Article 2 of 12 from Trees: Dispelling Myths & Misinformation, prepared by the arborists of the Southwest Division of Bartlett Tree Experts as a community education initiative.
In construction, as in life, if you don’t lay a solid foundation, everything else is more likely to be a problem.
It’s no different for trees, whose roots form the basis for their health and structural soundness. But what control do we have over tree roots that we can’t even see because they grow so deep into the earth? A lot. Because, generally, they don’t.
Much of the mistreatment of trees occurs because people don’t realize where the roots are located (or they do and think it doesn’t matter). Generally, most of a tree’s roots grow within the top foot or so of soil and take a horizontal path that extends well outside the tree’s canopy edge (the drip line). Obstructions, elevation changes, and other factors can influence this growth pattern, but knowing the natural character of root growth helps us make better decisions about what we do around trees.
Structurally, a mature tree is a wonder of engineering. Its roots and stem (trunk) must support a canopy whose network of branching and foliage sways with the breezes and confronts even more wind force during storms. No wonder people think that roots must grow deep. Rather, as James Urban explains in Up By Roots: Healthy Soils and Trees in the Built Environment, a tree is a vertical cantilever that “resists overturning by relying on the anchoring of the horizontal roots.”
Where the trunk flares at the base, it packs on extra wood to form a “bracket” that resists the stresses of weight and force. It then divides into buttress roots to distribute that loading force. The area that flares and transitions into buttress roots is called the root crown or root collar. This part of the tree is extremely important to structural stability.
Once underground, these roots taper quickly and transition to other types of roots that perform anchoring, storage, and absorbing functions. The absorbing roots hang out where there’s adequate air and moisture; that’s usually within the top twelve inches of soil. They are responsible for absorbing water and essential elements that the tree needs.
Picture a wine glass on a dinner plate. This classic model represents root growth of a tree allowed to grow in an open space. The glass’s bowl is the tree canopy, its base the root crown, and the dinner plate the root system. Keep it in mind, because the root crown and root system will come up over and over again as we explore other issues that impact proper tree care.
So does a great root system just happen? Nope. But before we look at how to encourage one from the start, in the next article, we’ll get better acquainted with the root crown. Then planting issues will make more sense.