How Deep Does Soil Have to Be for a Medium-Sized Tree?

The Root Zone

• Although when you look at a tall tree you might imagine roots growing deep into the ground, this is not the case. Most roots grow in the top 18 to 24 inches of soil, where water and nutrients are most accessible. And typically, regardless of tree size, more than half of a tree’s roots are found in the top 6 inches. Instead of growing deep, roots tend to extend sideways to make the most of this surface zone. At most, tree roots will grow downward to a depth of 3 to 7 feet.

Effect of Tree Size on Roots

• Most trees, especially in urban areas, do not develop taproots, which grow straight down beneath the trunk. Even large trees often don’t grow deeper than 3 feet. Because of this, there is no hard-and-fast rule about how deeply a medium-sized tree will grow its roots. Depending on the drainage, the tree type, and whether the setting is urban or rural, the tree may grow more deeply or more shallowly. The surface area required by a tree’s roots is more easily calculated than its depth: most trees' roots need two to four times the amount of space occupied by the tree’s crown.

Healthy Roots

• Because tree roots extend laterally over such a wide distance, it is more important to provide a wide unobstructed area of topsoil in which feeder roots can grow than it is to provide very deep soil. Generally, if your soil is several feet deep and drains relatively well, most trees will do well their within their U.S. Department of Agriculture hardiness-zone ranges, given proper care. Because disturbing or damaging a tree’s roots can affect the health of its branches, avoid upsetting major areas of the tree’s root zone.

Site Selection

• Many trees will grow happily in urban settings, assuming they are planted where their roots can extend over wide distances. Before you plant a tree next to a sidewalk, for instance, make sure the tree tolerates such plantings. Many will, because their feeder roots can grow underneath the concrete and the street, but some will not. Also choose a planting site where trees will not heavily compete with other trees for root space, light or water.