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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Monday - October 12, 2009

From: Hillsboro, OR
Region: Northwest
Topic: Diseases and Disorders
Title: Brown dead spots on arborvitae in Hillsboro OR
Answered by: Barbara Medford

QUESTION:

Hello. I live in Hillsboro, OR and have several mature arborvitae as a privacy screen in my backyard. They are on our side of a black chainlink fence separating our yard from a drainage area maintained by the city. Early last week we noticed that there were dead patches on all of the trees in our surrounding area. They are dead patches and not "dying". Apart from these dead patches, the trees seem healthy. This is inter-species as it seems to have affected pines, deciduous, broadleaves, etc. At first we thought it was just an irresponsible pesticide/herbicide job, but the dead patches are becoming worse and more noticeable and cover quite a large area. It has affected the trees in our small backyard, all of the trees in the drainage area, and across the street from that. Is there anything you know of that could possibly cause this?

ANSWER:

Sadly, you may be right about herbicide damage. Land maintained by a public entity, such as a city or county, may be sprayed for weeds, often by sub-contractors. If this is done carelessly or by untrained operators, the slightest puff of wind or bad aim can damage or kill other plants around it. The first step you should take is to call the city office responsible for maintaining that drainage area, and inquire about what has been done there recently. Certainly, you want to go to the drainage area and see if damage appears in the grasses or weeds in that area. Another thing to think about when you contact the city is that indiscriminate spraying of herbicide or pesticide, especially in a drainage area, can result in those poisons entering lakes in the area and, eventually, your glass of water.

Since this appears to be affecting more than one species of plant, if you eliminate the possibility of herbicide, we would recommend you contact the Oregon State University Extension Office for Washington County.  If this is happening in your neighborhood, it is happening in others and the Extension people are closer to the problem, and may be able to help you find a solution. 

 

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