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You're Thinking About Tree Roots the Wrong Way

They Probably Don’t Look Like You Think They Do

You probably have a mistaken impression of the way tree roots look underground.

Don’t worry – it’s not your fault.

The blame probably lies with a well-meaning but misinformed elementary school teacher or scout leader.

In an effort to inspire an appreciation of trees and open your eyes to the notion that things can be unseen yet incredibly important, this teacher or scout leader told you that a tree’s roots are like a mirror-reflection of its canopy.

And many people hold on to this image forever.

The problem is, it’s wrong.

We’ll explain why and draw a more accurate picture of root systems for you, below.

A Quick Aside: What Is a Tree, Anyway?

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Before delving further into the tree-root weeds, we need to discuss some vocabulary.

Believe it or not, there is no universally agreed upon definition for tree.

This is partially because trees don’t form a natural group that arose from a common ancestor. Indeed, trees spring up in all parts of the plant family tree. Additionally, some plants can exhibit tree-like growth form in some cases, and shrub-like growth in others.

And for the average person, none of this really matters.

Call that 10-foot-tall banana plant in your yard a banana tree if you want. The world will keep on spinnin’. You could say the same for bamboo, palm trees, gigantic shrubs or vines that are thicker than small trees.

But if you’re really drilling down into the nuts and bolts of trees, it helps to have a good working definition for “tree.”

Again, there’s no “official” definition for tree, but botanists usually require a plant to exhibit some version of the following characteristics to qualify as a tree:

  • It must have a single woody stem.
  • It must be perennial, meaning that it lives for more than one season.
  • It must have leaf-supporting branches some distance from the ground.
  • It must be anchored by a root system.

This weeds out most tree-like shrubs and plants, but there’s one more characteristic that’s important to note: True trees exhibit secondary or lateral growth.

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Let’s unpack that.

The branches and roots of a tree elongate over time in what is called primary growth. This is what makes tree trunks, branches, and root systems longer with each passing season.

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All plants exhibit this type of growth.

But, as mentioned, trees also exhibit something called secondary or lateral growth.

This occurs when cells divide in a layer called the cambium, thereby making trunks, branches, and roots thicker over time.

Some non-woody plants exhibit secondary growth, but monocots don’t.

This helps to eliminate palms, bamboo and banana plants (which are both monocots) from our definition of “trees.”

This only matters because we’re about to discuss tree roots, and some tree-like plants, including palms, bamboo and banana plants, produce very different kinds of root systems.

Bamboo, banana plants and palms (like the one you can see to the right) have small, fibrous roots that are often described as being “fingerlike.”

With that cleared up, we can get back to our original point.

The Organization of Tree Roots

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In some ways, the roots of true trees do resemble the branches in the canopy.

For example, they grow in the same manner. Roots and branches both exhibit both primary and secondary growth.

They get longer and get thicker with each passing season.

The roots are thickest near the trunk, and they get smaller and finer as you move outward — just like the branches of the canopy. That’s because the roots nearest the trunk are also the oldest, so they’ve had longer to increase in girth.

But their growth habit – which you can think of as the overall silhouette of a canopy or root system – differs pretty sharply from that of branches.

Whereas tree canopies often exist on a spectrum from thin-and-tall (think pine trees) to broad-and-oval shaped (think oak trees), root systems generally exhibit widely spreading but relatively shallow shapes.

They don’t form the lollipop shape we’re all led to believe as youngsters. Nor do they exhibit a carrot-like growth habit that would resemble the silhouette of a pine tree (at least not once they’re mature — initial taproot develop does look a lot like a carrot).

Instead, think of a flat-topped bonsai tree, only upside down.

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This isn’t to suggest that all of a tree’s roots grow in shallow soil.

Some, including the taproot and sinker roots, tend to head straight down or nearly so. But the vast majority of a tree’s roots — especially those actively involved in absorbing water, oxygen, and minerals — are in the upper layers of soil.

And this occurs because roots and branches serve different purposes for the tree and develop in vastly different environments.

The Harsh World Where Tree Roots Dwell

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Branches have it easy.

They’re bathed in sunlight, oxygen, and carbon dioxide, and the only things they have to “push” against when growing are gravity and air.

But roots have it tougher.

Roots have to force their way through the soil, sucking up what little oxygen is available (roots do need oxygen to survive). The soil in which they grow often becomes compacted by weather and foot traffic, and it’s occasionally flooded after heavy rains.

And we haven’t even touched on the sometimes-problematic soil chemistry characteristics at play. Macronutrient availability, micronutrient availability, and pH can all create additional challenges for the roots.

If they’re lucky, they’ll be colonized by fungi, which will help them do a little of the biological heavy lifting in exchange for a little sustenance in the form of sugars exuded from the root tips. But that doesn’t always occur in urban sites.

But of all these forces conspiring to make the roots’ lives a living hell, it’s the facts that roots require oxygen and must push through the soil that are most responsible for the overall root system’s shape.

Soil Is Thicker Than Water

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If the soil at the surface is tough to get through, the soil 1 foot down is even tougher. Travel more than about 6 feet down, and the soil is borderline impenetrable in many locations.

And that’s part of the reason most tree roots are found in the uppermost few feet of soil, and why they don’t exhibit the same growth habit that the canopy does — it’s hard to grow in soil.

It is important to note that this doesn’t mean roots never grow deeply.

Roots are sometimes incredibly good at forcing their way into places they don’t belong. There are instances in which they’ve pushed their way through rock crevices measuring only 100 microns wide, and stories of them wrecking underground pipes or pushing up driveways are numerous.

But compacted soil often seems to stop them in their tracks.

Roots Have to Breathe

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As mentioned, roots require oxygen.

Starved of it, as often happens in waterlogged soils, they’ll die. And because oxygen is far more abundant and available near the surface, this forms the second reason roots tend to grow in the upper reaches of the soil.

In fact, the vast majority of fine absorbing roots – the hairlike white roots that emerge from the tips of the root system – usually grow in the upper 12 inches of soil, where oxygen is plentiful.

Some trees have even developed adaptations for growing in areas with frequently flooded conditions.

For example, some trees produce small slits in the trunk called lenticels. These can absorb oxygen and direct it to the root system when water chokes off the air supply.

It’s even possible that the “knees” of bald cypresses play a role in air-exchange for the roots, but more research is needed before we can be confident we understand these enigmatic structures.

It’s Not Just Depth That People Misunderstand; It’s Root Spread Too

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Aside from the fact that roots don’t grow very deeply, the other major difference they have from the tree canopy relates to their spread.

Specifically, root systems often extend farther than the canopy does — typically about two or three times as far as the distance from the trunk to the drip line.

Now, recent research demonstrates that root radii are more strongly correlated with trunk diameter than tree canopy (38:1 being the recognized ratio for root spread to trunk diameter).

But the point remains: Root systems spread farther than branches do. And this means that once again, the mirror-image model of tree-roots-and-canopies many of us have is wrong.

Reaching Out


The taproot and sinker roots also play a role in keeping a tree upright, but the bulk of the tree’s anchoring strength is provided by the far-reaching lateral roots located in the upper portions of the soil.

Nature Loves Exceptions; This Is All An Oversimplification

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As with all things natural, there are plenty of exceptions to the root characteristics shared above. For that matter, we’ve simplified things greatly.

We haven’t discussed the five different types of root systems, which all exhibit slightly different growth habits, or buttress roots, which help support massive trees.

For that matter, the Guiness Book of World Records lists a fig tree in South Africa as having the deepest root system ever recorded. Stretching down just a wee bit more than most trees, this root system has reached nearly 400 feet below the surface.

But these caveats aside, the fact remains: The image most people have of tree roots is very different than they actually are.

Does it really matter if you think tree roots resemble a mirror image of the tree’s canopy?

Probably. Maybe.

You needn’t be an arborist or botanist to benefit from a better understanding of tree biology.

Maybe you’re just a homeowner wondering why your birch tree is utterly destroying your driveway or why your northern red oak just fell over, while your white oak remains standing. Or maybe you’re just wondering how deep you should dig a hole for your new hickory tree.

Or maybe, you just walked by a windthrown tree in the forest, and you’re curious about what happened.

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