Here’s the understatement of all understatements: tree roots are important to a tree’s capacity to survive. When one considers all of the benefits that tree branches and leaves provide (photosynthesis, carbon dioxide reduction, oxygen generation, aesthetic appeal, shade for outside activity, cooling for homes and buildings, etc.), none would be possible without a supporting root system.
Functions of roots
In no particular order of importance, the functions of the root system are threefold. First, the stability of the tree and nearby trees, especially in forests or multi-tree landscapes. In short, trees help other trees stand up. This is most visible on the edge of native stands, where the outside trees are more likely to fall and tend to grow shorter and have thinner canopies. In the first several years of growth, most trees develop tap or sinker roots to provide initial stabilization. Over time, these give way to lateral roots that extend horizontally akin to the spokes of a bicycle wheel laid sideways. These provide support for the tree and resistance to being blown over in windstorms.
The first root function that comes to mind is the absorption of water and nutrients, especially in the minds of non-plant people. Absorption occurs primarily through root hairs, fine finger-like structures that grow between soil particles to extract necessary elements and water. Once brought into the root, they are transported to the stems and leaves where sugars and carbohydrates are made through the process of photosynthesis.
Thirdly, and perhaps the most overlooked function, is erosion control. The best defense against the undesired movement of soil, pesticides and applied fertilizers on slopes is a thick, dense root system to absorb or retain these materials. Actually, turfgrasses with rhizomatous root systems, such as Kentucky bluegrass, bermudagrass and zoysiagrass, are the best erosion-reduction plants, but tree root systems are not far behind. For this reason, buffer strips containing grasses and trees/shrubs are often placed alongside crop fields where erosion is likely to occur.
Half the plant is underground
One of the first learning experiences that incoming graduate students in forestry encounter is a close-up examination of tree root systems. This is often accomplished by trekking to the nearest forest, where their professor hands them a shovel and instructs them to start digging horizontally until they find the end of the tree roots. This usually ends up with the student far, far away from the trunk, huffing and puffing, muttering something like “I wonder if there is time to change my major.” The point of the exercise is that roots in healthy environments grow far and wide.
In native stands and some older urban soils, mycorrhizal fungi help with nutrient uptake. Roots and mycorrhizae develop a symbiotic relationship in that they gain sustenance from the plant and they provide expansion of the root surface and interface with the soil particles. As a result, a larger and more robust root system develops.
Unfortunately, roots that are under the concrete/asphalt in urban environments don’t develop as well as they do in native stands. Many otherwise well-suited tree species perform poorly in shopping malls, near professional buildings and city centers. One of the most restrictive sites for tree root development is the narrow strip. Examples of these include the “hell strip” or area between the sidewalk and the street in suburban neighborhoods or the “tree pit,” commonly a 4-by-4-foot space in the midst of concrete.
When communicating about roots with your customers, the paradigm of the roots as extending two to three times the spread of the tree width is helpful. Another way is to think of half of the tree being above ground and half underground, taking into account the height, width and canopy.
Let’s take a peek
With regards to tree roots, the naturally inquisitive mind is likely to ponder how deep they grow, how far they spread, and where they are located. Considering that half of the tree is underground, the common notion to dispel is “out of sight, out of mind.” Perhaps the best way to do this is to make the invisible visible. How is this done? Just like many other investigative actions, there are simple and inexpensive ways as well as pricey and quite involved methods.
The first approach is one to take after Mother Nature has acted — to look at trees blown over in windstorms. Casualties of severe weather often serve as learning tools in that the trunk base and upper roots become exposed as a result of the severe weather event. Looking closely and noting where roots are attached to the rest of the tree, their diameter and their direction of growth is highly instructive. Next, making the same observations during soil excavation for construction purposes such as utility or sprinkler system trenching can teach us about their growth, location and orientation.
More involved approaches tend to yield more information. One such method is the use of an air spade, a device that blasts compressed air at tree roots to remove adjacent soil particles. The air spade can be helpful in that it uncovers and exposes most of the roots in the areas where it’s used. Being present when an air spade is exposing roots allows the observer to gain insights into where roots grow easily, where they grow slowly, and where they grow around obstructions in their way.
The most insightful tool to learn about tree root growth is a root observation chamber, such as the one at the U.S. Forest Service facility in Houghton, Mich. Called a rhizotron, it contains clear panels that allow for viewing of the growth of roots in a natural setting. First used to observe the growth of turfgrass root systems, observation chambers such as these provide the viewer with a noninvasive look into where and how extensively roots grow through the soil.
Protect, protect, protect
The first line of defense against damage to root systems is to prevent it from happening in the first place. One of the best methods to inform your clients of this technique is to draw lines on the ground near the approximate spread of the roots on their properties. Then, take simple steps to keep lawn mowers and other damaging machines away from the tree trunk and upper soil layers. Proper placement of mulch will go a long way towards the goal of protecting the tree roots from damage, while creating a more suitable growing medium as decomposition occurs over time.
Potential root system stressors
Especially in the urban environment, the number of potential root system stressors is high. Compaction of the soil in which the roots endeavor to grow is a significant stressor. In many situations, an air spade will reveal the extent of soil compaction. Another technique is to simply monitor the extent of water runoff after a rainfall event or irrigation cycle. If water is slow to infiltrate, it’s likely the soil is compacted.
Root removal is a highly stressful action. I often get the question, “Hey, these tree roots are lifting my sidewalk, can’t I just cut some of them out … won’t the tree be fine afterwards? After all, it’s a big tree.” Of course, the extent of the removal is a factor to be considered, but any root loss is problematic.
The lack of separation of turf and trees is not only a landscape design mistake in that it minimizes the valuable aesthetic effects of mass/void, it’s also a tree stressor in that it tends to cause trees to be overfertilized and overwatered due to the fact that the turf and trees are sharing the same rootzone. When not separated, tree roots compete with turf for nutrients, water and oxygen.
Perhaps the largest stressor occurs at planting time. Aggressive soil amendment and failure to correct girdling roots are mistakes that trees often never recover from. New planting stock often contains roots that are tangled and constricted in the ball or pot. As the tree grows, these roots enlarge in length and diameter, becoming girdled and restricting the flow of water and nutrients. Using peat moss or compost to amend backfill soils only makes this potential worse, as the new roots will preferentially grow in the amended soils, usually in a circular fashion.
Now what? Keeping the above information in mind, advise your clients of the following:
- where the roots are;
- how far they spread and how deep they grow;
- keep turf separate from trees;
- use natural wood chip mulch to modify the upper layers of soil and replicate Mother Nature;
- fertilize according to soil test reports and the potential absence of adequate soil and room for rootzone development;
- keep construction equipment away from roots; and
- how to plant trees correctly.
Planting trees correctly involves extending roots outward in a planting area (not a hole), and avoiding amending backfill soils as a way to get the tree off to a good start. Leaving them with the instructions to keep the new roots moist, not soggy or dry, and to water to the depth of the roots will go a long way toward future good tree health.
Editor’s note: This story was originally published in October 2012 and has been updated.