Many nurseries within Ohio and elsewhere in the U.S. and Canada have been experiencing severe bark splitting and scald type problems. To me, bark splitting was always easy to diagnose — but the amount and severity of damage seen in 2005 in particular, and continued into 2008, has been unlike anything I have seen before. Bark splitting is often associated with southwest injury and cold temperatures, but cold may not be the causal factor or the only reason for splitting.

The scale of this problem points to something more than an environmental problem. Major stock losses have occurred. Some growers have reported splitting losses exceeding five per cent of inventory, or 3,000 to 4,000 trees per nursery this past spring. At an average cost of $125 per tree, at the number of nurseries reporting problems, the stock losses in the state have been staggering — in excess of several million dollars. On the bright side, I have learned a lot about bark cracking, really more than I ever thought there was to know, and can now share my insights in this article.

Bark splitting
Bark splitting is most common on thin-barked trees. The trees that we have seen problems on have been Pyrus spp. especially Callery pears, (P. calleryana), Prunus spp. especially Yoshino cherry (P.X yedoensis) and Kwanzan cherry (P. serrulata ‘Kwanzan’), crab apples (Malus spp.), sycamore (Platanus occidentalis), serviceberry (Amelanchier spp.), hawthorn (Crataegus spp.) mountain ash (Sorbus spp.), black gum (Nyssa sylvatica), paper bark maple (Acer griseum), Japanese maples (,i>Acer palmatum) especially var. dissectum, Norway maple especially Acer platanoides ‘Emerald Queen,’ red maples (Acer rubrum), dogwood (Cornus spp.) especially kousa dogwood (C. kousa), magnolias, especially Magnolia ‘Elizabeth’ (Magnolia acuminata X M. denudata) and the yellow magnolias such as Magnolia ‘Butterflies’ (Magnolia acuminata X M. denudata ‘Sawada’s Cream’, Magnolia ‘Yellow Bird’ (Magnolia acuminata X (M. X brooklynensis ‘Evamaria’) or Magnolia ‘Yellow Lantern’ (Magnolia acuminata var. subcordata X (M. X soulangiana ‘Alexandrina’).

Again, bark splitting is often thought to result from very cold temperatures on sunny days and is often associated with southwest injury. Sunscald or southwest (SW) injury, as the name implies, is common on southwest exposures. On bright sunny days the southwest side of the tree heats up, absorbing the heat of the sun. When the sun sets or goes behind a cloud, there is sudden freezing in the heated up tissue. Snow cover, resulting in a reflection off the snow surface, and more extensive heating on the southwest side often aggravates this rapid freezing and thawing. Sunscald results in death of the exposed bark. The area subsequently dries out and dies, causing a large open wound on the southwest side of the tree. Unlike SW injury, bark cracking results in the formation of a longitudinal crack in the bark of the tree. It may also split the sapwood causing an area of the trunk to dry out and form an awl-shaped scar in the wood. Bark splitting in winter should not be confused with growth cracks from rapid summer expansion of the sapwood. Small growth cracks will callus over and heal rapidly (unless infected), but will leave vertical scars in the bark. However, these may become points for subsequent bark splitting from cold.

Although very cold temperatures on sunny days are thought to be the cause of bark splitting, other researchers believe that cold and frost are not the causal agents. The actual cause of the crack was pre-set by a wound, such as the improper removal of a basal sprout or leaving a branch stub (Butin and Shigo, 1981; Caspary, 1855; Shigo, 1963). The researchers listed above say that by knowing this, it makes it possible to prevent frost cracks by minimizing wounds, and by proper and early pruning of branches and basal sprouts. By thinking bark splitting is related to cold alone, there is little that can be done to prevent it, as we are subject to the climate. However, much care should be taken not to wound trees, especially when young (Butin and Shigo, 1980). The take-home message is, these trees may not be as tough as we all think they are, and TLC can go a long way to preventing bark splitting.

Tippet and Shigo (1980 and 1981) explain that after wounding, the cambium forms a distinctly different tissue called the barrier zone. The barrier zone is a very strong protection against infection, but it is also a plane of structural weakness, because it has a different anatomical and chemical makeup from normal wood. When internal stresses caused by rapid temperature changes or wind occur near the barrier zone, the wood may separate tangentially and longitudinally along the barrier zone (McGinnes et al, 1977), because this is the path of least resistance for the crack to form. Another way to explain it is, following a sudden temperature drop, the outer layers of bark and the wood cool most rapidly and are subject to appreciable tangential tension, which causes marked shrinkage and cracking to occur where the wood is structurally weakest (Larsen and Higgins, 1992). Again, however, the main point is that wounds and stubs — not frost — initiate bark splitting or frost cracks or what are called radial shakes in forestry. However, once started, the cracks may persist for the life of the tree because stress caused by many factors, including frost, leads to continuation of the crack.

Fewer cracks on the SW
This explains in part, why the cracking we have seen is not always on the SW side of the tree. In a study with apple trees, Larsen (1972) also found that more cracks were on the sector of the trunks facing south (S) than on any other portion. There were also a high proportion of cracks on the southeast (SE) and east (E) sectors of the trunks, with fewer cracks on the N and SW sectors (Larsen, 1972). Larsen had no explanation for the numerous cracks on the E, since prevailing winds in the study area were from the west (W). He also could not explain why fewer cracks were on the SW side where temperature fluctuations would have been the greatest. Again we have had similar findings of a predomination of cracks occurring on the S and E, as shown in Figure 4. However, in Larsen’s study (1972) he stated that low temperatures induced the cracking. He also stated the rootstock and fall maturity of the scion were very important in the development of longitudinal trunk bark cracking in apple.

Pruning wounds and caliper
Splits actually being the result of wounds, versus cold, also partially explains why some of the cracks we have seen are associated with pruning wounds (Fig. 5). Heavy pruning, late cultivation, thin or sandy soils and spray injury have also been long associated with increased cold injury, because all those factors disrupt acclimation. However, the association of pruning wounds with bark splitting is not an acclimation issue, it again concurs with Butin and Shigo (1980) that splitting is first caused by wounding and then continued by cold. There is also a strong relationship to caliper growth. In a study with young Asian pear trees, cracked trees were more prevalent (P< 0.0001) when mean trunk circumferences were significantly larger (Larsen and Higgins, 1992). Within cultivars, there has been a consistent tendency for larger trees to crack — we have also observed this in 2005.

However, the research that cracking is actually a result of wounds and not cold per se, has made us speculate that the removal of suckers and adventious shoots, especially with herbicides and/or the use of systemic postemergents shortly after mechanical removal of suckers, may be reasons for the increased severity and frequency of bark splitting. Howell and Weiser (1970) noted herbicides should be used with caution around previously injured young tissue, as re-injury will occur in the breaks. I will discuss herbicide injuries more in the next article, including their relationship to increasing cold injury. We no longer recommend the practice of tree sucker removal with glyphosate-containing products. Watch when removing suckers with a spade, or removing adventitious sprouts with pruning shears, that you do not damage the trunk.

Tree guards could increase splitting
There also seems to be a relationship between increased splitting and the use of tree guards (Fig. 6). Earlier studies have shown plastic trunk guards used to reduce SW injury, bark splitting or deer and rodent damage can actually intensify bark splitting (Spotts and Cervantes, 1994). If put on too early in the growing season, the guards inhibit the cold acclimation process in the fall, and result in an increased incidence of cold injury in the spring. The guards actually alter the microclimate of the bark in the protected region. Not only do the plastic guards reduce cold hardiness in the guarded region, but they also increase the moisture content in that region, resulting in increased susceptibility to mechanical injury, and thus splitting, caused by ice formation under the guard. Tree guards were originally used for cold injury protection and applied in late fall, however, in more recent years they are being left on year round for protection from deer. It is recommended that tree guards be removed in summer and re-applied in late fall or early winter. Leaving tree guards on year round increases the risk of SW injury and SW bark cracking.

Conclusions
Another problem that may be associated with bark splitting seen this year is internal bark necrosis (IBN), which results in a high density of bark lesions being present on certain trees such as pears. IBN only occurs when the soil conditions are acidic or in the range of pH 4.2. IBN is associated with manganese (Mn) toxicity or when foliar samples test at Mn levels of 86 ppm and higher. There is also a stem pitting-type injury associated with IBN. This stem pitting is another symptom we have seen with some of the split and scalded trees.

Dr. Hannah Mathers is an associate professor in the Department of Horticulture and Crop Science at The Ohio State University (OSU), Columbus, Ohio and working under contract with the Ontario-based Vineland Centre for Research and Innovation to help the nursery and landscape industry with research initiatives, industry projects and extension activities.