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Don Kinzler, Published July 05 2013

Growing Together: Crime against conifers: Understanding how to kill a tree without trying

I’ve noticed suspicious behavior among our younger trees around town, and I decided to do some investigative reporting.

Unfortunately trees don’t have Facebook sites or emails for us to scrutinize, and I don’t think they tweet. So we will have to do some Sherlock Holmes-type sleuthing to deduce what’s happening.

I’ve observed problems on trees ranging in age from newly planted to those growing for about 10 years.

The trees exhibit trunk and bark damage. On some, the rough, ridged, bark is injured slightly above ground level. Others have cracks, crevices and wounds running lengthwise up and down the trunk as high as the branches. Some of these cracks extend deeper than the outer bark, into the tree’s inner wood.

To start, let’s talk tree trunk biology. The outermost visible bark layer is the tree’s protective armor. The color, thickness and texture vary with each species.

Bark protects the lifeblood of the tree, the cambium layer. This thin, greenish-white layer is where the tree’s growth occurs and is immediately inside the bark. Tissues inside and outside this thin layer conduct water and nutrients up and down the tree.

Farther inside the tree trunk are the rings of “wood,” which structurally support the tree.

Possible suspects

E Lawn mowers and trimmers

The most obvious lower bark mutilation is caused by lawn mowers scraping and string trimmers whipping against the trunk.

Because trees can’t whimper, whine or whinny, it’s easy to mistakenly think of a tree trunk as we would a wooden fence post. We wouldn’t think of bloodying a horse’s leg by getting too close with a string trimmer.

If the outer protective bark is damaged, the cambium layer is exposed to injury, and the tree is left to figuratively bleed to death. Whether death comes slowly or quickly depends on the depth of the injury and whether the damage circles entirely around the trunk.

String trimmers are great, and I use ours weekly. But who among us, myself included, has never scarred a fencepost or the garage siding while trimming?

Years of tree growth can be ruined in seconds. The damage is also cumulative. A little nick this week, a little scrape next, and soon the damage is irreparable, and the tree can begin an irreversible decline.

A weakened tree becomes more susceptible to insects, disease and winter injury. The damage that began as bark injury can manifest itself as deep bark cracks, dead branches, overall decline in vigor and possible death over time.

E Tree wraps

Our next culprit is ironic because the damage is caused by our attempt to protect the tree.

Tree wraps have the goal of protecting the trunk from rabbits, lawnmowers, and winter sunscald. Most commonly seen are white tubular protectors and larger diameter black tubes.

Tree wraps, which should be removed before the trunk diameter touches them, are usually installed at planting and are too easily forgotten. There should always be at least an inch or two of space next to the bark. If the wrap is tightly enclosing the trunk, the unnaturally moist bark is a breeding site for rotting organisms and insects.

Tight tree wraps constrict and girdle the trunk. In time the tree declines in vigor, and death may occur.

E Planting depth

Thirty years ago when I was a young North Dakota State University Extension traveler, I remember a group of older and wiser horticulturists and foresters determining why tree trunks were splitting in Bismarck.

Many trees that had been planted during the previous eight years were exhibiting vertical cracks in the bark and sometimes deeper into the trunk.

The damage occurred on all sides of the tree (Winter sunscald injury can be similar, but occurs usually on the sunny south and southwest trunk exposures). A suspected cause was planting depth. It was believed the roots of the tree ended up under a too deep layer of soil.

I’ve observed similar symptoms on trees in the newer subdivisions of our cities. In doing a little research I found studies showing a nationwide problem. Apparently trees have commonly been planted too deeply.

Trunk cracking, bark problems, branch dieback and winter injury may result. Often these problems become slowly obvious as the tree ages, and a slow declining spiral occurs.

What a shame to see a tree that was planted 10 years ago declining due to planting depth. And what a loss of precious tree-years.

This mystery will be solved next week, when we discuss the solutions to these important tree problems.

Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler’s Greenhouse in Fargo. Readers can reach him at donkinzler@msn.com.