Answer: Watering depends on the soil where the plant is growing and also on the weather. Arborvitae prefers a soil that is evenly moist but not saturated/sopping wet. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you water, apply it slowly and thoroughly so it soaks down to the deepest roots. After watering, wait a few hours and then dig down to see how far it has soaked in; sometimes it is surprising. Using an organic mulch in a flat layer several inches thick over the entire root area (not up against the stem) will help keep the soil more evenly moist.
Unfortunately, based on your description I am not certain what is happening to your arborvitaes. Browning can be caused by many factors including pest or disease or incorrect watering. I would suggest you contact your local Cornell extension to obtain a specific diagnosis and based on knowing that, determine how to proceed. If it is something that requires a chemical control, they will have the most up to date information on what to use and how/when to use it.
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