Focus on Hemlock ~ Dwarf Mistletoe

/

By Kevin Zobrist, WSU Extension Forester

Summer’s a good time to take a walk in your woods for more than recreation and exercise. You can look for danger trees, areas to thin or harvest, and signs of disease. 

Western Hemlock’s Live-Aboard

Do your Western hemlock trees have structures that look like the ones shown in the photo? These are called “witch’s brooms.” They come in all shapes and sizes but are always characterized by swollen, deformed, and overlapping branching patterns. This is caused by hemlock dwarf mistletoe, a parasitic plant. 

Dwarf mistletoes are different from true mistletoes, but all mistletoes are parasitic. The plant roots into the branch of the tree, causing swollen, deformed growth. There are a number of dwarf mistletoes out there affecting different tree species (most are host-specific). Hemlock dwarf mistletoe is the only one of significant consequence in Western Washington, and it is specific to hemlock trees, though there is some evidence that it may occasionally inhabit Douglas-fir (rare at most). 

In the summer, mistletoe launches sticky seeds that can travel 20 feet. If the seed lands on a hemlock, it sticks and begins a new infection. This can eventually cause mortality in a heavily-infested tree by deforming the branches, and by robbing the tree of resources. 

What to Do if Your Tree has Mistletoe

What should you do? Not necessarily anything. Dwarf mistletoe is native, and it is a normal, natural agent in the forest. It has ecological benefits of providing some great structures for wildlife. If you are trying to grow hemlock though, it may cause concern as it can spread pretty aggressively from tree to tree. 

Young hemlocks in the understory are particularly vulnerable because mistletoe seeds rain down from above. If you have only a couple of affected trees, you can remove them to prevent the spread (important if you have other hemlocks not yet infected or young understory hemlocks you want to encourage). 

Actually, you don’t have to fully remove the tree--you can kill it and leave it in place as an excellent habitat tree (it will be a snag with diverse structures). You do this by girdling the tree. Killing the tree (the host) kills the parasite, so it won’t spread from the dead tree. If you girdle the tree near the base, that will be the point of failure in the future and eventually the whole tree will fall, so consider if this will pose a hazard. Another option is to hire an arborist to girdle the tree higher up, but you would have to prune off all the live branches below that point. This way it will fail up higher, and when that breaks off you still have the bottom portion available as a good wildlife snag.

And Then Again…

If you have a lot of hemlock trees and widespread infections, it may not be practical to try to eliminate the mistletoe. You can encourage a shift to non-host species by under-planting (or encouraging natural) Western red cedar seedlings, replanting non-host species after a harvest, or favoring the retention of non-host species when thinning. Otherwise, it’s really nothing to lose sleep over.

Girdling

Girdling, also called ring barking or ring-barking, is the complete removal of a strip of bark (consisting of cork cambium, phloem, cambium and sometimes going into the xylem) from around the entire circumference of either a branch or trunk of a woody plant. No nutrients can then be transferred down through the phloem layer. It's this severing of the phloem layer that kills the tree by starving the roots.

For more in-depth reading on dwarf mistletoe, check out:

  • http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5187427.pdf
  • http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_025978.pdf
  • https://www.for.gov.bc.ca/hfp/publications/00198/Hemlock_dwarf_mistletoe