Purdue Extension Service

Vanderburgh County, Indiana

Deep Planting Causes Deep Trouble

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, March 14, 2008

I recently sat in on a training session led by an expert on tree roots from the University of Minnesota. I already knew that stem girdling roots (roots that wrap around the tree's trunk and choke it) are one of the major causes of tree decline in urban landscapes. But what surprised me is the number of people, from the nursery all the way to the homeowner, who all contribute to this problem.

The single biggest cause for a tree to form stem girdling roots is because they are planted too deeply. The reason for this, briefly, is that tree roots require oxygen as well as water and fertilizer. If the root ball is planted too deeply, the existing roots (called the "first main order roots) begin to suffocate. The tree responds by developing new roots closer to the soil surface (these are called "adventitious roots").

Depending on many factors, these new roots will either grow straight out from the trunk (preferable, but not common), or will curve back toward the trunk (undesirable, but common). As these roots and the trunk grow and widen, they come in contact with each other. The root's location prevents the trunk from expanding, much like a piece of string wrapped around a balloon. As the balloon (tree trunk) tries to expand, the string (root) inhibits the growth.

This problem begins at the nursery. Most trees are dug up from the nursery with mechanical diggers, which deposit too much soil over the root ball. Containerized plants, which are started from bare root trees, are often planted deeply in the pot so they don't lean. A survey of trees planted by the Minneapolis Parks Department showed that 87% of the trees arrived with the at least two inches of soil above the first main order roots, and that 50% of them had over four inches of soil over the roots.

The solution is to assume that all nursery stock arrives planted too deeply in the container or root ball, and that the homeowner should remove the excess soil from the top of the root ball (until they can find the main roots) before planting.

Homeowners and landscapers are often guilty of planting too deeply. All the research from the last 30 years shows that one should not dig the planting hole any deeper than the root ball. Yet most holes are dug too deeply, especially if an auger is used to prepare the hole. This means that the tree is being set too deeply, which increases the problems from the excess soil being put over the root ball at the nursery.

Adding organic matter to the backfill almost guarantees a problem with girdling roots. Likewise, heaping too much mulch against the tree's trunk also promotes adventitious roots and girdling, or even wood decay of the trunk.

To learn more about the proper way to plant trees, contact the Purdue Extension Service at (812) 435-5287.