Hello -- I recently had the problem of my arborvitae turning black. I called MSU Horticultural Center and they weren't certain, but told me that it has been very dry and had a chart on the amount of water certain types of plants need. Jan told me that even this chart of water is on the very low side and should be significantly more. Every nursery I called couldn't answer what was going on but even growers of arborvitae said that it is very hard to overwater these trees. I have 65 trees that are about 7' tall that I just planted in April. I removed all the black and even the brownish leaves. We had a very rainy season up unto a month ago.. They looked beautiful until then. I was told a soaker hose for two hours couple times a week. I decided to water them heavily. I removed soaker hose and just used my open end of the hose. I put the hose up to the trunk of each tree and watered my trees for about 5 hours. I thoroughtly saturated with a deep watering and said a prayer. The very next day they looked like new trees. This blackening is definitely do to lack of watering. You need to water these trees about every 3 days in hot sunny weather, with a deep saturation. Watering often with small amounts of water doesn't get to the roots. These trees need to be watered just as much in the second year, and possibly even the third. Definitely this blackening is do to lack of water.