Arboriculture & Urban Forestry 37(5): September 2011 Arboriculture & Urban Forestry 2011. 37(5): 247–253 247 The Effects of Planting Depth on Windthrow, Stability, and Growth for Four Tree Species in Containers Chad P. Giblin, Jeffrey H. Gillman, Gary R. Johnson, David Hanson, and Patrick J. Weicherding Abstract. Establishing the effects of planting depth on tree stability and growth is critical in understanding the role nursery production plays in plant- ing depth issues at the landscape level. In this study, bare root Whitespire birch (Betula platyphylla var. japonica ‘Whitespire’), green ash (Fraxinus pennsylvanica), Snowdrift crabapple (Malus × ‘Snowdrift’), and bicolor oak (Quercus bicolor) were grown for 17 weeks in a container production setting with four levels of substrate over the first main-order root: 0, 5, 10, and 15 cm. Birch demonstrated the greatest instability of all species, leaning significantly more when planted at 0 cm than at 15 cm. In ash and crabapple, there were no significant differences in either the number of trees leaning or the amount of lean in all treatments throughout the study. Oak stems bent excessively, invalidating lean measurements. Stem cali- per increase was significantly greater in ash planted 0 and 5 cm deep than 10 and 15 cm deep. There was no significant difference in stem cali- per increase between planting depths in other species. Birch planted 0 and 5 cm deep had greater root volume increase than those planted 10 and 15 cm deep. Root volume increase in ash, crabapple, and oak did not differ significantly between treatments. Infrequent windthrow events were observed, but appeared random and apparently unrelated to planting depth. The perceived benefit of planting trees deep in containers to im- prove stability was observed in only one species (birch) at one depth (15 cm) and was at the expense of significantly reduced root volume increase. Key Words. Betula platyphylla var. japonica ‘Whitespire’; Fraxinus pennsylvanica; Malus × ‘Snowdrift’; Quercus bicolor; Stem-encircling Root; Stem-girdling Root. Container production of nursery stock is increasing in the United States. It is currently the dominant nursery production system, comprising about 50% of total nursery sales (USDA 2007). While offering production and merchandising advantages to the nursery grower, producing trees in containers has many chal- lenges, including planting depth issues, plant stability, root de- formities, root circling, stem-encircling roots (SERs), and stem- girdling roots (SGRs) (Harrington et al. 1989; Watson et al. 1990; Appleton 1993; Appleton 1995; Johnson and Hauer 2000). Several studies have examined tree stability and SGRs in field and landscape settings. Much of the early work focused on in- vestigations into poorly anchored or windthrown fruit trees. Ly- ons and Yoder (1981) found that deeply planted peach trees were generally smaller and more prone to instability and failure in wind loading events, even three to five years after planting, than trees with a first main-order root nearer the soil surface. They cautioned that the root system – not the graft union – should de- termine planting depth of nursery stock. Subsequent work (Lyons et al. 1982; Lyons et al. 1983; Arnold et al. 2005; Arnold et al. 2007) examined the effects of burying the first main-order root too deep on overall tree performance. This work showed, in many cases, how growth rate, stability, winter hardiness, and general vigor were improved if trees were planted with the first main-or- der root within 5 cm of the soil surface. Furthermore, Wells et al. (2006) reported a 50% mortality rate in deeply planted Yoshino cherry trees while all the control trees (planted at grade) survived. Stability of newly planted or juvenile trees seems to be the driving force that compels growers to plant trees deeply (Arnold et al. 2005). Staking trees has been shown to have negative effects on tree growth rate and stem taper (Eckstein and Gilman 2008), so alternative stabilizing methods are desirable. Consequently, it is not surprising that improved stability and reduced windthrow (without mechanical support) is a common rationale for planting trees deeply (Lyons et al. 1983; Arnold et al. 2007). However, not all researchers have had deep planting result in the positive effects desired by producers. Lyons et al. (1987) found that the influence of deep planting had exactly the opposite effect. Ap- ple trees with first main-order roots planted at shallower depths (approximately 5 cm) did not lean significantly more than those planted deeper (approximately 15 to 25 cm). Their research con- cluded that fruit growers would receive no benefit in tree stability by planting deeply. Lyons et al. (1987) recommended that pro- ducers of fruit tree nursery stock, when propagating, should place buds no more than 15.2 cm above the first main-order root, thus reducing temptation for the end-user to plant deeply. Soil oxygen levels may play a role in the development of SERs and SGRs. Partyka (1982) suggests that decreased soil oxygen concentration, especially when associated with poor or disturbed soils in urban areas, can result in root death and tree decline. Wells et al. (2006) speculated that roots growing ver- tically in deeply planted red maples may grow close to buried stems because of increased oxygen levels found there, creat- ing growth patterns that favor SERs and SGRs. Santoso (1987) also reports that mycorrhizal development was significantly reduced with increasing planting depth, further reducing chances of successful tree establishment and long-term survival. ©2011 International Society of Arboriculture
September 2011
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