. You cannot monitor soil moisture levels while using mulch very easily. When monitoring new plants for water, remember that the only roots they have are those in the root ball when planted. Surrounding soil may be moist and root ball very dry.