Some trees have a hard time making it through a winter. This is true whether they are in the open, in a new development, on an acreage, or protected in an old neighborhood. Winter sunscald and frost crack can be a real problem. Steve Schwab, retired City Forester, City of Lincoln Parks & Recreation Department, had a question about frost crack and sunscald at a meeting of the Lincoln Garden Club 6 years ago. I realized then that I had not written about this so I asked him to describe what they are, how to prevent them, and what trees are the most likely to be damaged. I am repeating his response as it is still applicable today:





          Long vertical bulges or cracks on trunks indicate a recurring stress. The common term "frost crack" is misleading because frost or cold does not initiate a seam or crack.

          Cracks or seams start at wounds or branch stubs. Serious damage can result from seemingly minor wounds. A sudden, sharp drop in winter temperature causes the outer layer of wood to contract more rapidly than the inner layer, which can result in a long vertical crack at weak points in the trunk.

          "Frost cracking" can occur repeatedly in the same place, causing a buildup of tissues and the formation of frost ribs or seams. Depending on the tree species, the tissues at the margins of the injury may grow so rapidly that they curl in on themselves, preventing complete closure of a wound.

          Avoid wounds to the trunk and properly prune branches to prevent the formation of frost cracks.


          Sunscald injury occurs during late winter or early spring when the temperature is above freezing during the day and below freezing at night. During the day the tree tissues, warmed by the sun, become active. Freezing at night kills this tissue, resulting in an elongated canker (wound) usually on the south and southwest sides of the tree. Sunscald injury is characterized by an elongated sunken, dried, or cracked area of dead bark.

          Thin barked trees such as maples, ornamental cherry and crab apple trees are most susceptible to sunscald injury.

          To prevent sunscald injury, shade the susceptible tree or use tree wrap on younger, susceptible trees to reduce the warming of tree bark during the day. Wrapping trunks of susceptible trees with protective tree wrap should be done after leaf drop in the fall and be sure and remove this wrap in late spring.

          Sunscald injuries to tree limbs can be minimized by avoiding heavy pruning of trees which have dense canopies, such as maple. Gradual thinning of limbs over a period of years is preferable, particularly on thin-barked trees.

          In Lincoln, sunscald injury is especially prevalent on ‘Emerald Queen’ and ‘Emerald Lustre’ Norway Maple, ‘Red Sunset’ Maple. and Sugar Maple if planted where it gets reflected sunlight off pavement and/or snow on the south or west sides  Street trees planted 5 feet back of the curb on the north or west side of streets, can have a greater incidence of sunscald. If new varieties of Sugar Maple such as ‘Green Mountain’ and ‘Fall Fiesta’, are planted further from the pavement (i.e. back of sidewalk or where there is no sidewalk), then sunscald doesn’t occur.

          Also, Sugar Maple has and is being planted on certain streets  as an “under story” street tree,(i.e. where large American Elms were removed due to Dutch Elm Disease) and because of their shade tolerance, these trees now are 35 feet tall and will eventually become the “over story” tree on such street streets.”

Copyright 2014

(Next week I will continue with Steve’s recommendations on winter protection of arborvitaes and evergreen trees.)