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Friday - April 09, 2010

From: Bucks County, PA
Region: Mid-Atlantic
Topic: Shade Tolerant, Trees
Title: Arborvitae thinning in Bucks County, PA
Answered by: Barbara Medford

QUESTION:

My arborvitae trees are about 11 ft. tall. I had them put in about 3 years ago. They were 8 to 10 ft. when planted. After the first year, I have noticed they are thinning to the point where you can see right through them. What can I do to make them full like they once were, or are they dying? Any help would be much appreciated.

ANSWER:

Bucks County in the southwestern corner of Pennsylvania and USDA Hardiness Zone 6b would seem to be the appropriate place for Thuja occidentalis (arborvitae) to grow.

However, we picked up some facts that may explain the thinning of your plants; the first was from our webpage on Thuja occidentalis (arborvitae). That offered the information that in a crowded environment, this tree is slender and not well-branched. In the open, it improves in form and density. This Ohio State University site commented that it prefers a moist, well-drained, loamy soil in full sun, but tolerates soils that are poor, rocky, clay, compacted, dry, and of various pHs extremely well, and is very urban tolerant to heat, drought, humidity, and pollution; however, not tolerant of shady situation. From that same article, we also learned that old foliage from the self-shaded middle of the shrub abscises (sheds) noticeably in Autumn, and falls through the interior of the canopy directly to the ground below.

Our conclusion is that if your arborvitae are not in an open area, with plenty of sun, that is probably causing their decline. They are not likely to be dying, but are not going to prosper as much as you might wish. Some of the openness is no doubt the result of the shedding of the interior branches, caused by the plant's own shade, which you can hardly do anything about.

Pictures of Thuja occidentalis (arborvitae) from Google

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