Hi, 

Unfortunately the yellow/brown Arborvitae are definitely dead and won't recover; once evergreens like this start changing color that markedly, they don't recuperate, so the dieback that began last summer or earlier won't grow again. (Evergreens take a while to fully desiccate and become brown, so it's hard to back-track when a problem began.)

The reasons for dieback are hard to determine but might be soil moisture/drainage-related. It sounds like you were doing watering at a reasonable amount, generally though you want to water less but for longer periods of time to fully soak the soil and reach the whole root mass. This helps develop deeper roots instead of just having mostly surface roots (which tend to develop when only watering for short periods of time) and will help plants sustain moisture in periods of drought and heat. Just keep this in mind for future watering needs. Always feel the soil at the base of the plant first, sticking a finger under the mulch about 4-6 inches down will give you an idea if the plants need water. While plants are establishing root systems for newer plantings, you don't want the soil to get bone dry , but you also don't want it to be muddy or soaking wet. This could mean maybe watering once for twice a week (during no rain or high heat) at a low slow garden trickle at the base of the plant for maybe 15-20 minutes to thoroughly soak the roots. Then letting the soil dry sufficiently before the next watering. So what might have happened is maybe the neighbor on the other side of that shared fence watered their own landscape more often or emptied a pool or something on their side, which seeps into the root zone of the plants along the fence line causing the root zone in this area to be too wet and not dry out in between your watering periods. 

Fertilization wasn't necessarily needed for the Arborvitae and, if over-applied, could stress the roots (though likely wasn't the only factor here, if it was a factor at all...something to keep in mind going forward, though). 

Were the root balls/burlap loosened upon planting? While this alone might not have explained the stark contrast in die-off, it may have been a contributing factor if burlap or a wire cage is still present or the roots weren't disentangled. Sometime in the container grown plants, the root systems grow in a circular pattern and it is hard to tell at the time of planting if they are loosened to an appropriate amount. This can cause a girdling root, or essentially the roots keep growing in that circular pattern and end up choking themselves and the plant. It can cause a slower death like this too. 

If you choose to replace the dead/yellowing Arborvitae, you may want to remove them first and see what the soil is like underneath. You can fill the old planting holes with water and perform a simple percolation test to see if it takes a long time for water to drain. There could be a large rock under the soil in this location where as in the other location the soil drains better. 

Let us know if you have further questions with this. Or discover other issues that you would like clarification on.

Emily

Hi Dan, 

Yes, that sounds like a probably a lot of water for Arborvitae. In general Drip Irrigation is often recommended since it help keep the foliage dry when water (if foliage stays consistently it can cause different fungal and disease infections) and drip irrigation is also great to help curb over watering and water evaporation, targeting the root zones and only water what is necessary. 

Drip irrigation isn't necessarily bad for Arborvitae but it seems irrigation paired with the other sources of water didn't allow for the soil to dry out in between. When roots stay consistently wet like that (on plants that aren't adapted for those conditions) they start to fail from a lack of oxygen. Missouri Botanical Garden has a simple explanation of this. 

It's possible that some trees might come back it the soil and roots dry out a little, but usually if a certain amount of the root systems have died and/or are dying, it is difficult to reverse this. 

Good luck! and hopefully they turn around. 

If you notice that the area is still consistently wet, then you can always consider planting some more water loving plants! (Not too many evergreens in this category, but a screening effect could be created by layering different plants.) 

Feel free to reach back out if that is something needed down the road. 

Emily