Yellowing Pines and Needle Drop
The following question was sent to the P&PDL diagnosticians here at Purdue University:
Question: Help! I have just noticed that the older needles on all of my pine trees, as well as my neighbor's pines, are turning yellow and dropping off the tree. This year's needles are still green. What is wrong? Does this mean my tree is dying?
Answer: No need to fear! The symptoms that you are observing are normal for this time of year. 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 Wood Plants (PDF 60K). 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).