As far as tree species go, the western red cedar is kind of a big deal here on the coast of British Columbia. Not only is the western red cedar (more formally known as Thuja plicata) touted as the provincial tree of British Columbia, but historically, the wood, roots and bark of the western red cedar were integral to First Nations culture.
Fall flagging is another notable characteristic of the western red cedar. If you’re not familiar with the process, fall flagging can look alarming—isolated branchlets of old, dying or dead foliage scatter over the whole tree and turn yellow, red or bronze in colour.
The good news is: fall flagging is a normal condition of the western red cedar. Because they prefer moist, well-drained soil, western red cedars are prone to flagging after long periods of hot, dry weather. Flagging is a defense mechanism to reduce water loss by ridding the tree of older, less productive foliage.
Thanks to Kevin Zobrist of Washington State University a lot of the mystery behind fall flagging is explained in this handy fact sheet: Seasonal Foliage Loss in Pacific Northwest Conifer Trees.
Here’s an excerpt below or click the link above to read the complete handout.
In the fall, after a long, dry summer, an evergreen conifer may not have enough resources to sustain all of its green foliage; thus, it will shed its oldest foliage (the foliage found on the innermost part of a branch). In doing so, the tree is prioritizing its resources. The oldest foliage is the least productive because it has become dirty over time and, being on the interior of the branch, receives the least amount of sunlight. The tree will sacrifice this older foliage in favor of the newer, more productive foliage.
Although the tree’s appearance may be somewhat alarming, this seasonal foliage loss is a normal part of conifer growth. The foliage loss is particularly noticeable in western red cedar, where it is referred to as “flagging”.
Since dieback is a seasonal phenomenon, it should resolve itself with the changing of the seasons. The foliage that has turned orange or brown will be blown out of the trees with the first big windstorms in November or December, and suddenly make the tree look much greener and healthier.
Other seasonal discolorations may appear as winter progresses. Again, red cedar is particularly prone to these discolorations—foliage may turn a bronze color due to cold, dry weather, but then green-up again in the spring.”
Nothing to worry about—business as usual for the western red cedar.