Are Your Pine Needles Brown, Red or Purple? It Could be Winterburn.
Information from this article comes from Dr. Peter Kolb (MSU Extension Forester). During dry and cold winters, evergreen trees growing in windbreaks or other exposed areas are afflicted with a bad case of needle discoloration that ranges from brown to purple followed by needle drop. Often this is most prevalent on the south side or windward side of the tree, and in some cases only last year’s new needles are impacted and in other cases most of the older needles are disproportionately afflicted. Multiple factors might be interacting on the tree, though the major cause is typically a winter where severe temperature shifts have occurred.
The general phenomenon is commonly referred to as “windburn” or “winterburn” and results from needles drying out. As mentioned, multiple factors can influence this condition. Green needles on evergreen trees are designed to function between 3-7 years on a tree, and thus must remain alive throughout the winter. To prepare for extreme cold, the cell structure and contents of woody plants undergo changes in the fall to winterize or “harden off.” Among these are increases in cellular sugars and lipids that both decrease the freezing point of cell cytoplasm but also act to break apart cell water content into micro-sized droplets that can “supercool,” reaching temperatures as low as -38 Fahrenheit before they freeze. As water freezes it expands and can result in cell wall rupture. As a result, trees or other plants not adapted to cold temperatures suffer from direct cell ice formation that kills living tissue.
Another form of cold damage is when water trapped in between cells freezes. Although this causes minimal damage as no cells are directly damaged, ice has the tendency to attract liquid water from surrounding tissue. Thus when trees go through multiple and rapid freeze thaw cycles, the ice between cells has a greater potential to pull the super-cooled water droplets out from inside the surrounding cells, thereby causing severe dehydration and cell death. This is the same mechanism that causes “freezer-burn” in produce or meat stored in a chest freezer too long or has thawed and refrozen several times. The outcome is almost always severe dehydration and disruption of the cell structure and a chemical change of many organic compounds that give freezer-burned foods their unique taste.
Good tree management begins with raising a healthy tree. Do not fertilize with nitrogen any time other than spring and then apply sparingly as fall nitrogen prevents trees from hardening properly. Keep your evergreens hydrated. Afflicted trees can recover. Do not prune until new branch buds have had a chance to sprout as buds are often much hardier than needles. Most pine trees can regrow needles, though a full complement may take three years. If no new needles have resprouted by mid-June then it is safe to prune off the branch if it has lost all its needles. Water and fertilize with a balanced fertilizer such as 10-10-10 in late May to help restore growth. Approximately one to two pounds for a fifteen foot tree should be plenty distributed evenly from the stem to 1 ½ times the distance to the edge of the canopy.
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