Trunk Cracking on Landscape Trees

Published: September 1, 2008

Extension offices around the state have received a number calls this summer describing development of large vertical cracks in trunks of landscape trees. Trees most commonly affected have been maples and crabapples. In some cases, cracks have spread two inches wide and extend several feet up the trunks of affected trees. Homeowners are understandably concerned and puzzled about this damage.

There is some debate among tree experts as to the causes of trunk cracking. The general consensus is that there are several contributing factors. First, bark damage of some kind may cause weakness in the affected area. For example, the Easter freeze of 2007 caused considerable damage to the bark of trees, some of which is not apparent from the surface. Our unusually high precipitation in 2008 may have also played a role. With relatively cool weather over most of the summer and ample soil moisture, many trees have grown rapidly. When this happens, radial trunk growth can put stress on the bark, leading to splitting at weak spots. A short dry spell, followed by excessive rainfall can intensify the splitting.

There is also debate about what, if anything can be done to help trees recover from trunk cracking. Bark is a terrific protective cover and any kind of bark damage to it provides access for diseases and insects to the inner bark and wood of the trunk. However, most tree experts agree that treating the cracks with a wound dressing of some kind has no significant effect on recovery and provides no real protection from insects and fungal pathogens. In most cases, the bark at the edges of cracks will begin producing callus tissue quickly and, if the trees are reasonably vigorous, the will be able to produce barrier layers under the wounds to prevent decay from moving into the trunk.

So, the best thing that one can do to help a tree recover is to maintain vigor. Mulching, watering during drought and moderate fertilization will help with this. If nearby turf is regularly fertilized, a tree may not need fertilizer application. If not, two to three pounds of actual nitrogen per 1000 square feet over an area 1.5 times the drip line diameter should be adequate. Avoid application of broadleaf weed killers to turf areas near recovering trees. Be particularly careful not to allow herbicide to leach downhill into the mulch over tree roots. Given a little TLC, most trees with trunk cracks will recover nicely and the damage will not be noticeable five years from now.