A:

This is an uncommon, but routinely observed, phenomena sometimes called stimulation broom. No one knows what causes this, but some type of stimulation must be involved. Brooming is a general term for branches that become abnormally clustered, or lose apical dominance and sprout lots of branches from a local area.

Douglas-fir dwarf mistletoe does not occur west of the Cascade Mountain crest in central and northern Oregon, so likely not that.

The classic pocket guide: "Common Tree Diseases of British Columbia" by R.E. Foster and G.W. Wallis from 1974. (Canadian Forestry Service) has a picture of this exact brooming. The photo is titled, "large stimulation broom in the crown of Douglas-fir, cause unknown." 

That sums it up. These occur throughout the NW, but no one knows what causes it. 

A mystery! 

Related Content from OSU Extension

Have a Question? Ask an Expert!

Ask an Expert is a way for you to get answers from the Oregon State University Extension Service. We have experts in family and health, community development, food and agriculture, coastal issues, forestry, programs for young people, and gardening.

Ask Us a Question