Buried Treasure: Exploring the Root Depth Controversy

Compiled by Amy Fulcher, Extension Associate for Nursery Crops

It has been said to read what your boss reads if you want to succeed. The root depth topic might be a good example of the importance of reading what your customers read! We have to be at least as informed as our customers. Staying aware of the issues that confront your customers can help you to understand what matters most to your customers and to provide the outstanding service and quality that will keep your customers.

Becoming familiar with the problem is essential because trees with deep roots are being grown and sold in the US. A multitude of images were shown and samples displayed of newly purchased trees with the root systems filling only the bottom 1/3 or ¼ of the ball. Tom Smiley of Bartlett Tree Company reminded us that depending on the ball size, a 6” increase in planting depth can reduce root mass at harvest by 1/3. The ongoing conversation of these upper Midwest landscape contractors and urban foresters during breaks was “I can't find trees that aren't planted excessively deep and without girdling roots.” The segment of the industry that plants and maintains trees has educated themselves about the potential consequences of deep roots and are informed consumers of green goods. These contractors and arborists are looking at root depth when they purchase trees and are comparing nurseries based on root architecture. Again, we have to be at least as informed as our customers. This is an opportunity to show our valued northern customers that Kentucky grows quality.

On November 10, 2005 the Morton Arboretum hosted a workshop featuring research that addresses the root depth controversy. In the last 5 years, the issue over root depth has grown, and thankfully been tempered by a collaboration between nursery producers, landscape contractors, and arborists.

With 13 presentations by researchers, consulting and municipal arborists, representing 9 states, the topic was thoroughly covered to the extent that current research results allow. The earliest effects of deep roots were assessed by examining 2” liners and ended with the examination of hundreds of 30+ year old trees that failed.

One common message from the research presented at the workshop is that there is a lot of variation in tree response to root depth. For example, results of research performed at the University of Illinois in a well drained, silt loam soil (with topsoil 3+ feet deep) showed no difference in caliper to planting green ash, white ash, Norway maple and Shademaster honeylocust with the root flare at grade or 6 inches or 12 inches below grade. However, in a study conducted in very heavy clay soil in Texas plants did much worse when planted below grade. (A species effect may have been part of the difference.)

Not only soil but also plant species can cause variation in tree response to deep roots. In a study conducted by Christina Wells of Clemson University, approximately 2” caliper October Glory® red maples and Yoshino cherries were planted at grade or with the root flare either 6” or 12” below grade. Foliar chlorophyll content was lower in both species when planted 12” deep. Defoliation and leaf scorch increased 6 month after planting the Yoshino cherries and 50% of the Yoshino cherries died within 2 years of the study. One year after planting, both species had similar root mass densities from soil sampled 0- 6” deep and 6-12” deep, and neither species had many roots in the upper 12” of soil if the tree was planted 6” or 12” below grade. In this experiment, root systems of both species responded similarly to deep planting, but the maples were more able to tolerate the conditions for the duration of the experiment. In addition, this deep planting was associated with stem girdling roots. Those trees planted at grade had stem girdling roots around 15% of the circumference of the trunk, those at 6” below grade had stem girdling roots around 50% of the circumference and those planted 12” below grade had stem girdling roots around 75% of the circumference of the trunk.

Research by Donna Fare, of the US National Arboretum, on planting depth during container production also supported a difference in tolerance among species. Zelkova looked no different regardless of planting depth and likewise no measurable difference in caliper due to deep planting was detected, however, the trees planted just 2” deep had reduced water use. Flowering dogwoods planted with the root flare 0-2” below grade grew well. Dogwoods planted 0-2 inches deep had sap flow 4-5 times greater than the trees planted 4” deep. Deeply planted (4”) dogwoods had significantly less leaf area and fewer leaves than those planted at grade. Dr. Fare offered a caution that it cannot be determined if the poor response of deeply planted dogwoods is due to the stem being covered by substrate (deep planting) or to the roots being in the perched water table of the container. During the conference, many of the impacts of deep roots were related to a landscape setting, but this was one instance where deep roots adversely affected the plants, and the bottom line, while in production.

Ed Gilman's research on live oaks showed a range of responses over the 3-year production of trade 45 gallon trees to various levels of planting depth. This research began with 2” liners in Accelerator pots, planted at varying depths at each transplanting into a larger container size. Liners planted at grade had slightly poorer performance that those planted slightly deep, which the researchers attributed to drying out between irrigation events. Caliper growth was slowed by deep planting for those 2” liners planted 4.5” too deep and those planted 2.5” deep at each of 3 transplanting steps into larger containers, but this was not detectable after 18 months into the production cycle. Researchers noted that in the first year of production, adventitious roots formed along the portion of the stem that was buried by the container substrate.

Gary Watson of the Morton Arboretum brought about a fact that may cloud the assessment of deep roots. Roots tend to respond to pruning in a more targeted and concentrated manner than above ground portions of a tree. After pruning, roots have a proliferation of growth just 1” or so from the cut end, as opposed to buds braking along 6-10” of stem tissue. These roots often begin at a depth of 6” or greater, simply reflecting where the cut was made, and that permanently affects the root architecture and the future planting depth of the liner. Those roots that developed as a result of the pruning appear to occur, at least in some species, at the expense of the continued development of the small roots that were located between the soil surface and the pruning cut. These small roots virtually disappear in “before and after” photos of root pruned liners. On the other hand, roots left unpruned had an even distribution of secondary roots along the length of the primary root.

Why have deep roots become such an issue? Several researchers discussed maple decline and failure of trees during storms, and in the absence of storms. Others working with nurseries cited instances when growers requested help due to dying or declining liners. These problems have contributed to the investigation of root depth in an effort to find explanations to these problems. Researchers from the University of Minnesota have been examining deep roots among established trees for about 10 years, with approximately 125,000 trees available for examination after the 1998 storms. In an initial study, root depth among trees planted in boulevards was measured. They found that 40% of hackberries were at grade, 32% of honeylocusts were 1” deep, and 25% each of green ash and lindens were planted 4” deep. In addition, circling roots occurred at a frequency of 25% when the root flare was 4” below grade. In 1998, due to the availability of trees lying on the ground with root systems exposed, researchers were able to inventory root characteristics. They found that 73% of the lindens that failed had 4” or greater of soil over the first lateral root and stem girdling roots and that 68% of trees that failed in the 6-10” diameter at breast height size range had stem girdling roots.

So how prevalent are deep roots and root flares that are buried under soil in nursery stock? Davey Tree Experts and Ohio State University surveyed nine tree nurseries and nine brokers that were located in a line that ran from NE Ohio to SW Ohio. They evaluated planting depth as well as propagation method to determine its influence on planting depth. The desire to cover the graft union and regrowth from a cut back was mentioned (by many presenters) as a reason for deep planting. Roots from nurseries ranged from 1.1” below grade to 3.7” below grade with an average of 2.4”. There was no significant difference among propagation types. There also was no statistical difference in planting date, with trees in year 1 averaging 2.6” deep and trees planted 3 years ago averaging 2.1” deep. On brokerage lots mainly the same species were sampled, with a few exceptions. The average depth of the uppermost roots was 3.4”, ranging from 2.6 to 4.2”. There was no significant difference between propagation types.

Research by Douglas Airhart of Tennessee Tech University and formerly of the University of Massachusetts examined of nine species of trees in field production. Root systems of 10 trees of each species were exposed with compressed air and the root depth in inches below grade was recorded. The results were: red maple 5.0”, redbud 4.9”, sycamore 6.6”, green ash 3.0”, pin oak 3.7”, zelkova 3.3”, sweet gum 5.4”, dogwood 5.3”, and hornbeam 1.8”. Dr. Airhart pointed out that the green ash trees were planted by carefully supervised horticulture students. The hornbeams were planted at a nursery with the tractor operating at its slowest speed and, not one, but two people walking behind it, resetting each tree to the ideal depth.

Some thoughts on the nursery related aspects of root depth:
Nurseries are often on locations with superior soil compared to the urban landscapes the finished trees will ultimately grow in. Planting depth may not cause nurseries a problem except on more sensitive species. We have observed extensive liner death in Kentucky in conjunction with the root flare 4-6” deep for both Cornus florida (container) and ‘Kwansan' cherries (field). This disparity in soil conditions may explain how landscapers experience problems more frequently while relatively few are experienced during production.

Likewise, container conditions are nearly ideal and create little stress with regard to moisture retention and air porosity, such that deep planting in containers may have little in common with deep planting in soils. Perhaps only the most sensitive species may succumb to deep planting in containers and perhaps extremely deep planting is required to elicit an effect.

The frequency of stem girdling roots appears to increase with increasing root depth and it is well established that girdling roots are a component of tree failure and decline in landscapes. The research and observations presented at the conference suggested deep planting of just 4” may be linked to the development of girdling roots.

We don't have an integrated industry like car manufacturers. Those corporations have control of the design and engineering of every aspect of the vehicle: manufacture of each part, assembly, sales, and even service after the sale, pointed out Ron Zillmer, former urban forester of Milwaukee, WI. The green industry may have several stops along the production, sales, installation, and maintenance “road”. With all of these steps being provided by a different company, in many instances, and individual businesses imposing their own philosophies and production standards on the products, a varied outcome is inevitable. Only through our knowledge of the impact of those before us on the production “road” and concern for our impact on those “stops” after us, can we improve the industry and ensure that we are doing everything possible to grow a superior plant.

My best effort was made to record the details of these presentations as accurately as possible. My apologies to the authors and readers for any errors that may exist. To my knowledge there are none.