Answer: It sounds like an evironmental problem. It's odd that only one side would turn brown, unless herbicides have been used near the plants and overspray, drift, or even chemical movement in the soil has affected the plants. A second thing to consider is the drainage on the west side. Overly wet or overly dry conditions could affect only one side of the plants. A third thing to investigate would be the possibility that reflected heat, improper pruning, or even an insect infestation on the affected side. For positive identification of the problem, take a sample (with both green and brown foliage) to your local Cooperative Extension office for analysis. If they can't diagnose the problem on the spot, they can send it to a WSU Plant Pathology Lab. Contact WSU Cooperative Extension at 425-338-2400. Best wishes with your arborvitae!
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