ISSUE 1   May 15, 2008

WINTER INJURY ON EVERGREEN TREES

Winter injury on evergreen trees has been reported in many areas of the state this spring (Fig. 1 and 2). It is most often seen on Colorado spruce and on certain pines, though junipers and arborvitaes are sometimes affected. The term "winter injury" actually encompasses a variety of needle-discoloration problems. Whole needles or portions of needles often turn brown or an orange-rust color, though spruce needles may get a purplish caste on their way to becoming rust-colored. Spruce trees are sometimes mistakenly diagnosed with Rhizosphaera needlecast, based on the needle coloration.

The cause that is most-often cited is having warm, sunny and windy winter days that allow the trees to begin photosynthesis and transpiration, while the ground is still frozen. The trees lose moisture from their needles and are unable to replace it. Other potential causes include de-hardening of needle tissue during warm winter days followed by plummeting temperatures. In those cases, trees can’t re-harden their tissues and needle damage results. Trees that have been stressed by insects, diseases or drought may be more susceptible to injury. Recently-transplanted trees, as well those growing in areas without snow cover are also prone to damage.

What can be done about winter injury? At this point, not a whole lot. The injury has already occurred. However, damage is usually (but not always) just aesthetic. If the buds are unharmed, then the trees will send out a flush of healthy new growth in 2-3 weeks. If the buds were damaged or killed, though, the tree will have a hard time recovering and will likely die. There are several steps that can be taken throughout the growing season to minimize this problem.

  • In late summer – mid-August to mid-September – allow the trees to become slightly drought stressed. This will start the winter hardening process earlier.
  • Following this stress period, keep trees well-watered right until freeze-up. This may help increase the hydration status of trees going into winter.
  • Anti-transpirants have been used in North Dakota with mixed results. Once applied, these products prevent water loss for about 3 months, and must therefore be applied some time in December.
  • Winter injury of evergreen trees        Winter injury of evergreen trees
    Winter injury of evergreen trees (photos by Sam Markell)

    Joe Zeleznik
    Extension Forester
    joseph.zeleznik@ndsu.edu


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