It’s time to clip, snip, trim, or cut back your conifer trees

The dead surface of this White Pine has blistered, which then created powdery orange rust fungus spores
The dead surface of this White Pine has blistered, which then created powdery orange rust fungus spores

If you need to prune your conifer trees, December is a good time. You can use the branches for holiday decorating, and the resulting wounds to the tree are less likely to attract wood-boring insects at this time of year than if you pruned in spring or summer.

Not all conifers need pruning, but one tree in particular benefits from this care: the western white pine (Pinus monticola). In addition to protecting the tree from disease, a little pruning will produce those great-looking wreaths and garlands featuring the western white pine’s characteristic long, fragrant needles and pitchy cones.  

Western white pine is a native tree that used to grow throughout much of Washington, Oregon, Idaho, and British Columbia, but it has been seriously afflicted by an exotic disease called “white pine blister rust.” The disease, caused by the Eurasian fungus Cronartium ribicola, infects white pine trees when tiny spores enter the needle surface and grow to the twig, then the branch, and then to the trunk of the tree. Tissues affected by the fungus die and, if enough of the trunk’s circumference becomes affected, the whole tree dies. In spring, the dead surfaces blister and emit additional powdery orange rust fungus spores.

Here’s how pruning can help.

One key to protecting white pine trees is understanding that the blister rust spores are very fragile and susceptible to drying out. The most humid part of the forest is near the ground. Pruning the lowest branches can remove branch infections before they reach the trunk, increase the drying effects of sunlight and airflow around trees, and reduce opportunities for new infections to start by moving the living pine needles further away from the damp ground. Western white pine trees, with their lowest living branches pruned to ten feet or more above the ground, are much less likely to die of blister rust than trees with low-growing branches and needles.

When pruning, remember:

  • Never prune a tree that doesn’t belong to you.
  • Prune close to the branch collar, not with the trunk.
  • Never remove branches on more than half the tree’s height.

Learn more about what DNR’s Forest Health Program is doing to protect forests in Washington State.


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