Answer: It is really difficult to diagnose a plant problem without being able to inspect the plant, but I'll give you some clues to look for and maybe you can determine just what might be going on with your arborvitaes. Usually when the new growth looks healthy, the plant is healthy. New growth is at the ends of the branches. Older growth is in the interior. Arborvitaes are evergreen, but eventually the oldest growth dies off, but the new growth remains green. What you describe may just be a natural shedding of the oldest foliage on your plants. If so, there's nothing to worry about. However, insect pests can populate a plant and cause problems. Spider mites especially like evergreens such as arborvitae because they prefer to live in places that are hot, dry and dusty - like the interior of your plants. Their feeding can cause the foliage to take on a stippled look and eventually can kill the foliage. To check for spider mites, take a piece of white paper, hold in under a few of the inner branche of your plant and sharply tap the branches. Debris will fall onto the paper. If you see specks that move, there may be spider mites. Look also for webbing on the affected branches. You can discourage spider mites by regularly hosing off your arborvitaes, directing the water spray towards the inner branches.
If you do not find evidence of insect pests, the dying and dropping foliage is probably normal.
One more suggestion: you may be overwatering your plants. One deep soaking is all they need each week. Watering every day or every other day can keep the root area too moist and can cause root rot. It's time to wean your plants off of frequent watering. Start by watering every 3-4 days for a few weeks and then reduce to once a week. Next spring, begin with a once a week deep soaking and increase that to twice a week in really hot weather.
Hope this information helps!
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