Answer: If the color is poor on your spiral arborvitaes inspite of proper fertilization, I suspect the plants are getting too much water or the soil is remaining too moist for extended periods of time. The poor color is an indication of root stress. I'd water as usual then wait 3-4 days and dig down to see how moist the soil still is. If it is moist 2-3" beneath the soil surface, you won't need to water for another 3-4 days. If it is on the dry side, go ahead and water. If the irrigation system has changeable heads, reduce the size of the heads to reduce the amount of water your arborvitaes are getting.
The emerald arborvitaes sound as though they are acting normally. Although they are considered evergreen, the oldest foliage does eventually die. That's the foliage on the inner part of the plant. You can reduce the appearance of this dieback by lightly pruning the plants each spring which will force growth behind the pruning cuts and will also encourage new growth at the ends of the branches. Together, all this new growth with make the shrubs fuller and greener throughout.
Best wishes with your landscape!
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