Arborvitae dying in only one spot-why?

Asked June 23, 2015, 1:16 PM EDT

Good morning! I planted a row of 3' arborvitae five years ago. From the attached pictures you can see the row is doing well, except the three at the end that have died. The original three in this spot died three years ago. I replaced them in the fall of 2013, this time with 5' trees to keep up with growth of others. They never thrived and gradually began to die and now you can see what I'm left with. They receive the exact care and water as the others, which are gorgeous! Do you have any idea or even a theory why the arborvitae die there? The holes were dug out really large for the root ball and back filled with the appropriate, good quality soil. There is a septic tank about 30 feet away, and the whole arborvitae row runs alongside a drain field. There has never been any kind of gas/oil tank buried nearby or anything that might contaminate the soil. This mystery is driving me bonkers! My primary goal is to fill in that gap to create what will soon be a filled in hedge. Do you have any suggestions how I can make the arborvitae work there, or can you suggest a different dense hedge like shrub or tree that might be a suitable replacement? My deepest appreciation for trying to help solve the mystery of the dying arborvitae!

Multnomah County Oregon

1 Response

The most common cause of arborvitae turning brown all over and dying is root rot caused by Phytophthora, a fungus which is widespread in local soils. Early in the process, when only one or several roots are affected, the portion of the tree they supply with water will die. Eventually, the entire root system is involved. When the tree is dug up, the roots are black, brittle, shortened and break easily. During a heat wave such as we’ve had recently, a tree with an affected roots typically dies suddenly. Perhaps the most discouraging part of this is that the fungus persists in the soil for many years. The only “treatment” is to avoid planting susceptible plants in the same area.

If your first trees died of the root rot, the replacements are dying of the same cause. Even so, it’s worth knowing that arborvitae differs in susceptibility to Phytophthora root rot. Western red cedar (Thuja plicata) is most resistant, T. occidentalis 'Pyramidalis' is intermediate, and T. occidentalis 'Smaragd' is most susceptible.

You might want to contact several Certified Arborists for first-hand on-site evaluations. We suggest Certified Arborists because they must pass a test and must acquire continuing education. You can locate such individuals at http://www.treesaregood.org/findanarborist/findanarborist.aspx.