How Tree Roots Work Part I

How Tree Roots Work Part II: Digging Deeper  |  How Tree Roots Work Part III: Mycorrihizae

When we see a tree in the landscape, we rarely think about what’s happening below ground. We know those tree roots are there, but we can’t see them beneath the soil surface. Tree roots tend grow in the top 2-3’ of soil and the smallest roots grow in only the top 6” of soil. Those small roots are the ones that take up water, nutrients, and oxygen. How do they manage to grow so close to the soil surface? Well, these small, fine roots grow well beyond the extent of tree branches and the larger roots closer to the trunk anchor the tree in place. Tree roots are actually quite fascinating when you think about it!

  Drawing of a tree showing the roots are shallow but extend outward well beyond branches

Although tree roots have the ability to extend hundreds of feet out in search for water and essential nutrients, they don’t always get that chance. Living in the urban environment is stressful on trees. Imagine a tree growing downtown, along the highway, or even that tree growing between the road and sidewalk in the front of your yard. Space is limited and tree roots are constantly competing with man-made structures to survive. If you are thinking of installing a driveway, an irrigation system, or even additional plants near a tree at your home, consider where those tree roots are growing. Plan before you change the tree’s environment and follow these guidelines when working around your tree.

Drawing that shows a tree's critical root zone is a circle one foot in radius for each inch of tree diameter

Measuring the Root Zone is Easy

Measure the diameter of the tree at 4 ½ feet above the soil surface. For every one inch you measure, measure out from the trunk one foot. For example, a 20 inch tree has a 20 foot radius root zone. This is called the Critical Root Zone or CRZ. This is the area you want to focus on protecting.


Learn how to renew, nurture, and protect our City's urban forest on our Tree Resources webpage.

How Tree Roots Work Part II: Digging Deeper  |  How Tree Roots Work Part III: Mycorrihizae

Article by Regina Ramos, Forester with the Parks and Recreation Urban Forestry Program

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This information is sponsored by the City of Austin. Learn more about trees and resources at the Tree Information Center!