5 Tree Root Myths Explained

Steve Nix is a member of the Society of American Foresters and a former forest resources analyst for the state of Alabama.

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Tree root systems are seldom on the minds of forest owners, tree planters, and general dendrophiles. Roots are rarely exposed, and so misconceptions about how they grow and function can influence tree managers into bad decision making. For example, not giving a juvenile tree enough space to develop roots that will spread a width equal to 38 times its potential trunk diameter could hinder its development. And pruning root tips prior to planting? Another big no-no.

Trees simply grow healthier when their planters understand their root systems. Here are several tree root myths that may change—improve, rather—the way you plant.

Myth 1: All Trees Have Single Tap Roots

Most trees do not have tap roots after the seedling stage. They quickly produce water-seeking lateral and feeder roots.

When a tree is grown in deep, well-drained soil, these trees will develop many deep roots directly surrounding the trunk. They should not be confused with what we think of as a taproot similar to other vegetable plants like carrots and turnips or the tap roots of tree seedlings.

Shallow, compacted soils will eliminate deep roots altogether and you will have a feeder root mat with very few deep roots. These trees get most of their water above the water table level and are subject to damaging windthrow and severe drought.

Myth 2: Tree Roots Will Grow Only to a Tree's Dripline

There is a belief that roots tend to stay under a tree's leaf canopy. In reality, this is seldom the case. Trees in a forest develop roots that sprawl well beyond their individual branches and leaves in search of water and nutrients. In fact, some experts today speculate that tree roots grow laterally to a distance equal to the height of the tree or up to five times the radius of the canopy.

The best way to determine root spread, though, is by looking at trunk diameter. Studies have found the ratio of root radius to trunk diameter (at breast height) to be about 38 to one. Trees standing together in a forest send roots beyond their individual limbs and intermingle with the roots of neighboring trees.

Myth 3: Damaged Roots Cause Canopy Dieback on the Same Side

This does happen sometimes. Dr. Edward F. Gilman's 1991 paper, "Dispelling Misperceptions About Trees," said that "roots on one side of trees such as oaks and mahogany generally supply the same side of the tree" with water and nutrients. "Dieback" of individual branches and limbs will occur on the damaged root side. But there are exceptions to the rule.

Maple trees, for example, do not seem to show injury or drop leaves on the side of root injury. Instead, branch death may occur anywhere in the crown with some tree species.

Myth 4: The Deeper Roots Secure Water and Nutrients

The "feeder" roots in the top three inches of soil, not the deepest roots, supply the tree with water and food. These delicate finer roots are concentrated in that upper soil and duff layer because that's where immediate nutrients and moisture are quickly available.

For this reason, minor soil disturbances can injure feeder roots and remove a large portion of the absorbing roots on a tree, setting it back significantly. Major soil disturbances due to construction and severe compaction can kill a tree.

Myth 5: Root Pruning Stimulates Root Branching

When planting a tree's root ball, It can be very tempting to cut back those roots that circle the ball. It is even often thought that a dense root ball will stimulate new feeder root growth, but that is just not the case. Most new root growth occurs at the end of existing roots, which is why root tips should not be pruned.

Plant nurseries will often prune roots to accommodate packaging and encourage growth before the final sale. When you go to plant your tree at its final site, you should gently break up the root ball to allow for normal root development, but avoid pruning the tips.