In the most severe cases, deep root systems can lead to decline and death of the tree, sometimes very quickly. Deep roots are subjected to greater soil saturation and insufficient oxygen, and symptoms can be similar to drought stress. Loss of willow oaks was five times greater when planted six inches deep. (See Smiley [pdf])

View larger

Under less severe conditions, growth will be slowed and the tree will be stressed. The tree may appear fairly normal without comparison to a tree with roots at the correct depth, as the photo of container grown trees shows. (See Fare [pdf]) Live oak trunk caliper and height growth increased more slowly as planting depth in containers increased. (See Gilman [pdf]) Deep planted trees can have greater defoliation, chlorosis and leaf curl. (See Wells [pdf]) (photo by: D. Fare)

View larger

Symptoms of deep planting may also be delayed in developing under moderate conditions. There was little response to planting depth in the first year on green ash and live oak, compared to subsequent years. The delay in symptom development is likely related to tolerance of these species to periodically wet soils. (See Bryan [pdf])

Over time, a tree with deep roots will grow slower and stress will develop earlier than a healthy tree. Sometimes girdling roots can develop after deep planting in the landscape and make the symptoms even more pronounced. Even a mild reduction in growth will reduce the ecological, economic, health and psychological benefits provided by the urban forest.

View larger

Deep roots problems are magnified on sites with heavy, poorly drained soils. This includes most of our neighborhoods, commercial properties, and parks developed in the past 40 years–virtually anywhere the land has been reshaped with heavy equipment. Areas of the country with clay soils will have the most severe problems.

View larger

Even if the structural roots are too deep, good drainage and aeration may allow the trees with deep roots to grow fairly well, at least for a while. Growth of southern magnolias growing sandy soil was not reduced by deep roots. The roots grew back up to the surface quickly from the buried root ball. (See Gilman [pdf]) (photo by: E. Gilman)

View larger

Deep planting did not impair establishment of Turkish hazelnuts in well-drained soil, but a deep root system may make the tree more sensitive to occasional stresses it could otherwise endure—in particular, exceptionally high rainfall. (photo by: S. Day)

View larger

This was demonstrated when the Turkish hazelnuts were subjected to flooding. One with deep roots died and stress levels were clearly greater than before flooding. (See Day [pdf]) (photo by: S. Day)

View larger

A study conducted on heavy clay loam soil resulted in reduced growth and/or survival of multiple species when trees were planted as little as 7.6 cm (3 in) below grade. Root quality was improved by amending the native heavy clay soil with 30% by volume sand or peat moss. (See Bryan [pdf])

Site quality also explains why trees can grow nicely in the nursery with deep roots, and ultimately be shipped to landscape sites with roots too deep in the root ball. Nursery sites are chosen because of the high quality soil. Under these conditions, there was no growth reduction of liners planted with the graft union up to six inches below grade. (See Jarecki [pdf]). A tree growing vigorously in the nursery with deep roots will usually perform poorly on an urban site if planted at the same depth.

View larger

Tree species vary in tolerance to soil poor conditions. In a study comparing the effects of deep planting on hypoxia tolerant and intolerant species, planting 7.6 cm (3 in) below grade reduced the survival of green ash (F. pennsylvanica) by about 40% after 3 years and bougainvillea goldenraintree (Koelreuteria bipinnata) over 90% within only 2 years. (Bryan paper link) Within two years of installation, one half of deeply-planted cherries died, while there was no impact on survival of red maples. (See Wells [pdf])

View larger

Deep root systems increase tree failure rates. Thirty-three percent of trees with excess soil over the roots failed, compared to only eight percent of trees without excess soil over the roots. (See Smiley [pdf])

In a Minnesota study, 73% of the Little Leaf Lindens that suffered total failure had four inches or more of soil over the structural roots. Most of these trees also had girdling roots causing stem compression where the trees broke off. (See Giblin [pdf]) (photo by: T. Smiley)

View larger

Deep roots have been associated with greater winter injury in conifers. (See Smiley [pdf]) (photo by: T. Smiley)

View larger

Some species have the ability to produce adventitious lateral roots above existing lateral roots if planted too deep, but these are the exceptions. Most do not. Production of shallower adventitious roots could allow a tree to adapt to deep planting.

Four months after bing planted too deeply in a container, all of the roots above the finger are adventitious roots formed after planting. (See Giblin [pdf]) (photo by: E. Gilman)

View larger

Cathedral Oak™ developed adventitious roots along the stem above the top-most root present at the initial planting, but only on very young trees. The ability to develop adventitious roots was lost as the trees aged. (See Gilman [pdf]) (photo by: E. Gilman)

View larger