There is a popular misconception that tree roots grow deep down in the soil in order to anchor the tree in the ground, much like a mirror image of the canopy. This belief  is wrong, the majority of roots are near the surface of the soil, providing a wide but shallow plate. This is in order to carry out its primary functions efficiently, which is to draw up water and nutrients, and to anchor the tree in the ground. The bulk of rain water is in that part of the soil and so are nutrients from dead matter such as rotting leaves.  The Buttress Roots are the large roots which can be seen on all sides, at the base of the main stem or trunk. There function is to provide structural stability/anchorage. The fibrous roots which can range from 1-2 mm in length and 1.0 – 0.07 mm in diameter are found just under the surface and draw up the water and nutrients.

How Far can tree Roots Spread?

– In temperate trees the roots can spread away from the trunk usually 2-3 times the radius of the canopy, or 1.5-2 times the height of a tree. For example a mature Oak tree could have roots up to 30 meters away from the trunk.

Ref -Trees Their Natural History Peter Thomas Cambridge University Press 2000