Roots are always underfoot, whether it’s a forest or prairie, your yard, or a local park; these all-important plan structures fill the upper layers of soil and quietly do their work to support the plant world. Since they are often “out of sight, out of mind,” there are many common misconceptions about these vital plant parts.
Roots and leaves are connected
Tree roots have always fascinated me when considering the sheer number needed to support a full canopy of tree leaves. Roots are directly connected to leaves by conductive tissue that transports water and soil nutrients upward. Many describe this conductive tissue like a wick that draws moisture upward to exit through minute pores in the leaf epidermis, called stomata.
These stomata open and close depending on the tree’s needs, and the moisture they release is used to cool leaf surfaces and regulate temperature for optimal leaf function. On a hot day, stomata are wide open, and the wicking effect is constant, drawing soil moisture upward continually.
When it’s a hot and windy day, air blowing across the leaves intensifies the wicking effect further. Under these conditions, an enormous amount of moisture is drawn out of the soil, and a staggering amount of root biomass is required to support the moisture needed to cool leaves. That doesn’t account for the water needed to fuel other vital leaf functions, such as photosynthesis.
How big are tree root systems?
While many figures of roots may show a mirror image of tree branches above ground and roots extending deep below ground in symmetry, that is far from reality. The large majority of tree roots are confined to the upper soil layers where oxygen, water and nutrients are readily available. Over 90% of tree root biomass occurs in the upper 18 inches of soil, with almost 50% of that biomass concentrated into the upper 6 inches of soil. So, the mirror image of branches above ground and roots below is entirely incorrect. Tree root systems are very shallow and wide.
The large width of tree root systems cannot be understated and has been estimated to be up to 3 times the canopy spread, with some studies finding even larger root systems based on soil conditions. Heavy clay soils tend to support a smaller root structure, often only 0.5 times the canopy width. In contrast, looser sandy soils have been noted to support root systems up to 5 times the canopy spread.
These extensive root systems are often hard to imagine in an urban environment, where impervious structures, such as sidewalks, drives, and building footprints, restrict their available space. In addition, urban soils vary widely, even in the space as small as one front yard. So, tree roots have a tough job finding all the moisture and nutrients needed to support a healthy tree.
While tree root systems are primarily wide, some roots extend to great depths, with tap roots being mentioned most often in this context. If you aren’t familiar with this term, tap roots extend directly downward in alignment with the trunk to provide stability when a tree is young. However, not all tree species have a tap root at maturity, and the reality is that the majority do not. It is true that every tree has a tap root as a seedling, but it is simply not advantageous to have them at maturity because lateral roots usually provide adequate support, and resources such as oxygen and nutrients become more limited deeper in the soil.
Oaks, hickories, and other nut-producing species are generally deciduous trees that have tap root structures later in life. Pine species comprise most of the evergreens which support a taproot as mature trees. Most other trees develop a lateral root structure from their expanding roots as they gain biomass with age, but, in all cases, taproots will not develop if soils are restrictive or overly disturbed from past human use.
In the case of transplanted trees, which is really any tree that you purchase at a nursery in a pot or balled and burlapped, the tap root is destroyed in the production process. So, most trees in the urban environment lack a taproot from the transplanting process because their species don’t support taproots later in life or simply because soil conditions prevent it.
Roots concentrate at the dripline
Another misconception about tree root distribution is the common assumption that the area under the tree canopy houses most roots. In reality, most of the roots are concentrated around the dripline, which is defined as the area located around the outer circumference of the tree canopy, or the area just outside the branch spread.
This alignment makes a lot of sense because the tree canopy intercepts a large amount of annual rainfall. Think about times you’ve run under a tree during a rainstorm for shelter. It's bone dry under a tree during the initial few minutes of rainfall. As the leaves and stems slowly become saturated, some rain drips through the canopy, but trees with a dense canopy structure can intercept an amazing amount of rainfall, leaving the roots beneath with little moisture.
How can I protect tree roots?
There are certainly some practical applications that tree root knowledge can inform. When mulching or watering trees, the most effective location to add or preserve moisture is at or just outside of the dripline. I often explain the optimal mulch alignment as a doughnut, with the empty space in the middle being the tree trunk and the mulch being centered around the dripline.
Construction can have heavy impacts on tree roots, and most folks consider protecting their trees somehow. While it is important to ensure that equipment, materials, or even humans don’t contact the trunk of the plant, most of the roots lie at the dripline. I often see trees fenced off in construction sites, but the fencing mostly protects the trunk and doesn’t extend out far enough to preserve roots at the dripline.
Understanding tree root distribution can really help us understand the best practices in caring for these long-lived perennial plants. After all, trees are the highest-valued perennials in the landscape, so it really pays to understand their care.