Pearlbush has beautiful white flowers in the spring… and older specimens show attractive bark in the winter. Photo: www.pepiniere-bretagne.fr & landscapeplants.oregonstate.edu
Question: I have a shrub called pearlbush (Exochorda × macrantha ‘The Bride’) which has been losing its bark by bits and pieces for a few years now. However this year, almost all the bark of the trunk is sloughing off in patches: there is hardly any left on the trunk! It was magnificent in May, covered with white flowers as in previous years, and showed no other signs of illness. Is this normal? Or if not, should I cover the trunk with something for the winter?
Answer: Peeling bark is actually quite normal for this species. The pearlbush’s bark is said to be “exfoliating”: it comes off in strips over time. This trait is, however, only seen on more mature specimens, usually after many years of culture. And rather than worrying you, it’s supposed to please you. It’s one of the attractions of the shrub. After masses of white flowers in the spring and attractive green leaves all summer, it now offers stunning peeling bark in the winter.
Under the bark that peels away there is already fresh, healthy bark, so the plant doesn’t suffer at all. Obviously, there is no need to cover or otherwise protect the exfoliating trunk: just relax and enjoy the show!
The pearlbush isn’t the only woody plant with exfoliating bark, by the way. This is a fairly common trait and seems to have evolved independently in many families of plants. For example, birches are well known for their attractive peeling bark.
Very often the trees and shrubs with exfoliating bark need time to start performing. Often their bark is smooth on young specimens, but changes in texture as they mature and that can be decades later. Even young birches, though planted specifically for their “papery” bark, have smooth bark that clings to the trunk in their youth.
What Is Exfoliating Bark For?
No one is sure exactly why so many woody plants have bark that naturally exfoliates, but there are a few theories and, in some cases, there might be more than one reason. Here are a few:
- Bark peeling off might allow the plant to get rid itself of its enemies: insects (mealybugs, for example), fungi, invasive climbers, heavy epiphytic plants, etc.
- Some trees photosynthesize through their bark and it’s therefore logical that the plant would get rid of its outer bark, which may have become more opaque over time or maybe covered with lichens or mosses, in order to expose younger bark to the sun. This is the case, for example, of gumbo limbo (Bursera simaruba), whose older flaky bark is red, but new bark is green.
- The sloughing off old bark may allow a better exchange of atmospheric gases between the plant and the surrounding air or even improve transpiration.
- For some species, the exfoliating bark appears to be a protection against fire. Either the bark serves as a flame retardant and protects the trunk, or it burns quickly, with fire moving rapidly up the trunk while leaving the living bark underneath intact.
But in most plants with peeling bark, the reason for this exfoliation is really not fully understood.
Trees and Shrubs with Exfoliating Bark
I thought I’d include a short list of trees and shrubs with exfoliating and often very decorative bark with this article. I mean, how many can there be? Several hours of research later, I came up with nearly 50! Here they are!
- American hop hornbeam (Ostrya virginiana)—hardiness zones 3–9
- American sycamore (Platanus occidentalis)—hardiness zones 5b-9
- Amur Cherry (Prunus mackii)—hardiness zones 2b-9
- Bald cypress (Taxodium distichum)—hardiness zones 5–9
- Beautybush (Kolkwitzia amabilis)—hardiness zones 4–8
- Black Cherry (Prunus serotina)—hardiness zones 2–7
- Chilean myrtle (Luma apiculata)—hardiness zones 8–9
- Chinese birch (Betula albosinensis)—hardiness zones 3–8
- Chinese elm (Ulmus parvifolia)—hardiness zones 5–9
- Cinnamon clethra (Clethra acuminata)—hardiness zones 6–8
- Climbing hydrangea (Hydrangea petiolaris, syn. Hydrangea anomala petiolaris)—hardiness zones 4–8
- Common ninebark (Physocarpus opulifolius)—hardiness zones 2b-8
- Cornelian cherry (Cornus mas)—hardiness zones 5–8
- Crepe myrtle (Lagerstroemia indica)—hardiness zones 7–9
- Dahurian birch (Betula davurica)—hardiness zones 4–7
- Dawn redwood (Metasequoia glyptostroboides)—hardiness zones 5–8
- Eastern red cedar (Juniperus virginiana)—hardiness zones 3–9
- Eastern redbud (Cercis canadensis)—hardiness zones 6–8
- Eucalyptus (Eucalyptus spp.)—hardiness zones 8–11
- Gumbo-limbo (Bursera simaruba)—hardiness zones 9–12
- Himalayan birch (Betula utilis)—hardiness zones 4–7
- Hybrid strawberry tree (Arbutus × andrachnoides)—hardiness zones 8–10
- Japanese dogwood (Cornus kousa)—hardiness zones 6–8
- Japanese Zelkova (Zelkova serrata)—hardiness zones 6–8
- Kalm St. John’s Wort (Hypericum kalmianum)—hardiness zones 4–7
- Katsura (Cercidiphyllum japonicum)—hardiness zones 5–8
- Lacebark pine (Pinus bungeana)—hardiness zones 6–8
- London plane (Platanus × acerifolia)—hardiness zones 5b-9
- Manchurian birch (Betula costata)—hardiness zones 4–7
- Oakleaf hydrangea (Hydrangea quercifolia)—hardiness zones 5–9
- Pacific madrone (Arbutus menziesii)—hardiness zones 7–9
- Paper birch or canoe birch (Betula papyrifera)—hardiness zones 2–7
- Paper mulberry (Broussonetia papyrifera)—hardiness zones 7–9
- Paperbark (Melaleuca quinquenervia and other species)—hardiness zones 9–11
- Paperbark maple (Acer griseum)—hardiness zones 5b-8
- Pearlbush (Exochorda spp.)—hardiness zones 5 or 6–9
- Peking lilac (Syringa reticulata pekinensis)—hardiness zones 4–7
- Persian ironwood (Parrotia persica)—hardiness zones 5–8
- River birch (Betula nigra)—hardiness zones 3b-9
- Scots pine (Pinus sylvestris)—hardiness zones 3–7
- Seven sons flower (Heptacodium miconioides)—hardiness zones 3–9
- Shagbark hickory (Carya ovata)—hardiness zones 4–9
- Silver birch (Betula pendula)—hardiness zones 3–7
- Southern wax myrtle (Myrica cerifera, syn. Morella cerifera)—hardiness zones 7–10
- Stewartia (Stewartia spp)—hardiness zones 6 or 7–9
- Strawberry tree (Arbutus unedo)—hardiness zones 7–10
- Three-flowered maple (Acer triflorum)—hardiness zones 5–7
- Yellow birch (Betula alleghaniensis)—hardiness zones 3–7
- Yellow buckeye (Aesculus flava, syn. A. octandra)—hardiness zones 5–8
When Bark Shouldn’t Be Exfoliating
So much for species for which exfoliating bark is normal and even one of their attractions. But for most trees and shrubs, when bark starts sloughing off, that’s not good news!
You can see this effect following or during a fungal or insect infestation, after an extreme cold snap, after severe drought or when the bark has been physically damaged in some way. In such cases, any bark that comes free is likely dead or dying. It’s often a sign that the tree or shrub is in poor condition or maybe even dying.
In such a case, it might be best to have the damage seen by a certified arborist to see what can be done to save it, especially if it’s a tree you truly want to keep.
If that’s too much of an expense, at least cut away the peeling bark and clean the wound up to any healthy bark, exposing it to fresh air. Regularly sterilize any cutting tool in rubbing alcohol as you work. Do not apply wound dressing: that could make things worse! Then let Mother Nature see what she can go to fix things. If the tree is reasonably healthy, it will be able to cover the bare patch with fresh bark.
So much for trees. In the case of shrubs, try cutting it back severely, either just the damaged branch or all the branches. Make the cut well below the wound, up to to only inches from the ground. Most shrubs will resprout rapidly from the base. This is called rejuvenation pruning.
Exfoliating bark: it can either a beauty or a beast! It’s up to you to determine which.
cork oak is the really weird, because it is so spongy.
The paperbark melaleucas are a main street tree downtown, and the spongy bark has been an advantage for protecting both the trees and cars from vehicular altercations.
I’m not sure you could count cork oak as having exfoliating bark. Sure enough, humans exfoliate it, but lumpy as the bark may be, it doesn’t seem to exfoliate on its own.
Oh of course; it stays attached. Otherwise, it would not get so thick.