Arborvitae can lose color and brown for many reasons. Your plant is browning in the middle, which is normal up to a point (evergreens drop up to a third of their needles each year, though they don’t drop all at once as with deciduous trees). It’s true that either too much/too little water or poor drainage can take a toll. Check the trunk at the soil line to make sure it’s not injured or damaged. Look for insects or disease patterns in the foliage. If you can help us narrow down what’s going on and send more photos, we may be able to offer more advice.
Occasionally pests such as spider mites are an issue – you can check for them by placing a white sheet of typing paper under a branch and shaking the branch slightly. If present, the spider mites will fall onto the paper and should be easily spotted. Look for other signs, such as webs for spiders, tunneling on the stems and leaves, leaf miners and wet sticky sap that might be a sign of aphids. If you discover a pest, your local garden center or nursery can help you determine effective solutions. (Never spray without knowing what your pest is, because that, too can cause browning!)
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