Do you have Winterburn on your Evergreens?

 

April 25, 2019



By MSU Extension Forestry Specialist Peter Kolb

Some years evergreen trees growing in windbreaks or other exposed areas are afflicted with a bad case of spring needle discoloration that ranges from orange to brown or purple. On some trees only the needle tips are discolored and right next to them the entire tree is orange. This variability might be the result of individual tree genetics, microsite, soil water and an individual trees physiological vigor prior to going into winter. Different tree species will also show different affliction. This year pines and junipers appear to have been particularly impacted whereas most spruces showed minimal impact.

The general phenomenon is commonly referred to as “winterburn” and results from needles freeze-drying from sudden temperature shifts and extreme wind, or a combination of the two. Green needles on evergreen trees typically remain functional between 3-7 years on a tree, and thus must remain alive throughout the winter. In the fall tree cells “harden off” by increasing cellular sugars and lipids that both decrease the freezing point of cell cytoplasm but also act to break apart cell water content into microsized droplets that can “supercool”, reaching temperatures as low as -38 F before they freeze. When water freezes it increases in volume by about 9% - which is also why ice floats on liquid water. If cells have too high a water content or have not increased their sugar content, water within the cells turning to ice will rupture cell walls, which results in dead tissue. This is the same as “frostbite” in humans or animals. Each plant/tree species has its own unique ability to harden off to some minimum temperature and in response to certain stimuli. Some tree species respond to cooling temperatures in the fall and other harden off in response to shorter day-length. Plant hardiness zones indicate how adapted each tree species is to extreme cold before they begin to suffer from freezing damage.

So is a discolored tree dead? Orange or brown needles indicate that the discolored part of the needle has been damaged beyond repair – except for junipers. However, that does not mean the tree is dead! As long as the buds are alive, the tree will survive because in the spring the bud will elongate and produce new needles. To check buds, just peel a little of the brown coating off and if the bud in green and juicy inside it is alive. Dead buds are brown and dried out inside. Even needles that are partially green near their attachment to the twig or branch will remain somewhat functional – providing the tree with sugar from photosynthesis, even if it is not at optimal production. Within a year or two the tree will have regained much of its former “normal” look.

Juniper trees are much harder to diagnose because they have scale like needles and their buds are much smaller and harder to evaluate. In general, juniper species are very hardy and typically will turn brown or orange in the winter. In the spring they usually green up again.

Late season extreme cold, fall drought, excessive wind, fall fertilizer, and mid-winter warm spells all can contribute to winter burn phenomenon. Needle discoloration does not mean the tree is dead. When in doubt, wait until the end of June. If there is no new green growth from the buds by then, the tree is probably irreversibly damaged.

 

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