well established (over 40yrs) arborvitae trees that are turning brown and thinning drastically.?

Question

Added iron and additional watering to combat dry conditions.

Answer 1

Check for spider mites and other misc pest problems. Spider mites.......take a piece of white paper, hold it under the foliage and shade the branch. A lot of misc stuff will fall onto the paper. Watch carefully. If some of that trash moves, you've got spider mites.

Arborvitae leaf miner mines out the leaf tips,
causing them to turn brown. Translucent areas are seen
where the miner has been active.

Several scales infest the stems and foliage....they appear as strange specks that can be easily picked off. They can be hard shelled or fluffy.

Bagworms get into them, but you'd see the little "sleeping bags" withing the tree.

Also 40 years old is getting rather old for the tree. If it isn't spider mites or one of the other insects, yes, continue extra watering, fertilize with a balanced fertilizer, iron isn't a bad idea unless over done. Remember tree roots are far reaching, so water under the entire canopy and beyond.

Answer 2

It has been my experience with arborvitae trees that as they get older they turn brown and start to die.

Answer 3

Pull off a brown twig and seem intently. in case you notice a trojan horse there, you ought to spray it, or it is going to die. We had one die and did not comprehend it became infested until my husband shrink it down and had it interior the mattress of his pickup truck to haul away. Then we observed them, they curl the little fronds around them and are just about impossible to make certain.