In the Garden | Shedding light on bark loss

In the Garden | Shedding light on bark loss

Our native sycamore tree (Platanus occidentalis) is known for its extraordinary exfoliating bark that peels off to expose the beautiful creamy white and greenish colors beneath, creating a camouflage-like appearance.

This summer, many area sycamores are shedding bark at alarming rates, which has prompted folks to question the health of their trees.

In most cases, shedding bark on a sycamore is no cause for concern. In fact, it may actually be a sign that the tree is extremely healthy and growing rapidly.

To better comprehend exfoliating bark, it helps to understand the fundamentals of woody stem growth.

We all know that trees produce annual rings, and by counting the rings, you can estimate the age of a tree. So, why does tree growth occur in concentric rings?

Just beneath the bark of every tree is a layer of cells called the cambium. Every year, this specialized tissue gives rise to xylem cells on the inside and phloem cells on the outside, creating the growth rings we count.

Xylem tissue transports water and nutrients from the roots to the growing tips of each limb. This supply of water and nutrients is used by leaves to photosynthesize, creating sugars the tree will use for energy.

Phloem tissue is responsible for distributing sugars (and other compounds generated from the sugar energy) back down the stem to other plant parts, ultimately storing any unused energy in the roots for next year.

As trees enter winter dormancy, cambium growth stops for the year. The following spring, the tree will use energy stored in its roots to initiate growth again, creating another annual growth ring.

As cambium tissue creates new xylem and phloem, the older layers are pushed aside, becoming wood to the inside and bark to the outside. Over time, both of these layers become non-living portions of the tree stem. We often refer to the non-living inner wood as heartwood, a term I am sure many are familiar with.

Bark is retained to protect the stem, although it remains nonliving and does not exhibit further growth. Therefore, plates of bark tissue begin to separate as the tree stem enlarges over time.

Based on individual characteristics or genetics of each tree species, a unique exterior bark pattern develops. Believe it or not, bark is distinctive enough that many tree species are readily identfied by bark pattern alone.

Since sycamores are known for their exfoliating bark, a pattern unique to their species, excessive bark shedding may just be a sign of excessive growth. With adequate moisture, it is reasonable to expect some of the highest growth rates of the season this time of year.

Thus, an otherwise healthy sycamore shedding bark is simply a sign of rapid stem growth, which is a good thing.

It is important to note that exfoliating bark on sycamores typically occurs on younger stems. As the tree ages and the trunk becomes large, some bark is actually retained, which creates a vastly different appearance than younger, vigorously growing stems up in the canopy.

If bark on the lower trunk begins to fall off or slough, this may be a sign of health problems.

For example, wetwood is a condition caused by a complex of bacteria that produce internal pressure in the tree, creating a waterish material, called slime flux, which oozes out through wounds in the bark.

These wounds, along with the persistent flow of slime flux, can cause more extensive tissue death, resulting in bark falling off or sloughing.

Thankfully, the constant presence of slime flux oozing down the trunk is a blatant symptom of this condition and will often precede any bark sloughing, making it easy to identify.

Other serous conditions that create sloughing bark on lower tree stems are also associated with telltale signs of health problems, such as foliar symptoms on leaves or dead limbs in the upper tree canopy.

So, if your sycamore is otherwise healthy, but shedding a lot of bark from the canopy this summer, rest assured that is a natural part of your tree's annual growth and little more than a yard maintenance issue.

Ryan Pankau is a horticulture educator with the University of Illinois Extension, serving Champaign, Ford, Iroquois and Vermilion counties.

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