Tree root structure is often misunderstood. Roots do not grow deep underground as some mirror image of the tree’s crown. Our urban soils are generally compacted and without the necessary pore space for optimal water and oxygen infiltration. Since roots must respire, they cannot function normally without oxygen. Roots grow where conditions are best, and this is usually close to the surface where oxygen and moisture are available. Close to 90% of fine absorbing tree roots are located in the top 6 – 12 inches of soil.
Tree Root Functions
The three main function of roots are to store sugars, provide anchorage, and absorb water and essential elements. Roots do not make food nor do they absorb food from the soil. There are no feeder roots. You cannot feed a tree with fertilizer regardless of what glitzy packaging claims. Only by using the sun’s radiant energy can a tree photosynthesize sugars—or make tree food—by combining atmospheric carbon with essential elements. Fertilizers can add these essential elements.
Although roots don’t grow deeply, they will grow a great distance if conditions are favorable. Different species will exploit varying soil volumes, but as a general rule, mature tree roots will spread horizontally, at least 2 – 3 times the radius of the tree’s canopy if conditions allow.
Soil Compaction Limits Tree Roots
These basics of tree root structure have important implications for urban tree care. Because roots respire, because they need oxygen, we must take steps to minimize soil compaction. Minimizing soil compaction will allow a tree’s roots to explore greater soil volumes for water and nutrients. Roots will be able to grow deeper into the soil and provide greater anchorage for the tree. If we manage the soil skillfully, our trees will be healthier and have increased capability to withstand high winds. An important step we can take to minimize soil compaction is to mulch a wide area under the tree, 2 – 4 inches deep. Fine absorbing roots have been shown to be much denser under a properly mulched tree.
We must also be careful not to raise the soil level over tree roots. Tree roots can easily be smothered and suffocated.
Trenching Can Severely Damage Tree Roots
When we understand that overall tree root structure is more like a plate than an inverted tree crown, that roots generally grow widely but not deeply, we can recognize the problems associated with trenching. Even relatively shallow trenches dug for the installation of utilities or irrigation systems can sever and destroy vast numbers of roots, particularly if trenches are made close to tree trunks. A tree with roots destroyed this way can easily die from water deficit and can more easily be blown over.
We can prevent these problems by understanding tree root structure. If you’re having irrigation installed or construction activities near your trees, give us a call and we can work with your contractor to make sure your trees are not severely damaged. All too often we’re called to diagnosis trees that have already been irreversibly damaged. Plan ahead and provide proper care for your trees.
