There are places where trees don’t belong -- their strong roots can probe the joints in pipes, grow into cracks in foundations and damage underground structures. A compromised basement wall, septic system or sewer connection might be the expensive consequence of buying and planting an ornamental tree without checking to find out how deep its roots will grow and how aggressive they’ll be in their search for sustenance.
Form Follows Function
Tree roots serve two vital functions; they gather nutrients from the soil and they support the weight of the top growth of the tree. Like an iceberg, a large tree requires a large substructure -- in the tree’s case, that structure consists of perennial primary roots and shorter-lived secondary roots that grow in waves as the tree’s leaf canopy expands. Some primary roots extend almost as deep as the height of the tree. Secondary “feeder” roots often extend far beyond a tree’s drip line -- the imaginary line around the tree where water drips off the perimeter of its canopy. Genetic mutations and environmental considerations aside, tall trees with huge, wide crowns require deeper, stronger root systems than smaller trees.
Permanent Root Types
Primary roots are permanent roots that support the weight of the upper tree. The deepest primary roots, called taproots, emerge as radicles from germinated seeds and continue to grow as the tree matures. White oaks (Quercus alba), hickories (Carya sect. Carya), walnuts (Juglans microcarpanigra) and hornbeam cultivars have strong, deep taproots. Heart roots are multiple primary roots that may be visible at the base of the trunk and drop numerous secondary roots in the search for water. Red oaks (Q. rubra) and sycamore (Platanus occidentalis) have thick heart roots. The bigtooth maple (Acer grandidentatum) has both taproot and heart primary roots. Flat roots spread far beyond drip lines; when planted too deeply, flat primary roots easily wrap around the base of the trunk in a pattern called girdling, which threatens the health of the tree. Cottonwoods (Populus sect. Aigeiros) have aggressive, flat primary roots.
Roots and Soil Conditions
Some trees, such as evergreen figs (Ficus spp.) and poplar (Populus spp.), are notorious for aggressive roots that damage septic fields. Soil conditions can limit or attenuate growth in any tree’s root system, however. Where the soil is sandy or loamy or the water table is low, tree roots grow deeper in search of moisture and the minerals it carries. Where soil is clay-heavy or water tables are high, even deep-rooting varieties like the big oaks grow shallower, more numerous roots.
Some trees may not be particularly deep-rooted, but still threaten pavement, utilities and structures because of the sheer volume of roots they produce. The strength of weeping willow (Salix babylonica), Eastern cottonwood (Populus deltoides) and golden bamboo (Phyllostachys aurea) comes from their invasive root systems that spread rapidly, sprouting new trees as they invade surrounding areas.
- North Carolina State University Horticulture: Tree Anatomy
- Iowa State University Forestry Extension: Roots in Depth
- Virginia Department of Forestry: Introduction to Trees: Root
- Lady Bird Johnson Wildflower Center: Ask Mr. Smarty Plants
- Kansas Forest Service: Deciduous Tree Seedlings
- University of Minnesota Extension: Native Trees for Landscape Use
- Colorado State University Extension: Diagnosing Root and Soil Disorders on Landscape Trees
- Burke's Backyard: Trees to Avoid
- Northscaping.com: Invasive Root Systems
- City of Richmond Hill Georgia: Oak Tree Care
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