Understanding the Importance of Tree Root Structure

Perhaps the importance of tree root structure seems obvious but oftentimes our misconceptions related to the root structure can compromise both the health of our trees and the health and integrity of the landscapes around these trees. Making good species choices above ground AND below ground is the first step in having trees that survive and thrive in the landscape. We have all seen the potential damage done by tree roots as they crack sidewalks, clog sewer lines, and penetrate foundation walls. The unfortunate consequence has been that trees are becoming a consumable ingredient in the landscape, living 20 years at best and then being replaced. However, with knowledge of species choice and a great understanding of what root systems need, we can reverse this trend.

One of the first misconceptions is that particular species of trees will have predictable root structures and while this may be generally true in ideal growing conditions, huge variations occur based upon availability of nutrients, oxygen and water.

‘Root growth is essentially opportunistic in its timing and its orientation. It takes place whenever and wherever the environment provides the water, oxygen, minerals, support, and warmth necessary for growth.’1

If compacted soil exists below the ground then the tree will continue root growth above that layer of compaction. This is very typical in urban situations where compaction and poor drainage is a serious issue. Similarly, with a perched or high water table. However, the root structure of trees growing in sandy soils can grow to great depths.

A second misconception is related to the overall size of the required root structure for a healthy tree. Tree protection requirements in most jurisdictions require only protection of the tree root structure to the dripline but as you can see below a healthy root system may extend significantly larger than that!

‘The major roots and their primary branches are woody and perennial, usually with annual growth rings, and constitute the framework of a tree’s root system. The general direction of the framework system of roots is radial and horizontal. In typical clay-loam soils, these roots are usually located less than 20 to 30 centimeters (8 to 12 in) below the surface and grow outward far beyond the branch tips of the tree. This system of framework roots, often called “transport” roots, frequently extends to encompass a roughly circular area four to seven times the area delineated by an imaginary downward projection of the branch tips (the so-called drip line).’1

Again, size of a root structure will be depending on the tree species but more importantly soil conditions and access to resources. If a tree species would normally grow a tap root but runs into heavily compacted soil, it will simply continue the growth out instead of down. When designing for trees in constrained spaces it is important to think about your ‘container’ below ground. Where are the barriers on the container and what will the tree roots do once they encounter those barriers?

Certain species of trees are just considered a ‘no-go’ when designing in constrained spaces such as urban plantings, small backyards, and near sidewalks and utilities. These species are off limits for different reasons. For example, populous, ficus, Silver Maple, and poplar have aggressive flat (meaning they are growing mostly flat and shallow to the ground) primary root structures. Whereas, other populus, salix, American Elm, bamboos and robinia are generally off limits simply because of their love of water and how prolific their root structure is. Remember that roots are going to grow where they are welcome! Oftentimes the issue with sewer lines, water lines, or ditches is that there is a small break in the line already and the nearby roots simply make a beeline for the moisture!

The best defense to damaging tree root structures (in addition to making great tree choices in the first place) is to define and maintain a fantastic ecosystem for the root structure so it need not go far for everything it needs. Because the majority of the root structure is taking up nutrients and water in the top 6-12” of soil, it is imperative to remember that anything done within the dripline and say 2-4 times greater than that is where the tree root structure will get everything it needs (or doesn’t). Herbicide applied in this area can have an immediate effect on a tree.  While this may be up for debate, fertilizer spikes, etc. are unnecessary for most trees and fertilizer, mycorrhizae amendments, water, etc. can all be applied to the surface under the tree.

Using structural soil, Silva Cells, root barrier, etc. are all great tools in constructing the correct ‘container’ for the tree’s root structure. If these are not required in urban settings where you work, consider doing some research and incorporating. This can take the longevity of an urban street tree from 5-8 years to 30+ years!

Lastly, consider covering larger areas of the root system with mulch when there is no snow on the ground but significant cold snaps are anticipated. These large areas of mulch can help with moisture retention and preventing roots from freezing during winters with vast temperature swings.

We’ve written a separate blog about planting under evergreens but really ALL trees will aggressively compete for the resources within the root structure of that tree. Being conscious of the additional resources needed both for the tree and for the plants under it is critically important. If the tree or the plants are failing, consider that one or both need additional resources – light, water, nutrients and oxygen.

  1. http://arnoldia.arboretum.harvard.edu/pdf/articles/1989-49-4-tree-roots-facts-and-fallacies.pdf Thomas O. Perry


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