As aspens and cottonwoods in the Arkansas River Valley change from green to gold, many evergreen trees are going through a similar process. Their interior needles are changing from green to yellow or brown before dropping off as part of a natural shedding process, and not due to bark beetles or tree disease.
Every autumn, many Colorado evergreen tree species shed some of their older, interior needles as part of an annual growth cycle. Needles on the lower portion of the crowns or closest to the trunk are most commonly shed, but trees stressed due to drought or root damage may shed more needles to keep the tree in balance with its root system. Soon-to-be shed needles typically turn yellow first, then a reddish-brown color before dropping off; very small branches with few needles on them also may die.
In the CSFS Salida District, which includes Chaffee, Lake, Gunnison and Hinsdale counties, ponderosa pine, lodgepole pine and Douglas-fir are tree species that commonly shed needles in September and October. Pine trees tend to be the most conspicuous when shedding, because their needles are longer than spruce and fir needles.
Evergreen trees that shed fall needles have a different appearance than trees infested by bark beetles. The needles on a beetle-infested tree typically change color throughout the entire tree, initially starting with an off-shade of green and turning to reddish-orange by the following summer. In addition to changing needle color, bark beetle-infested trees will show other signs of attack, such as fine sawdust at the base of the tree and popcorn-shaped masses of resin on the trunk.
Fall needle drop is frequently mislabeled as “needle cast,” but the term actually refers to a fungal disease of evergreen trees.
For more information about tree and forest health, contact the CSFS Salida District at 719-539-2579 or visit csfs.colostate.edu.