More than three-hundred-fifty feet deep and thriving in a clammy South African cavern are the roots of a fig tree native to that county. The 1923 discovery remains the world’s record for the deepest known roots. Dotting the arid regions of California, Arizona and New Mexico, velvet mesquite trees sometimes have taproots that dip into aquifers more than one-hundred-sixty feet below the surface. Standing tall along a central Texas watershed, native pecans send roots through more than three feet of soil to sip from mineral rich water delivered by the Brazos. Deep roots like these may be the picture many have in mind about most trees but they’d be wrong.
Roots are mysterious features of trees, literally out-of-sight and too often out-of-mind. Researchers have used a variety of clever methods to reveal them from carefully rinsing away soil to using ground penetrating radar. One thing they’ve concluded for certain is that most roots aren’t very deep. Instead, they form a wide plate in the top foot to eighteen inches of soil with more than half of them found in the upper six inches. An average of five to eleven buttress roots grow horizontally away from tree stems like spokes on a wagon wheel. These are usually the deepest roots of the tree. As the grow laterally they quickly taper just a few feet away from the trunk moving closer toward the surface and giving rise to secondary root branches which in turn give rise to more until terminating with almost microscopic hairs.
Many of the outer cells of small roots jut into the surrounding soil where they absorb nutrients and leak a little carbon and sugar into the surrounding soil. Nearby fungi and bacteria devour this leaky ooze and in return give the roots some of the nutrients they’ve picked up, particularly phosphorous. These microorganisms often improve the partnership by tapping directly into root cells. Small roots are not permanent; they grow and retreat depending on conditions. All of the activity contributes to a friable soil which has a rich earthy smell in this area of the root zone.
The depth and spread of roots are under genetic and environmental control with soil conditions exerting the greatest influence. Roots follow the path of least resistance. When one bumps into a boulder, compacted soil, concrete or some other sold object, it will go under or around it until it gets to the other side where it resumes its normal path. Despite stories to the contrary, roots are rarely responsible for damage to foundations although they often lift and crack shallow concrete structures such as sidewalks, driveways and patios. However, they may occasionally crack foundations of older homes which were not built to modern standards.
Roots grow where conditions meet their needs. They will not grow in soil that is low in oxygen, moisture or nutrients. In heavily watered lawns, tree roots grow just below the grass and not very far from the trunk. They become “lazy” and respond too slowly to support the tree when suddenly deprived of water. In sandy soils they grow far from the trunk but developing fewer fine roots. In clayey soils, root systems often have a somewhat restricted development. In forests, roots develop extensively in the upper soil surface where nutrient dense humic acids and decomposing organic materials are thick.
Trees can be acclimated to survive drought by gradually exposing them to deep watering cycles stretched over longer and longer periods of timer. Trained in this way, roots will grow stronger, longer and deeper. Unlike the velvet mesquite of the arid southwest, river bottom pecans of Texas and wild figs of South Africa, deep roots are rare and practically imaginary in urban landscapes. When it comes to tree health, compromised roots cause more than eighty percent of tree problems. Decline results from ditching, poor irrigation, compaction, grade changes, herbicide application, fertilization or disease. Most of these things are under our control. One could argue that trees do well in spite of what we do for them rather than because of what we do for them.