Container and Landscape Planting Depth and Root Ball Shaving Affects Magnolia grandiflora Root Architecture and Landscape Performance

Nursery Planting Depth

Bending stress to winch trees up to five degrees trunk tilt 40 months after landscape planting was not impacted by planting depth into nursery containers (data not shown), similar to Quercus virginiana Mill. (Gilman and Weise 2012). Planting deeply into nursery containers had a negligible impact (3 mm less, Gilman et al. 2010a) on trunk diameter of these trees while in 170 L nursery containers, and had no impact on any measurement of growth (trunk diameter and tree height) up to 68 months after planting into the landscape (data not shown). Others have also shown a small impact of planting depth in nursery containers on two year’s post-planting growth (Bryan et al. 2010; Gilman and Wiese 2012). This may be the reason many growers have not recognized planting depth as a horticultural issue.

Despite no growth differences after landscape planting attributable to planting depth in nursery containers (data not shown), trees planted 128 mm deep (64 + 64 mm) into 57 L nursery containers and finished in 170 L containers had more roots growing tangent to and touching trunk bark at landscape planting (Gilman et al. 2010a) and 68 months after planting (Table 1) than trees planted shallowly in containers. This indicated that tangent and circling roots present at landscape planting persisted 68 months later. Root pruning by shaving the 170 L root ball at landscape planting did not affect this (root pruning × nursery planting depth not significant, P > 0.20), indicating that shaving did not correct root deformations on the interior of the root ball. Defects over the root collar must be remediated at planting by removing roots at the top of the root ball (Harris and Day 2010). Few roots appeared to emerge from the buried portion of the trunk or root collar, also noted on the same set of trees in 170 L containers at planting (Gilman et al. 2010a). Moreover, root circle rating 68 months after landscape planting was higher (indicating more circling roots) when trees were planted deeply in containers than shallow regardless of landscape planting depth (Table 1). This data adds to existing evidence that the root collar in nursery containers (Harris and Day 2010; Gilman et al. 2015) and in field nurseries (Hewitt and Watson 2009) should be close to the substrate surface, especially for trees not capable of forming new roots from the root collar after planting. In apparent contrast, certain species capable of generating adventitious roots along the buried stem when planted into field soil from propagation containers (Schwan 1994) perform best when planted a few cm deep. Young seedling-sized trees of those taxa can continue forming adventitious roots after planting—a capacity often lost among older trees planted from shade tree nurseries (Lyons et al. 1987).

Root Ball Shaving

Root ball shaving—which removed roots from the periphery of the root ball as trees were planted into the landscape—statistically decreased trunk diameter growth three (2011) and four (2012) years after planting, but the small differences (3 mm and 4 mm, respectively) may be of little practical importance (Table 2). Tree height increase the first year after planting was suppressed 10 cm as a result of root ball shaving; however, there was no difference in height increase after that (data not shown). Earlier studies measured impacts of root pruning on trees less than two years old at planting; some found reduced growth (Arnold and Young 1991; Arnold 1996), increased growth (Krasowski and Owens 2000), or were unaffected (Persson 1978). Tree age, soil, irrigation management, and weather likely account for at least some of the variation in results among these studies. Recent studies on larger, landscape-sized trees showed there was little impact on post-planting growth from root remediation when planting 57 or 170 L containers (Harris and Day 2010; Gilman et al. 2015). Combined with the current data, this suggests that practitioners can feel fairly confident that long-term growth will see little impact from root pruning done to remediate root defects at planting of these taxa and size under these conditions.

A more striking change occurred in root system architecture as a result of root ball shaving. The visual container imprint rating on the root system in response to root pruning was cut to one-third of that without root pruning (Table 2). A lesser imprint has been associated with improved root systems (Salonius et al. 2000; Weicherding et al. 2007; Gilman and Harchick 2014). In apparent contrast, percent trunk circumference with tangent or circling roots touching the trunk increased by threefold in response to shaving. This may have been a response to removing all roots at the periphery of the root ball, which invigorated existing roots proximal to the cut. Some of these roots were likely already growing tangent to and close to or against the trunk, although this was not measured. Invigoration of existing roots proximal to root pruning cuts has been found previously on other taxa (Watson and Sydnor 1987; Gilman and Kane 1990). Although not measured, no tangent or circling roots appeared to arise from new roots growing from cut root ends. Root ball shaving may be combined with root remediation at the top of the root ball to reduce occurrence of circling and tangent roots (Harris and Day 2010; Gilman et al. 2015).

Root pruning by shaving root balls at planting reduced bending stress (by approximately 15%) required to winch trees up to five degrees trunk tilt 40 months after landscape planting (Figure 2), and shaved trees had 10% greater rest angle (1.0 versus 0.9 degrees, Table 2) after winching to five degrees than trees not shaved. This suggests a measurable reduction in anchorage as a result of shaving exhibited by increased trunk rest angle. Winching to five degrees probably represented an extreme wind event rarely encountered in most landscapes according to winching tests calibrated to wind speed on slightly larger trees of another taxon (Gilman and Masters 2010). Previous studies on other taxa showed various root pruning treatments at landscape planting either increased anchorage by 10% to 15% when planting from containers of various sizes (11 L, Gilman and Paz 2014; 57 L, Gilman and Wiese 2012; 230 L, Gilman 2013) or had no effect (170 L, Gilman et al. 2015). Improvement in root architecture, expressed by a dramatic reduction in root system imprint (Table 2), likely outweighs—in the long-term—the rarely encountered downside of slightly less anchorage in an extreme weather event occurring 40 months after planting.

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Figure 2.

Bending stress required to winch trunks up to five degrees tilt 49 months after planting. Differences were significant for each angle at P < 0.05.

Root pruning increased root CSA growing up at more than a 20 degree angle for both landscape planting depths, but only for trees planted 64 mm deep in both nursery container sizes (total 128 mm deep, Table 3). Soil conditions for root growth from trees planted at shallower depths may have been suited for horizontal growth; lower oxygen availability deeper in the profile may have been the impetus for more roots growing up at an angle (Coutts and Nicolle 1991). This response from root pruning could enable establishment under certain soil conditions by quickly generating roots closer to the soil surface compared to trees not able to generate surface roots quickly.

Landscape Planting Depth

Like Ulmus parvifolia Jacq. from a container (Bryan et al. 2010) and Corylus colurna L. from a field nursery (Day and Harris 2008), landscape planting depth had a negligible effect on magnolia trunk growth and height growth in the 68 months after planting (Table 4). In contrast, other researchers (Lyons et al. 1987; Arnold et al. 2007) found reduced trunk diameter growth with increasing landscape planting depth when installing container-grown trees. However, landscape planting depth affected root system radius and root to shoot spread ratio (Table 4). Eight months after planting (May 2009), roots on magnolia that were planted 5 cm high in the landscape extended farther into the landscape than those planted deeply, generating a larger root to shoot spread ratio. But 14 months after planting, roots were beyond the edge of the crown (119 cm between trunk and root tips), and the difference between treatments was no longer significant (data not shown). Depth to the root collar was seven times greater for trees planted deeply in the landscape than shallow for all combinations of planting depth in nursery containers (Table 4); this was not surprising given the greater landscape planting depth. This strongly suggests that magnolia did not have the capacity to generate roots from the root collar after landscape planting under conditions of this study. Others showed that roots ceased growing from the collar as trees became 24 to 60 months old (Coutts et al. 1983; Lyons et al. 1987; Gilman and Harchick 2008).

Shallow roots growing tangent to the trunk over the main roots have been associated with reduced health on certain taxa attributable to setting the root collar deeply into the landscape (Wells et al. 2006), although other taxa have not been affected (Wells et al. 2006; Gilman and Grabosky 2011). Fewer roots in the current study grew tangent to or circled the trunk when trees were installed deeper in the landscape (Table 4). A similar experiment in the same soil with Quercus virginiana (Gilman and Grabosky 2011), and other studies (Wells et al. 2006; Day and Harris 2008), clearly showed the opposite; i.e., more roots growing over the root collar when planting deep than shallow. Total CSA of roots growing up from deeper in the soil at more than a 20 degree angle increased nearly threefold when trees were installed deep (10 cm) compared to shallow (5 cm high) in the landscape (Table 4). Perhaps roots growing up from the deeper portion of the root ball suppressed growth of surface roots close to the trunk, which could explain the fewer tangent roots growing over the root collar. Roots assembled at the bottom of a container root ball are often the oldest and largest (Gilman and Paz 2014), representing an established xylem connection to the root collar and trunk tissue. Therefore, these would be the roots expected to grow aggressively once planted, at least initially.

Trees planted 5 cm above landscape grade were slightly—but significantly—less stable in the landscape than trees planted deeper (10 cm deep), as indicated by a 35% larger rest angle following release of the winch after pulling to five degrees trunk tilt (Table 4). This suggested a slight shift of roots in the soil as a result of winching 40 months after landscape planting. The entire root ball sides on deeply planted trees rested against landscape soil providing greater resistance to winching than trees planted with 5 cm of the ball above landscape soil level; the soil over the root ball on deeply planted trees would add to root plate mass and therefore overturning resistance (Danjon et al. 2005). In contrast, the top 5 cm of the ball on shallowly planted trees would be pushing against the remnants of mulch, which would provide less resistance to winching than soil. However, the hinge point was farther from the trunk (18 cm) for trees planted high (5 cm above landscape grade) than those planted 10 cm deep (12 cm). The greater distance between trunk and hinge point suggests that anchorage could increase as trees grow (Danjon et al. 2005). In support of the hypothesis that trees can become better anchored with time, Gilman and Grabosky (2011) found Quercus virginiana (1.5 cm trunk caliper) installed at or slightly above grade were as well anchored 72 months later as trees planted 15 cm deep into field soil from 95 L containers. Others have also found reduced differences in anchorage between planting treatments as roots compensate over time (Nichols and Alm 1983).