The Importance of Tree Root Structure

Whether you’re dealing with a pansy or a ponderosa, everyone understands that healthy roots are crucial for a healthy plant. But nowhere is a healthy root system more important than in trees, the highest value and most long-lived asset in any landscape. Making good species choices for above ground AND below ground conditions is the first step in designing a landscape that will thrive into maturity.

We have all seen the potential damage done by tree roots as they crack sidewalks, clog sewer lines, and penetrate foundation walls. An unfortunate consequence of this destruction is that mature trees are often removed to salvage the surrounding hardscape or infrastructure. This can be avoided with a better understanding of structure and behavior of tree roots.

One Size Doesn’t Fit All

One of the first misconceptions is that each particular species of trees will have a predictable root structure. While this may be generally true in ideal growing conditions, huge variations occur based on underground obstacles, soil quality, and localized 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 typically continue lateral root growth above that layer of compaction. This is very typical in urban situations where compaction and poor drainage is a common issue. Trees will root to similarly shallow depths if placed above rocky shelves or perched/high water tables. However, the roots of  same species may grow to great depths in sandy soils.

Our Roots Don’t Run THAT Deep

A second misconception is related to the overall size and depth of the root structure for a healthy tree. Tree protection requirements in most jurisdictions require only protection of the tree root structure to the dripline but 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

The size of the root infrastructure is affected by the tree species, but more significantly impacted by soil conditions and access to resources. If a tree species that would normally grow a tap root but runs into heavily compacted soil, it will simply continue the lateral growth out instead of down. When designing for trees in any conditions, think about your ‘container’ below ground. Where are the barriers at and below the surface, and what might the roots do once they encounter those barriers?

https://www.trees.org.uk/Trees.org.uk/files/61/6181f2b7-e35d-4075-832f-5e230d16aa9e.pdf

Rascally Roots

Certain species of trees are traditionally considered a ‘no-go’ when designing in constrained spaces such as urban areas, small backyards, and near sidewalks and utilities. These species are off limits for different reasons. For example ficus, some poplars, and silver maples, have aggressive flat (meaning lateral and shallow) primary root structures. Trees of this type are known for creating knobs in lawns and heaving adjacent paved surfaces. Other trees such as willows, American elms, and robinia are generally off limits because of their love of water and their extremely extensive root systems. Trees of this type are often responsible for invading leaky water or sewer pipes, or sending up problematic shoots a great distance from the mother tree.

 

Problematic roots.

Stop Problems Before they Start

Solving for problem roots is difficult and frequently results in significant damage to mature trees. It is better to avoid problem roots altogether. The best way to do this, no matter the tree type, is to prepare and maintain a fantastic habitat for the root system so it doesn’t need to travel far to provide everything the tree needs.  If possible, minimize compaction and provide sufficient water and nutrients within an area approximately 2 – 4 times larger than the tree’s dripline. And remember, anything you apply to the ground in this area, including herbicides. can have an immediate impact on the tree. If you are planting small trees take into account the mature width of the tree when planning for its root system. Try to keep tree trunks at least 4′ off of pavements or obstacles to allow for root spread.

When this is not possible (for example if you are working in urban or constrained areas) structural soil, Silva Cells, etc. are all great tools in constructing the correct ‘container’ for the tree’s root structure. These types of systems prevent compaction which can encourage roots to grow at the surface causing heaved pavements, etc. Even if these elements are not required in urban settings where you work, consider doing some research and incorporating them anyway. This can take the longevity of an urban street tree from 5-8 years to 30+ years!

If there is an area you do not wish tree roots to invade, consider including a vertical root barrier. But be aware that if a tree’s root develop too lopsided the tree is more prone to blowing over. Lastly, make sure the root zone is protected with an appropriate depth of mulch or vegetative cover to maintain soil temperature and prevent drying. With a few simple precautions your trees AND the surrounding hardscape can all enjoy a long happy life!

 

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

 

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