Contrary to popular belief most trees send the majority of their roots out to the sides not deep into the ground. We simply assume when we look at a towering oak that its roots must run hundreds of meters into the ground, but this isn’t true. The majority of its roots are only 46 cm (18 inches) below the surface, keeping them close to the nutritious top soil and where they can take advantage of the slightest moisture.
If you have ever seen a tree that has uprooted and fallen over in the forest you will have noticed how shallow its root system actually is. You can appreciate how important it is to think about the area that surrounds the tree and not just what is directly below the trunk.
The aforementioned oak will send its tap root 1.2 meters (four feet) into the ground and then spin out lateral roots to widths of 27 meters (90 feet)!
In nature there are always exceptions and when it comes to trees with deep roots that exception is the Boscia albitrunca or African shepherds tree. A specimen measured in Kalahari in 1974 had sent a root 68 meters (223 feet) into the ground!