IMPROVING ROOT DISTRIBUTION IN NURSERY TREES

Transcription

1 IMPROVING ROOT DISTRIBUTION IN NURSERY TREES By Mark A. Webber Years ago, I attended a Buckeye Yard and Garden Line ( BYGL Live ) educational workshop in Cincinnati, Ohio. The workshop featured a demonstration using compressed air to expose a trees root system. The demonstrator used a device called an Air Knife. At the time, I had no idea that this single event would become a career- changing experience for me. I learned that these misdirected roots were called stem-girdling roots. At that very moment, I realized that some the things that I had learned or assumed to be true about tree root systems were inaccurate (Johnson & Hauer, 2000). The operator uncovered the roots of a flowering dogwood (Cornus florida) that was showing symptoms of decline. There was dieback in the crown and growth was obviously stunted. Once the upper layer of soil was removed, I could clearly see how deformed the surface roots were. I had never seen anything like this before. The roots appeared twisted and fused, and rather than growing radially away from the trunk, they grew at odd angles and completely encircled the trunk. The roots had literally placed a choke hold on the cambium and phloem of the tree, and it was being killed by it own roots! Stem Girdling Roots Photo Source MkWebber 2007 Improving Root Distribution in Nursery Trees Mark A. Webber 1

2 From that day forward, whenever I inspect a tree for a client, I look not only at the crown for structure and symptoms of restricted root function, but also check the roots for signs of roots defects. If a tree appears unhealthy, I try to determine if its condition is related to root dysfunction. Arborists need to understand how tree roots develop and function to determine if the cause of the decline in trees. Furthermore, they must understand the differences between how tree root systems develop in natural forests, versus urban landscapes where trees and other landscape plants are grown in nurseries. Moreover, then planted there. The main difference is the roots of nursery trees are manipulated and altered by various propagation practices. In this article, I will provide some insight into managing or training tree root to prevent or mitigate defects. Tree roots that develop naturally vs. those produced at the nursery Trees roots in the natural environments are not confined to growing in containers with limited soil volume, or dug up and transplanted when they ve reached the desired size. Root defects often develop because trees many trees are propagated in containers. They can also develop when trees are planted below grade, or when roots are not properly positioned, such as when bare-root trees are planted. These defects will create defects below ground that can affect health and longevity. It should be noted that root defects do indeed occur in natural forests. Rooting depth is often restricted by impenetrable layers, and rocky soils can impede normal root architecture. The roots of one tree can interfere with the development of a neighboring tree or even girdle it. Tree root defects - roots growing the wrong way! Just like the upper portion of a tree, tree roots can be easily deflected when they contact a solid object, impenetrable layer or encounter an unfavorable environmental condition, such as a high water table, compacted soil, etc. Significant defects that develop in the root system when the tree is young can affect performance, reduce longevity, and result in decline or increase risk potential. As the fruit growing industry has shown, trees pruned and trained into a central leader are more structurally stable and can handle dynamic loading better than those trees that were not trained in that method (Lespinasse & Delort, 1986). Dr. Edward Gilman (2012) describes the 5 primary causes of root defects: Deflection due to containers or impenetrable fabric Improving Root Distribution in Nursery Trees Mark A. Webber 2

3 Deflection by hardscape, compacted soil, curbing, pavement, rocks, etc. Planted too deeply Upward or lateral root growth from cut roots in field grown conditions Mulch or soil placed over the root flare Gilman, Harchick, & Paz, 2010b). Such defects can girdle the main stem, interfere with natural root development and potentially shorten a tree s lifespan. Developing more natural root systems The training needs of root systems of young trees are not significantly different than the training requirements of the crown. The issue of SGR s begins in many cases in the handling and propagation of young trees. Many of the containers used in the production of trees will result in root growth in a circular or descending direction rather than mostly radially and/or obliquely downward because they are deflected by the container (Hudler & Beale, 1981). In addition, species that naturally form taproots trees are altered and lost during propagation. Tree roots that are unnaturally oriented (bent, kinked, or circling) occurs when the root tips or secondary (lateral) roots are deflected by the root ball container of a solid object in the soil, e.g., rock, root barrier, foundation, curb, etc. Root defects (unnatural orientation) are typically the result of the propagation in containerized stock (Gilman, Harchick, & Paz, 2010a; Trees that are grown in container systems are more likely to develop defective roots. Careful management or training is often needed to eliminate significant defects and improve root orientation. (Struve, 1993; Nelson, 1996). Tree roots in native tree stands typically grow radially away from the trunk. Some grow obliquely downward. With the exception of the taproot and sinker roots, roots generally Improving Root Distribution in Nursery Trees Mark A. Webber 3

4 don t grow vertically downward unless soil conditions permit. Correcting root defects in containerized planting stock can be challenging. Most root defects can be resolved with root pruning. I ve outlines 4 steps in training defective root balls or mitigating problems in existing trees. The first step in the training of tree roots is to inspect the root ball and to check for visible defects. It is often necessary to remove the potting soil placed over the root flare during propagation. The root flare is where the root originates from the trunk. It is critical that the tree is planted so that the root flare is slightly above grade. Be careful not to confuse the root flare with the trees bud union if the tree was grafted. The second step in container grown trees is to rem ove matted or circling roots at the periphery and bottom of the root ball. This process is called Root Shaving and can be done easily with a sharp nursery spade; hand pruning saw, hand-pruners, or battery powered saws-all (Gilman, Harchick, & Paz, 2010c). The vast majority of the circling kinked, bent, or descending roots are located in the outer regions of the container. The roots that form after shaving the root ball will grow radially outward from the root ball. In some cases, there are defects deep within the root ball that developed before trees were shifted to larger containers. These defects cannot be seen or corrected unless all the soil in the root is washed away. Although defects within the root ball can then be seen and sometimes corrected, this approach is not very practical. It would be more prudent to purchase planting stock from producers that consistently root prune when re-potting to larger containers. Photo Source ufl.edu/woody/removeside.shtml Improving Root Distribution in Nursery Trees Mark A. Webber 4

5 The third step involves repotting or planting the treated root ball (steps 1 & 2). Be sure to place the root flare at the soil line and do a final visual inspection of the root system for any additional defects that can be corrected before adding the potting mix or backfill soil. The final step is to mulch the area surrounding the root ball properly. Planting trees below grade or placing thick layers of mulch around the trunk can lead to root disease issues or favor the development of adventitious roots that can potentially girdle the stem. According to ANSI A300 (Part 8) Root Management Standard, stem girdling roots can often be cut, but removal can be difficult of cause significant injury, particularly if the roots are large or have fused to the trunk. Over mulched red oak (Quercus rubra) caused stem girdling roots to form. (Photo source MkWebber 2013) Improving Root Distribution in Nursery Trees Mark A. Webber 5

6 Longer Living Trees with Defect Free Roots The role of the professional tree and landscape specialists is to manage properly trees and other landscaping, both the roots and crown, to ensure plant health and longevity, thus increasing the benefits they provide to their clients and community. Arborists Spreading the Word about the Best Practices in the Production and Tree Installations It is incumbent on arborists to do more to promote the importance of inspecting the roots of planting stock as well as the roots of existing trees for defects, and mitigating significant problems whenever possible. Most homeowners and many within the landscape and nursery industries are unaware that root systems of containerized nursery trees often need to be managed for structural defects, commonly created during propagation. The simple 4-step Tree Root Training process, outlined above, can reduce and even eliminate root defects generated during nursery production, and help mitigate problems that were not addressed at the time of planting, or that developed later in the landscape. References: Anonymous. (2012). American national standard for tree care operationstree, shrub, and other woody plant management-standard practices (Planting and Transplanting). ANSI A300 (Part 6). Tree Care Industry Association, Inc. Londonderry, NH. Anonymous. (2013). American national standard for tree care operationstree, shrub, and other woody plant management-standard practices (Root Management). ANSI A300 (Part 8). Tree Care Industry Association, Inc. Londonderry, NH. Gilman, E. F., Harchick, C, & Paz, M. (2010a). Effect of container type on root form and growth of red maple. Journal of Environmental Horticulture, 28(1), 1-7. Gilman, E. F., Harchick, C., & Paz, M. (2010b) Planting depth affects root form of three shade tree cultivars in containers. Arboriculture and Urban Forestry, 36(3), Gilman, E. F., Harchick, C, & Paz, M. (2010c). Root ball shaving improves root systems on seven tree species in containers. Journal of Environmental Horticulture, 28(1), 13-18Gilman, E.F. (2012). An Illustrated Guide to Pruning (3rd ed.). Clifton Park, NY: Delmar. Improving Root Distribution in Nursery Trees Mark A. Webber 6

7 Hudler, G. W., & Beale, M.A. (1981). Anatomical features of girdling root injury. Arboricultural Journal, 5(2), Johnson, G. R., & Hauer R.J. (2000). A Practitioner s Guide to Stem Girdling Roots of Trees. St Paul, MN: University of Minnesota Extension Service. Lespinasse, J. M., & Delort, J.F. (1986). Apple tree management in vertical axis: appraisal after ten years of experiments. Acta Hortic, 160, Nelson, W. (1996). Container types and containerized stock for New Zealand afforestation. New Zealand Journal of Forestry Science, 26(1/2), About the Author: Mark A. Webber is an ISA Board-Certified Master Arborist with nearly 40 years of relevant professional experience. He is a nationally known practicing arborist and provides forensic investigations, reports, and testimony in matters related to tree maintenance and removal, plant value, herbicide injury, management of nursery and landscaping operations, and an assortment of horticultural issues. He can be contacted at mwebber@ webberlandscaping.com. Struve, D. K. (1993). Effect of copper-treated containers on transplant survival and regrowth of four tree species. Journal of Environmental Horticulture, 11(4), JANICK J Horticultural Science. 2nd ed. San Francisco: W.H. Freeman and Company. 586 p. Improving Root Distribution in Nursery Trees Mark A. Webber 7