I have a hedge of emerald arborvitae that I planted about 20 years ago. About a year ago the east side of the hedge started dying (loss of green) and it kept moving north from tree to tree. It is still moving north and now the other side of the trees are dying. The garden center a year ago suggested fertilizer. That hasn’t stopped it. Cooperative extension suggests wilt proof since they believe it to be an exposure problem. Perhaps relevant is that I had 2 large pine trees removed which sheltered the arborvitae somewhat. The trees were in the same line and to the south of the arborvitae. Lrabideau3@nycap.rr.com
I was approached many years ago about a similar problem. As it turns out, the planter never removed the plastic string from around the trunk or root ball. As the plants grew, the twine became tighter and tighter and was cutting into the bark of the plant cutting off the flow of nutrients and moisture from the roots to the foliage above so the foliage was dying. I’d suggest getting under there and checking out the trunks for twine that wasn’t removed…plastic or otherwise. Piling soil or mulch up against the bark of the arbs also slowly strangles the plant by hindering the flow of moisture and nutrients through the bark. While your checking for twine down there you should also brush away any dirt or mulch until you find the original soil level that they were planted into and always keep it several inches away from the trunk to let the bark breathe and allow moisture and nutrients to flow unimpeded.