Arboriculture & Urban Forestry 35(4): July 2009 structural roots 7.5 to 30 cm (3 to 12 in) deep in the root ball. Both Maynard’s report and an earlier report by Berrang et al. (1985) attributed excess soil over the root collar to soil cultiva- tion for weed control in the nursery. More recently, changes that occur to the structural root architecture during nursery produc- tion have also been identified as a possible cause (Hewitt and Watson 2009). Recent studies in Indiana, Ohio, North Carolina, and Tennessee (U.S.) indicate that the average depth of struc- tural roots in nursery fields and harvested root balls ranges from 6 to 21 cm (2.5 to 8 in) (Airhart 2005; Rathjens et al. 2008). Depths of individual trees were not provided in these reports, but it is clear that many trees leave nurseries “deep in the ball.” Deep Structural Roots in the Landscape Although several observational studies indicate that deep struc- tural roots are prevalent in landscapes, each study is essentially a snapshot of a particular situation, and there is not sufficient in- formation to identify either upward or downward trends in the prevalence of deep structural roots. Inspection of a planting site in the Chicago area in 1989 found 28% of recently planted trees had the first two structural roots at depths greater than 7.5 cm (3 in) while an inspection at a second site in 2004 found 63% of trees with roots at these depths (Hewitt and Watson 2009). Employees of Bartlett Tree Research Laboratories (Charlotte, NC, U.S.) excavated 417 recently planted trees and found 93% had buried root collars, defined as the root flare not being vis- ible above the soil. Average depth to first root varied from 7.5 cm in Charlotte, NC and Cape Cod, MA, to slightly over 15 cm (6 in) in Long Island, NY (Smiley 1991). In a Minnesota study of street trees with trunk diameters of 7.5–22.5 cm (3–9 in), 20%–58% of trees had the uppermost structural roots more than 7.5 cm below ground (Giblin et al. 2005). None of the studies examine the relative contribution of nursery production methods and planting practices to deep structural roots in installed trees. It is not clear how deep roots became so prevalent in landscape tree nurseries and in established trees, but propagation practices and reforestation techniques may have contributed. In reforesta- tion plantings, it is common practice to plant seedlings [usually 20–30 cm (8–12 in) tall] much deeper than they existed in the nursery, sometimes with only the terminal bud above ground (Slocum and Maki 1956; McGee and Hatcher 1963; Stroempl 1990), unless the site is poorly drained (Koshi 1960; Switzer 1960). Deep planting has been shown to increase survival and growth in these circumstances, presumably because of better ac- cess to moist soil and protection of the collar. “Deep” planting of such small stock, however, usually results in roots being placed only a few centimeters deeper than normal, and is therefore not comparable to deep structural roots in landscape-sized trees. In addition, this practice with these very young plants may be com- parable to the nursery propagation technique of rooting stem cut- tings, where stem tissue is plunged several centimeters deep into propagation media to initiate rooting. In this practice, root pri- mordia usually form in the phloem parenchyma cells of the stem (Hartmann et al. 2002). In some species, more mature stock can lose this ability to produce new roots from the stem. Small rooted cuttings (<1 year old) of live oak (Quercus virginiana) retained their ability to produce new roots from the stem, but the same plants did not produce new roots from the stem a year later when buried deeper in container media (Gilman and Harchick 2008). 183 Other species, such as coast redwood (Sequoia sempervirens), can retain this ability throughout their life (Stone and Vasey 1968). Other specialized horticultural practices also use deep plant- ing to achieve specific results. For example, trees in fruit or- chards may be planted with the roots up to 20 cm (8 in) deep so that the graft (bud) union can be buried and consequently pro- tected from desiccation and physical damage (Lyons and Yoder 1981). These authors noted that changes in grafting technique in which trees were budded 13–18 cm (5–7 in) up from the root collar compared to earlier practices of < 5 cm (2 in) resulted in deeper planting when growers used the bud union as a reference point for planting depth. Deep planting will slow growth of fruit trees (Lyons et al. 1982; Lyons et al. 1983), and is sometimes done deliberately for this purpose (Lyons et al. 1987). Although it is not recorded in the published literature, these historically common forestry and horticultural practices may have served as precedent for structural roots being placed deep during nurs- ery production and in subsequent planting into the landscape. Professional Response to Deep Structural Roots In the early 2000s, industry concern over the effects of deep struc- tural roots in urban and suburban landscapes reached a critical level. Because deep structural roots are so prevalent, it is assumed that problems associated with these buried roots may ultimately have widespread and long-lasting effects on urban forest health and safety. Municipalities, in particular, rely on transplanting large trees (>2 m in height) to build their urban forests (Harris 2007). Unlike reforestation seedlings and naturally occurring seedlings, there are multiple opportunities for large planted trees to have their structural roots placed too deep in the soil, both during multiple steps in nursery production and at transplanting. Because the long- term health of these trees is critical to achieving survival and can- opy coverage goals, a national green industry working group was formed in the United States to address the issue (Watson 2005b). Green industry professional organizations, including the Ameri- can Nursery and Landscape Association, the American Society of Landscape Architects, the International Society of Arboriculture, and the Associated Landscape Contractors of America (now part of the Professional Landcare Network) opened dialogue concern- ing prevention of deep structural roots in landscape trees. During the same period, language was added to the American Standard for Nursery Stock (ANSI Z60.1) addressing root flare depth of balled-and-burlapped nursery stock (ANSI Z60.1 American Stan- dard for Nursery Stock 2004). As a result of the working group, a Best Management Practice (BMP) booklet for identifying deep root systems in nursery stock was developed (Watson 2005b). Another component of this initiative has been a coordinat- ed effort to fill knowledge gaps through research. The Morton Arboretum (Lisle, IL, U.S.) convened a meeting of research- ers in November 2005 (Watson 2005a) to open communica- tion between researchers investigating aspects of deep root problems. This meeting was funded in part by the USDA For- est Service upon the recommendation of the National Urban and Community Forestry Advisory Council. New research continues to emerge and, partly as a result of this initiative, information is now available that may reshape our views on deep structural roots and the implications for arboriculture. ©2009 International Society of Arboriculture
July 2009
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