Dec. 28--At this season, one of the most important features of trees and shrubs really shines: bark. "In winter, with the leaves gone, it's more exposed, and we can really see it," says Kris Bachtell, vice president of collections and facilities at The Morton Arboretum in Lisle.
A tree's bark protects it from cold, sun and rain, but it's more than a shield. It's a living organ where many of the most essential processes of a tree take place.
Just below the outer bark is a crucial layer of cells called the cambium. That's where the tree adds new cells each year, with every new layer making the trunk or branch wider. The cambium includes specialized cells that carry water up from the roots to the branches and leaves and also transport the sugars produced in the leaves throughout the plant.
Any damage to the cambium harms the tree's ability to function and grow, Bachtell says. So it's no wonder trees have a tough outer layer for protection.
That outer bark layer is a tree's armor and insulation, but for gardeners, it's also an ornament. Each kind of tree has a distinctive pattern that's formed as the trunk grows wider and the bark grows thicker.
On some trees, such as paperbark maple (Acer griseum), sycamore (Platanus occidentalis), plane tree (Platanus x acerifolia), Peking lilac (Syringa pekinensis) and many birches, the bark splits and peels, often revealing inner layers that have a different color.
Other species have bark that develops deep furrows, like that of bur oak (Quercus macrocarpa), or long flat plates, such as that of shagbark hickory (Carya ovata). Some trees, such as beeches (Fagus), yellowwood (Cladrastis kentukea) and American hornbeam (Carpinus caroliniana), have bark that can stretch around the expanding trunk and stay smooth until the tree is old.
When he chooses which trees to plant where, "I always think about the bark," Bachtell says.
He knows, for example, that smooth, glossy bark like that of Amur chokecherry (Prunus maackii) shines in afternoon sun. "Between 4 and 6 o'clock, the sun really lights it up," he says. He especially likes paperbark maple when its peeling bark is backlit by the low winter sun.
If you're thinking of planting a tree in the spring, you should first consider your site conditions, including the soil, the sun and how much space you have, Bachtell says. (The Arboretum's Northern Illinois Tree Selector at mortonarb.org/tree-selector can help.) Consider the form of the tree, the leaves and perhaps the flowers. But if you're choosing between two suitable species, he says, think also about bark that will be attractive all year long.
Beth Botts is a staff writer at The Morton Arboretum in Lisle (www.mortonarb.org).
For tree and plant advice, contact the Arboretum's Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).
