How Do Tree Roots Grow?

To learn about how tree roots grow, stay healthy, and where they exist we’re going to reference a few articles that you can find at these locations if you desire to learn more:


Different Types of Roots

We’re going to start by defining the different types of roots and what purpose they serve to the tree. There are taproots, structural (or lateral) roots, sinker roots, and fine roots.

Taproot

In our current water saving culture, we flock towards trees with a “deep taproot” in order to decrease irrigation frequency and quantity. While taproots can extend to astounding lengths in nature, they rarely do in the urban setting. Martin Dobson explains in Tree Root Systems:

“Development of the taproot then declines with the result that only a small proportion of trees have a sizeable taproot at maturity. In fact, it is hard to distinguish a taproot at all in many mature trees, as injury to the juvenile taproot tip often occurs, for example, by the browsing of soil fauna, root rot, failure to penetrate hard or compact soil layers, or for nursery stock, by undercutting/transplanting.”

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

Compaction is the major contributor to why we don’t see long taproots in urban settings.

But what about oaks you say? Don’t they have really deep taproots? Dobson continues:

“Species often thought of as ‘taprooted’, such as oak, pine, and fir, appear to have a stronger inherent tendency to retain a distinct taproot, than species such as poplar, willow, and spruce, but frequently the taproot does not persist even in these species. Intact taproots are usually largest just beneath the trunk and taper until they reach a depth of 0.5-1m, where they often divide into several smaller, but nevertheless downwardly growing roots.”

https://www.trees.org.uk/Trees.org.uk/files/61/6181f2b7-e35d-4075-832f-5e230d16aa9e.pdf
tree root system depiction
Courtesy of Tree Root Systems by Martin Dobson, Arboricultural Advisory and Information Service

There are also something called “Sinker Roots” which are small vertical roots that grow down from lateral structural roots. These are typically about 2cm in diameter, grow downwards, and divide into non-woody roots. These help to anchor the tree in addition to the large structural roots that grow laterally.

Large Structural Roots

Large structural roots are usually the roots responsible for moving the precious hardscapes of many homeowners. These roots grow large and can protrude through the surface of the soil. They are responsible for anchoring the tree. There are between 4-11 of these lateral roots and they can grow as thick as 12″.

The size of these lateral roots taper off quickly as they grow away from the tree. Dobson writes:

“They taper rapidly until at 2-3m (6-9′) distance they are usually only 2-5cm (0.787″-2″) in diameter”

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

Knowing this can help us to keep these large roots from damaging hardscapes such as driveways or exterior walls by planting the trees further than 10′ from them. We can encourage larger structural roots to grow deeper in the soil by mulching the soil with 2-4″ of wood chips (or other organic materials) to help reverse compaction of the soil and provide a more fertile soil.

Fine “Feeder” Roots

Fine roots are primarily located within the top 12″ of the soil. This is where the soil is most aerated. These roots are responsible for the uptake of water and nutrients. When the soil surface is damaged or disturbed, the fine roots typically experience the most damage. These roots are also the primary beneficiary of mulch. Mulch allows the soil to hold moisture longer and increases the microorganisms in the soil surface which in turn aid the roots in disease prevention, water retention, nutrient uptake, and drought resistance.


Tree Root Health

When it comes to the health of our trees the health of the roots is of most importance. Frequently we complain about beetle borers, diseases, and other problems that arise in our trees. These are frequently secondary problems with the real problem being stress on the tree from the lack of health in the root system. Below we’ll discuss the best ways to take care of your tree’s root system.

Root Flare

Having the root flare of your tree uncovered is the easiest thing you can do to improve the health of your tree. This requires uncovering the soil until you see your tree base widen.

For more information on how to do expose your root flare visit this post: https://www.natesnursery.com/exposing-the-root-flare-of-your-tree/

Compaction of Soil

Soil compaction is the reason that tree roots don’t grow deeply in urban areas. Heavy machinery from construction of concrete, homes, roads, etc. and dragging of yards for weeds with machinery are two of the most common practices that compact urban soil in the high desert. Doing these things while the soil is wet is additionally damaging as soil is more prone to being compacted when wet. Below Larry Morris shared this table about the difference in compaction of humans vs. cars vs. machines:

The top 14 inches that are being compacted are the area where a significant amount of tree roots reside. When the soil is compacted tree roots grow closer to the surface of the soil. This more shallow root system will have a lower tolerance of drought, higher potential to cause damage to hardscapes, and be more easily damaged by surface soil disturbance (dragging, digging, weed killer, etc.).

How To Reverse Compaction

To combat compaction there are a few options, but many of them are not feasible over a large area or do not address deep soil compaction. The options for treatment prior to planting include:

  • Using a backhoe (I know, the irony) to break up deeper portions of the soil
  • Rototilling (only addresses the top few inches of compaction)
  • Subsoiling
  • Radial trenching when planting helps trees establish better

Addressing compaction when there is already trees and plants present can be done by:

  • Using a high powered air stream to break up surface soil (doesn’t damage roots)
  • Vertical mulching which involves drilling holes in the soil with consistent spacing
  • Mulching with an organic mulch

While there are several options to reverse soil compaction my favorite is mulching with wood chips. There are so many benefits of mulching with wood chips that it should be done regardless of compaction.

  • It keeps weeds down
  • Helps retain moisture and lower water usage
  • Increases water infiltration
  • Prevents erosion
  • Provides nutrients to the plant/tree
  • Prevent splashing which keeps soil-borne diseases from spreading
  • Decreases watering frequency
  • Helps roots grow deeper, which enables better drought tolerance
  • Increases organic matter content in the soil
  • Reverses compaction (albeit not immediately)
  • Encourages mycorrhizae and other beneficial bacteria and fungi
  • They can be free from tree trimming companies

To learn more about mulching you can view this article by Linda Chalker-Scott, Ph.D. – https://s3.wp.wsu.edu/uploads/sites/403/2015/03/wood-chips.pdf

Proper Watering

Watering of recently planted trees can be tricky. The rootball needs to stay moist without drowning the tree, but native soil that doesn’t drain well can cause problems with that. One of the best ways to make watering less of an exact science is, yep you guessed it, mulching. The benefits of increasing water infiltration, lowering water amount and frequency, preventing evaporation of water from the soil from the sun, and keeping weeds down all directly lower stress of a newly planted tree.

tree root system depiction

Watering a mature tree is not the same as watering a tree you just planted. Not only does it need to be watered significantly less than a newly planted tree, but watering at the base of a mature tree is like drinking an tablespoon of water when you’re thirsty. As the photo above illustrates, trees roots extend much further than the ‘drip line’, or canopy width, and could benefit from water as well. These far out areas are where fine roots that absorb water are produced. A solid soaking of the area considered the drip line, which is where the canopy extends to, and a few feet out would be the proper way to water a mature tree. This can be done with a soaker hose, leaving your hose trickling for an extended period of time, or by hand watering the area.

It bears repeating that mulching the area with wood chips would increase the effectiveness of watering, decrease the frequency, and hold the water in the soil longer.

Soil Disturbance

Seeing how tree roots are commonly found in the upper 4″ of compacted soils shows us how harmful disturbing the soil can be. The common practice of dragging a yard to remove weeds can easily destroy at roots within the top 4″ of soil and potentially deeper.

Martin Dobson states:

“Tree roots may extend radially a distance equivalent to at least the height of the tree and are located primarily in the upper 60cm of soil. The main structural roots are usually found in the upper 30cm, and taper substantially within about 3m of the trunk. The vast majority of fine absorbing roots are even closer to the soil surface. Thus, any soil disturbance within the rooting zone will damage tree roots and should be avoided.”

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

Trenching can be severely damaging to trees if not properly assessed before proceeding. Consider a Fruitless Mulberry with a canopy width of 45′ (a mature tree). This can take up a large portion of your yard, so you may not have a lot of room to trench around it. You can see below that digging a trench 12′ away from the tree can sever 30% or more of the tree’s root system.

If cutting a root more than 4″ thick, you can start to damage the tree’s ability to stabilize itself in the ground. This can result in a blown over tree if you carelessly cut or trench too close to a tree. If you require a trench close to your tree, consult an arborist on ways you can help nurture the tree to recover or stabilize it.

Soil Fertility

People tend to think that desert soil is terrible and has a lack of nutrients and that is why things don’t grow here. Desert soil typically has enough nutrients, but construction and grading destroys the microbiology needed to help the plants absorb it efficiently. When you have a good presence of beneficial organisms you decrease the need for fertilization significantly and increase the soil’s fertility. Organic mulch does so much to increase the fertility of your soil. As the mulch breaks down, beneficial organisms start to occupy the soil and improve plant health. Some of these organisms include:

  • Bacteria
  • Fungi
  • Protozoa
  • Earthworms
  • Nematodes
  • Arthropods

Leaving the soil bare and regularly disturbing the soil drastically reduces the presence of these beneficial organisms which leads to more frequent maintenance in the forms of fertilization, fungicides, insecticides, herbicides (weed control), watering, and plant replacement costs.


Misconceptions About Tree’s Roots

Roots Chase My Water and Septic Tank

This is a misconception that prevents many homeowners from planting trees or choosing certain species. The reality is that roots are opportunistic. This means they grow where the soil is least compacted, where there is moist soil, and the path of least resistance. When trees do get in water pipes it means there was an issue before that happened. If a PVC pipe cracked or started leaking, then a tree will take advantage of that constant moisture. This can be exacerbated by stress. If a tree is stressed from drought, it will be seeking water and look for available water in the soil.

Some tree species are water loving and thrive in wet compacted soil. Willow, Maple, Poplars, Mulberry, and Elms all fit this category. This makes it important to plant these trees far enough away from the septic tank that it will not be a problem. When doing so, keep in mind that the size of the tree and the watering regimen can drastically change how far it needs to be planted. If you are planting a Fruitless Mulberry 30′ from your septic tank and were hoping to not have to water it at all after 5-10 years, I’d suggest moving it farther away as the roots will be extending farther than 30′ looking for water. On the other hand, if you’re planting a Maple tree in a lawn, you have a much better chance that you have no issues at 30′ away from the septic tank.

Deep Rooted Trees

Jim Urban, FASLA a noted tree and soil expert said of deep rooted trees: “Trees are genetically capable of growing deep roots, but root architecture is strongly influenced by soil and climate conditions.”

Any tree can become a deep rooted tree if the soil condition allows, the problem is that few urban sites have soil that allow any tree to have a deep root system. Trees that are referred to as ‘deep rooted’ trees have roots genetically different that can withstand difficult soil conditions like compaction and lack of oxygen. Some trees that fit this description are Palo Verde, Chinese Pistache, Ash, Poplus, Sweet Gum, Elm, some Pines, and various Oak trees.

Roots Will Damage My Foundation

Christopher Overbeke goes into details about roots and their potential to damage property and foundations. He makes two distinct points in his article:

“Roots encountering a solid object will divert and follow the course of least resistance, thereby causing no damage. The continuous radial expansion of trunks and structural roots in contact with a structure and in a restricted space, however, may exert sufficient pressure to displace heavy structures.”

https://link.springer.com/article/10.1057/jba.2008.6

This can be easily avoided by planting a tree far enough away that the radially expanding structural roots do not come in contact with your foundation. We learned above that these roots are typically 2″ in diameter or less once you get 9′ or further from the trunk.

His next point talks to the indirect damage that trees can cause.

Indirect damage is caused by the influence of a tree on soil moisture levels of a substrate prone to shrinkage and expansion. The ground is dehydrated through the transpiration of leaves abstracting moisture from the ground and…the ground contracts and settlement occurs. Usually winter rainfall results in full recovery so that the process is seasonal. Conversely, removal of a tree results in long-term recovery and expansion of the ground.”

https://link.springer.com/article/10.1057/jba.2008.6

This can be a large problem in soils high in clay due to it having a high plasticity index. Plasticity index measures how likely a soil is to be molded or shaped by external phenomena. However, the plasticity index is significantly lower in sandy soil. Below is a chart of the potential of a soil to shrink based on its plasticity index.

https://link.springer.com/article/10.1057/jba.2008.6

“Soils are highly relevant since vegetation cannot cause indirect damage to a property unless it bears upon a substrate that has potential for volumetric change. A root system abstracting soil moisture is likely to have rather less impact on the volume of a founding clay substrate with a P.I. of 15 per cent than that of a clay with a P.I. of 60 per cent.”

https://link.springer.com/article/10.1057/jba.2008.6

The question then becomes “What is the plasticity index of my soil? Rather than go into another 1,000 words explaining the process of how it is performed, I’ll give you links to find out what it is based on tests that have already been performed through University of California Davis and the United States Department of Agriculture.

Using the chart below you can find the listed plasticity index of your specific location.

plasticity index of soils in high desert california mojave desert

You can find your soil area’s number by locating it on this map (visible below): https://casoilresource.lawr.ucdavis.edu/gmap/ or the less user friendly version at https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx. Take that number and compare it to the list above. You can also learn more about your area’s general soil properties through this map: https://casoilresource.lawr.ucdavis.edu/soil-properties/

Of the area of interest (AOI) below (limited by acreage, otherwise I would’ve done the whole high desert) only 2.1% of the area had a plasticity index over 10. This means that it is unlikely that you will experience damage from swelling and shrinking of the soil. If you find you have a number not on the list or need help applying this information you can send an email to [email protected] for help.

As I encounter other topic fitting to this massive page of information I will continue to add to this page. Thanks for making it this far, it’ll help trees everywhere!

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