Cregg and Rouse are involved in a research project at Michigan State University on “Improving transplant success of container-grown landscape trees.” Here are some of the findings they’ve shared with Turf Tree Services.
Trees provide a myriad of functions and benefits in landscapes including shading, screening unwanted views, serving as focal points, and even capturing air pollutants and mitigating urban heat island effects. No wonder clients want to plant more of them!
One of the first steps in the tree planting process is deciding on whether to plant ball-and-burlap (B&B) trees or trees grown in containers. Increasingly, landscapers and homeowners are opting for container-grown trees because they weigh less and are easier to handle and plant than conventional B&B trees. However, a major downside to growing trees in containers is the development of circling roots during production. The production of most container-grown shade trees starts with bare-root tree liners (or whips) that are planted in plastic containers, which typically range between 7 and 25 gallons. As roots grow, they eventually encounter the wall of the nursery container, deflect, and begin to circle. Container trees also commonly produce a heavy mat or “pancake” of roots at the container bottom.
Both the circling roots around the sides as well as the pancake on the bottom of the root-ball are defects that can limit root egress into surrounding soil after planting. Root egress is essential to successful long-term establishment and for providing tree stability. Moreover, when roots continue to circle around the base of the tree, they have the potential to girdle the stem and effectively “choke out” the tree.
Nurseries have grown trees in containers for decades, and landscapers have observed container-related root defects for nearly as long. Over the years, researchers have investigated a variety of approaches to mitigating container effects including slicing, butterflying, and teasing apart roots—often with mixed results. More recently, research has shown that shaving—removing the outer periphery of the root systems of container-grown trees—can eliminate circling roots and improve root egress into soils.
Shaving can be done with a pruning saw, reciprocating saw, or sharp spade. In some cases, the container root-ball is cut into a square and is referred to as “box-cutting.” Over the past decade, our lab has conducted research trials on the response of container-0grown trees to an array of pre-plant root modifications. Here we discuss these trials and some of the key implications for landscapers planting container-grown trees.
A Close Shave
In 2012 we installed a trial to investigate the impacts of planting practices on survival and establishment of container-grown Bloodgood London planetrees. Before planting, we divided the trees into three groups and assigned them to three root treatments:
- Shaving, in which we removed the outer 1 1/2″ of the periphery of the root system along with the bottom pancake of roots;
- Teasing, where we “teased” or pulled apart any circling roots; and
- Control, which we planted as-is.
We planted all the trees in landscape test plots at the Michigan State University Horticulture Teaching and Research Center and monitored their survival and growth. We excavated the root systems of a subset of trees two and four years after planting. We found that shaving increased root egress into the surrounding soil and reduced circling roots. Teasing also improved root system quality and increased root egress compared to the control, but not to the same extent as shaving.
However, one concern with root shaving is the potential negative impacts of root removal on subsequent tree stress and growth. Yet in our trial, tree height and diameter growth was comparable among all three root treatments.
How Does It Work?
While root shaving can improve new root growth into the surrounding soil, how exactly does it happen? And why isn’t teasing apart circling roots just as effective? When thinking about root modification strategies, the first thing we have to do is eliminate the circling. If circling roots are not mitigated, they will continue to circle and ultimately become woody. This further limits growth into the backfill and can con-tribute to tree instability down the road. When done properly, both shaving and teasing can reduce circling roots and improve long-term root quality. But one challenge with teasing roots is that it can be physically challenging to pull apart roots, especially on large, densely matted root systems. We also find that when we straighten out roots, the roots tend to spring right back to their original position.
Another advantage of root-ball shaving compared to teasing is that pruning roots helps to stimulate new root growth, similar to how pruning branches can result in new growth below the pruning site. When we cut through roots, callus tissue forms at the point of pruning, and there is a proliferation of new roots formed that can increase root egress into backfill soil.
Slicing & Bare-Rooting
In talking with tree care professionals, we find slicing—making a series of vertical slices through the root-ball—is a frequent practice when planting container-grown trees. Although slicing is widespread, there is relatively little research to support the practice and the few studies available yield conflicting results. Dr. Ed Gilman at the University of Florida found that slicing roots of container-grown live oak trees reduced development of circling roots, but also decreased tree stability during controlled “pull tests.” Researchers at the University of Minnesota found that slicing root systems of littleleaf linden and weeping willow trees did not improve new root growth compared to untreated controls.
More recently some have advocated bare-rooting container-grown trees prior to planting. In bare-rooting, all container substrate is removed in order to access the entire root system and correct any root defects, including circling roots and other root deformities. Again, little systematic research has been conducted to support bare-rooting. We have trialed bare-rooting trees prior to planting and found that tolerance of trees to bare-rooting varied widely among species (planetrees were tolerant while tulip poplar, hornbeam, and hophorn-beam had extensive dieback and mortality following bare-rooting) and by season (bare-rooting resulted in severe dieback and mortality when performed during Summer).
A preventive approach to circling roots is growing trees in alternative containers. Over the years various container types have been developed with air-pruning slits or other designs to minimize circling. Fabric grow-bags or root pouches have also been promoted. Dr. Alison Stoven O’Connor with Colorado State University Extension evaluated root development after transplanting Callery pear trees that were grown in either standard black plastic containers or fabric grow bags, and found that trees grown in fabric bags had fewer circling roots and better overall root quality.
While circling roots can negatively impact tree establishment and long-term structural issues, root-ball shaving is an effective technique for eliminating circling roots, improving root quality, and stimulating new root growth. Fortunately, the availability of trees grown in alternative containers, which may reduce root circling, is increasing. Landscapers should consider trialing trees grown in alternative containers if they are available in their area.
Cregg is a Professor and Extension Specialist in the Department of Horticulture and Department of Forestry at Michigan State University. Rouse is an ISA Certified Arborist and a Research Assistant in the Department of Horticulture at Michigan State University. Collaborators on their research include: Michigan State University Project GREEEN; J. Frank Schmidt Family Charitable Foundation; ICL Specialty fertilizers; Rainbow Scientific; J. Frank Schmidt and Sons Nursery; Renewed Earth, Inc.; and Nursery Supplies, Inc. For more on Cregg’s research, visit canr.msu.edu/people/bert_cregg.
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