Answer: The older, inner leaves of arborvitae (Thuja) naturally turn brown and drop (similar to the loss of leaves on deciduous trees). This shouldn't cause alarm, and does not indicate disease. If the tips of the branches are turning brown, look for infestations of spider mites (they leave tiny webbing between the junctions of the leaves and stems), or consider cultural problems such as over- or under-watering. Sometimes the leaf tips will turn brown because they've been exposed to sudden temperature changes. Since your arborvitae were planted last fall, I'd rule out transplant stress - they should have had plenty of time to adjust. I'd give them about an inch of water every week, and wait to see if new growth appears. If so, you can prune off the brown tips to make room for healthy green growth.
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