What's behind die back, brown patches in arborvitaes?

Question: I have an arborvitae hedge. The south side of each shrub is turning brown, and there are dead patches/dead spaces in the hedge. What is the problem?

Answer: If the south side of your arborvitae hedge turns brown, it may be caused by some sort of stress. It could be too little water, drying winds, high temperatures, injury to the roots or damage to the trunk. According to the PNW Plant Disease Handbook, die-off of arborvitae from physical causes is quite common in the Pacific Northwest.

One-sided die off on a hedge sometimes can be traced to a history of nearby chemical applications, such as drift from a neighbor’s use of herbicide. 

Since the damage to the south side of your hedge is probably more likely from a physical problem than a biological cause such as a disease or insect pest, there’s no silver bullet cure. Here is what the PNW Handbook recommends:

• Prune the affected foliage to remove the unsightly tissue. Pruning allows more light in and stimulates growth of new foliage.
• Water regularly at the base of the plant during dry periods.
• Fertilize in spring to stimulate production of new foliage.

Master Gardener Q&A: Arborvitae turning brown

You mention dead patches in your arborvitae hedge as well. When there is die-back of individual branches here and there, it may indicate root problems. It is time to think about what is going on below the soil surface. Root rot, a set of fungal diseases, could be the problem, as they are fairly common with arborvitaes in our area. 

Sadly, the only way to tell if a plant has root rot is to dig down and actually examine some fine surface roots. So get your shovel and dig shallowly until you run into fine roots of one plant. What do the new fine roots look like? Cut into one with a sharp knife. Healthy fine feeder roots should be brown on the outside but have a white internal core and white tips. If your fine roots are soft and brown and can be peeled or broken off easily, your hedge may have root rot. 

More:Two garden chores you can do mid-winter

Do you water your hedge in the summer? Heavy watering contributes to root rot. Does your hedge get hit with an overhead sprinkler? This also may contribute to the problem. Several fungal species cause a whole shovel full of different root rot diseases.  

Poor drainage, saturated soil conditions and warm temperatures encourage root rot in arborvitae. If your hedge plants are planted too deep, or your soil is compacted or pure clay, this also encourages root rot diseases. Is your hedge mulched thickly? Could be a culprit as well. 

Many modern houses are built on terrible soil, subsoil or fill that is heavy with clay. If this is the case, your hedge may be dead within a few years. 
Consider planting a row of shrubbery that can tolerate clay soil. Or build a berm of well-drained soil and plant new shrubs into the raised and amended soil. 

My all-time favorite clay-tolerant shrubs for our climate include Pacific wax myrtle (Myrica californica). It is evergreen, native and grows thickly. Also, there’s several hardy Viburnum species that make great hedges in clay soils.

More:Give houseplants boost with fertilizer, but not too much

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