Trees and tree roots seem to cause a lot of confusion
By far a common misconception or myth is: Tree roots only spread out underground to the dripline or the edge of the tree canopy.
Tree roots will grow more than three times the distance to the dripline, and some trees can easily grow to 10 times wider than the canopy of the tree. The trees we often call bad trees are due to the root system growing very wide. Mulberries, cottonwood and elm trees fall into this category.
It has been recommended for years not to plant these trees no closer than 200 feet from a septic tank or leach field. This means the roots from one of these trees can easily cover over 3.5 acres of ground.
I had a cottonwood tree that started growing from a root in my backyard. The nearest cottonwood tree was three houses down the street and in the next row of houses behind mine.
In most trees the main roots grow only about 5 to 6 feet deep into the soil. The soil usually prevents roots from going very deep into the soil, due to how hard the soil is and less oxygen in the soil. Even the largest redwood tree grows only a few feet into the soil until they are stopped by the bedrock. To hold up these large trees, the roots grow between boulders and wedge themselves into the ground.
One tree is known for its deep roots is the salt cedar or tamarisk. They have been found to have roots growing to over 200 feet deep.
Also, while not a tree, alfalfa is considered one of the deepest-rooting plants; they can go down to over 400 feet. The pioneers started coming to the Antelope Valley or western United States looking for a native plant called ephedra, commonly called Mormon tea. This native plant has very deep roots, which usually signaled a good place to dig a water well.
Cottonwood, elms and mulberry trees are drought-tolerant. I see them growing in the desert or I don’t water mine and it is doing just fine.
The aggressive root system can find water and utilize it. The tree in someone’s yard that never gets water and is growing just fine is because all the neighbors are watering the tree.
Just think of that cottonwood tree that grew in my yard. If it grew from the cottonwood three houses down the street, it was also being watered from the three houses the other way. Then the six homes behind all of these homes and probably six to 10 houses on the other side of the streets. All of these homes were watering that one tree.
The cottonwood trees alone in the desert are the one or two remaining from the thousands planted as windbreaks for farming. These fast-growing trees are very thirsty and use a lot of water to grow in the Antelope Valley.
Water your tree at the trunk or base of the tree.
If you look up into the tree, you will see large branches coming off the trunk. These branches do not have any leaves. They divide into smaller and smaller branches that finally have leaves. The leaves collect the sunlight and make sugar. The branches are just plumbing to transport the sugar, water and nutrients through the plant.
The roots closest to the truck are the same; they are just plumbing and do not absorb water, just transport sugar and water. At the ends of all the roots is a small region of cells that absorbs most of the water the tree needs. Also, what you see plant tissue that is growing above the ground, trunk, branches and leaves would be the amount of tissue growing below ground.
Water next to the trunk is hard for the tree to absorb and often causes diseases that can kill it. This is why most trees planted in parking lots never survive. They just have a small area that they grow in, and the water is flooded around the trunk. The water sits in the soil, depleting the oxygen, and the tree roots rot and die.
Some trees are better for growing in lawns.
There is no such thing as a good lawn tree or a tree that will not make surface roots in your lawn. Even in nature, most trees are shallow-rooted, but when you plant them in a lawn, it is impossible to prevent surface rooting.
When you are growing a lawn, you water it to only 6 inches deep and you are fertilizing your lawn frequently. Both factors cause the tree roots to grow closer to the surface, and as the roots grow larger in diameter, they pop out of the ground.
If you head to the forest, be sure to notice at all the roots coming out of the ground. If you are heading to a tropical area, there are even more roots at the surface, due to all the moisture they are looking for in a region with more oxygen. So they grow close to the surface, and as the root grows bigger in diameter, it pops up above the soil surface.