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Gail Ruhl and Jeff Burbrink
Click on small photo to see larger image.
(Photos by Jeff Burbrink.)
Note that the needles turning brown are the older needles on the inside of the tree.This is natural needle drop! This is not problem!
In some years, the natural needle drop will occur in a relatively short period of time, with rapid and noticeable browning of the older needles, which leads to fears the tree is dying. In other years, people don't even notice it happening, because the browning is spread over a long period of time.
The symptoms are normal for the fall. Pines and arborvitaes normally shed their oldest needles in the Fall. White pines usually retain their needles for one to two years. The older needles, those towards the center of the tree, turn yellow before dropping. Older arborvitae leaves turn a reddish brown. Natural needle discoloration may be more noticeable on trees that have experienced root stress due to less than optimal growing conditions. Extended dry periods during the summer months, as well as sites with poorly drained, heavy clay soils may accentuate root stress to pines. Since the newest growth (this year's needles) are still green and healthy in appearance, you can be assured that this yellowing phenomenon is a natural one.
To improve the growing conditions around your trees you may want to refer to HO-140, Fertilizing Woody Plants (PDF file). In addition, deep-core aeration may help improve root growth by improving the soil structure and the percolation of water into the soil. To apply one inch of water a week (the minimum needed during drought periods) you may need to run a hose and sprinkler or soaker at full capacity for 1.5 hours under the branch spread (drip line area).
Plant and Pest Digital Library and Digitally Assisted Diagnosis, Purdue University, West Lafayette, Indiana.