Fall Needle Drop
Gail Ruhl, Sr. Plant Disease Diagnostician, Purdue University
Every year in late summer and fall, the P&PDL receives inquiries from concerned homeowners about conifers that are exhibiting discoloration and loss of inner needles. The older needles on white pine, those towards the center of the tree, are observed to turn yellow and drop while older arborvitae leaves turn a reddish-brown before being cast to the ground. The symptoms are very striking and tend to cause alarm.
No need to fear! Although evergreens provide green color all year long this does not mean the individual needles live forever. Evergreens do shed their older needles to make room for new growth, but what makes them evergreen is that they retain some green foliage all year long, instead of shedding all of the leaves at once. However, some species of evergreens, such as white pine and arborvitae, have a more noticeable leaf drop than others, dropping their 2-year old needles all at once in the Fall. This can be quite alarming, if you don't realize that it is perfectly normal.
The environment may also play a role in how quickly this natural phenomenon occurs. Trees under stress from adverse environmental factors such as drought may lose more than one year’s foliage at a time, or the discoloration may occur more quickly than in healthy trees.
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).