The Hidden Half of Trees: How Roots Work
- Cheryl Ames
- May 1
- 5 min read
We tend not to think about the huge part of the tree that is underground—until a
tree overturns, lying on the ground with its root system exposed. The bigger the
tree, the more we are amazed by this hidden half—far larger and shallower than
most people realize.
Tree roots also bring attention to themselves when they begin causing
problems—getting in the way of digging for new plantings, uplifting sidewalk and
creating walking hazards, or even encroaching the foundation of a house and
creating cracks.
Understanding how tree roots really work can help us make better decisions about
where to plant trees and what to do when they become mature and begin causing
problems.
The Architecture of Roots: More Than Meets the Eye
The anatomy of a tree consists of approximately 5 percent leaves, 15 percent
branches, 60 percent trunk, 15 percent large transport roots, and 5 percent fine
feeder roots.
Contrary to what many people imagine, most tree roots do not grow deep like a
carrot. Instead, they spread widely through the top 12 to 24 inches of soil, where
oxygen, water, and nutrients are most available.
There are two main types of roots:
• Structural, or framework, roots: Large, woody roots that anchor the tree and
radiate outward from the trunk. They increase regularly in diameter and even
display annual rings. It is this increase in size that swells the base of trees,
raises the earth around them, and lifts sidewalks.
• Fine feeder roots: Hair-like roots responsible for absorbing water and
nutrients. These roots can be as small as .008 inch. They provide the major
portion of the absorption surface of a tree’s root system. Symbiotic fungi
called mycorrhizae aid this absorption of water and minerals, competing
with grass and shrubs.
A tree’s root system can extend considerably wider than its canopy, forming a
broad, shallow web beneath the ground. The framework of major roots usually lies
less than eight to twelve inches below the surface but can extend outward to a
diameter one to two times the height of the tree.
How Tree Roots Differ Among Species
One way they differ is shallow versus deep root systems:
• Trees with shallow root systems (fibrous) are like the ones described above.
They spread outward near the soil instead of going deep. These roots quickly
absorb rainwater but can be more sensitive to drought. Examples are maples,
willows, and spruces.
• Some trees do grow a dominant central taproot that goes straight down. The
taproot grows 3-7 feet or more until they encounter impenetrable soil or rock
layers. This helps them reach deep water sources and stay stable in dry or windy
environments. Examples include oaks, pines, and walnuts, especially when
young.
Another way trees differ is with wide-spreading versus compact roots:
• Some species extend roots far beyond the tree’s canopy—sometimes two to
three times the tree’s height. This is common in species that compete for surface
nutrients.
• Other trees keep roots closer to the trunk, often in environments where space or
soil depth is limited.
Wide-spreading root systems are very common in the Pacific Northwest. These
include the Douglas-fir which may start with a taproot, but this usually fades over
time. Eventually Douglas-fir roots can spread two to three times the width of the
crown. Western hemlock, which has a fibrous root system, is prone to wind throw
(blowing over) because roots stay near the surface. Sitka spruce roots are very
shallow and often rely on root grafting with nearby trees for stability. This is
common in the Pacific Northwest because frequent rainfall and oxygen-rich
surface soils make shallow roots more efficient than deep ones.
What this means for PNW landscapes:
• Shallow roots dominate, even in very tall trees.
• Trees often rely on root networks and soil structure rather than deep anchoring.
• Windthrow risk increases in saturated soils common in winter storms.
• Urban conflicts (sidewalk lifting, pipe intrusion) are frequent because roots stay
near the surface.
When Roots Struggle to Thrive
While roots are resilient, they depend on the right conditions to support a healthy
tree. When those conditions are limited, the entire tree can suffer.
Some of the most common challenges include:
• Compacted soil: Heavy foot traffic or machinery can compress soil, reducing
the air spaces roots need for oxygen.
• Poor drainage or waterlogged soil: Roots require oxygen as well as water;
overly saturated soil can suffocate them.
• Drought conditions: Extended dry periods limit water availability, especially
for fine feeder roots.
• Limited soil volume: When roots are confined—such as in small planting
areas—they cannot spread or access enough resources.
• Nutrient-poor soils: Without adequate minerals, roots cannot supply what the
tree needs for growth.
When roots are stressed, trees may show symptoms above ground such as sparse
leaves, dieback in branches, or slower growth.
Why Conditions Matter for the Whole Tree
A tree is only as healthy as its root system. When roots struggle, the effects ripple
upward:
• Reduced water uptake leads to drought stress in leaves.
• Limited nutrients affect leaf color and growth.
• Weak anchoring can make trees more vulnerable to wind damage such as
loss of limbs.
Healthy soil—loose, well-drained, and rich in organic matter—is one of the most
important factors in supporting strong roots and, in turn, a thriving tree. Thriving
trees are necessary to avoid damage to structures if limbs fall or roots travel
upward to find nutrients or moisture.
Supporting Healthy Root Systems
Fortunately, there are simple ways to help roots succeed:
• Avoid compacting soil around trees.
• Provide consistent, deep watering during dry periods.
• Add mulch to retain moisture and regulate soil temperature.
• Improve soil with organic matter when possible.
• Allow adequate space for roots to spread.
A Living System Worth Understanding
Tree roots are not just structural supports—they are living systems that respond to
their environment. When we understand how they function and what they need, we
can better support trees from the ground up:
• We can keep trees healthy and select the right trees for planting near sidewalks
and buildings.
• Tree size can be matched to the width of the space in the lawn.
• Tree species that are more tolerant to root damage can be planted nearer to
sidewalks, since the roots can be trimmed as the tree grows. Roots can also be
forced downward with compacting soil under sidewalks or by installing barriers.
• Deep-rooted trees like oaks might be the best choice to grow near sidewalks.
• Planting at the correct depth can avoid unnecessary swelling at the tree base as
the tree grows.
Trees can certainly conflict with human spaces, since their roots extend so far. But
if we want the advantages they provide us, such as cooler environments and great
natural beauty, we need to make good decisions about what to plant. Ultimately,
loss of mature trees is felt for decades as we wait for new trees to grow. We need to
be able to preserve what we have!
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Resources and Further Reading
• ArborTrue, Understanding Tree Root System: The Hidden Half of Trees, True
Tree Talk, March 7, 2025, www.arbortrue.com
• Everything Arboriculture, Tree Roots Anatomy: Everything You Need to Know,
• James Fazio, Resolving Tree-Sidewalk Conflicts, Tree City USA Bulletin,
estry/Resolving%20Tree%20Sidewalk%20Conflicts.pdf
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