The Effects of Planting Depth on Windthrow, Stability, and Growth for Four Tree Species in Containers

Windthrow

Windthrow was not a major problem for any of the species or treatments (data not shown). Three oak and one birch tree chronically windthrew. Other windthrow events occurred during the study but were infrequent and random. There was no statistically significant reduction in the occurrence of windthrow when these tree species were planted with their first main-order roots deeper in nursery containers. Reduction of windthrow frequency in nursery container production is more likely a result of staking practices than planting depth.

Lean Frequency

There were no significant (P < 0.05) effects of planting depth on lean frequency in any of the three species measured (Table 1). In birch, there was a significant (P < 0.05) interaction between planting depth and time after planting, implying that, while the effect of planting depth on lean frequency was not statistically significant, time after planting had varying effects on trees planted at different depths (Figure 1A). In ash and crabapple, there were no significant (P < 0.05) interactions between planting depth and time after planting for lean frequency (Figure 1B; Figure 1C). Lean measurements on the bicolor oaks were terminated early in the study due to excessive stem bending, which made accurate and objective measurements impossible.

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Figure 1.

Lean frequency (percent leaning) observed in three tree species: Whitespire birch (Betula platyphylla var. japonica ‘Whitespire’), green ash (Fraxinus pennsylvanica), and Snowdrift crabapple (Malus × ‘Snowdrift’), planted at four depths in nursery containers over the course of 17 weeks (n = 5).

Average Lean

There were no significant (P < 0.05) interactions between depth and time after planting for any of the species measured. The overall effect of depth was significant (P < 0.05) in birch, with trees planted at 0 cm leaning significantly more than those planted 15 cm deep (Table 1). At its greatest, the significant difference in lean resulted in a tree that is straighter by 4.7 degrees (Figure 2A). In ash and crabapple, there was no significant (P < 0.05) effect of planting depth on average lean (Figure 2B; Figure 2C).

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Figure 2.

Average lean in centimeters off plumb for three tree species: Whitespire birch (Betula platyphylla var. japonica ‘Whitespire’), green ash (Fraxinus pennsylvanica), and Snowdrift crabapple (Malus × ‘Snowdrift’), planted at four depths in nursery containers over the course of 17 weeks (n = 5).

While the total number of trees leaning is important in a production setting, the amount that each tree leans (centimeters off plumb) is also critical in proper management of nursery stock and the production of a marketable product. These results show that planting depth had no significant effect on the number of trees leaning, and that the angle of lean is significantly reduced by planting 15 cm deep only in birch trees.

Tolerance of deep planting in hypoxia-tolerant or floodplain tree species, like some species of birch and ash, has been suggested by Arnold et al. (2005) and Gilman and Harchick (2008); however, the Whitespire birch used in this study is considered to be relatively flood-intolerant, especially when compared to a common floodplain species like green ash. These results in birch are difficult to explain. One possible explanation might be found in the amount of wind resistance of different tree species. Larger trees, and those with a denser canopy, may also offer more wind resistance and thus have more potential for lean in container production. Further research in this area would be very beneficial, as tree stability is usually considered to be a function of root development. Additionally, a survey into perceptions of acceptable lean in containerized nursery stock would quantify the needs and desires of both the grower and consumer of containerized nursery stock.

Growth Rate

Tree caliper differed significantly only in ash, where caliper increase was reduced in deeply planted trees (Figure 3). There were no significant caliper increase differences in birch, crabapple, or oak.

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Figure 3.

Mean stem caliper increase in centimeters in four tree species planted at four depths in nursery containers. Note: Within species, bars with the same letter are not significantly different at P < 0.05.

There was no significant difference in root volume increase in ash, crabapple, or oak (Figure 4). In birch treatments, however, trees planted at 0 and 5 cm depths had significantly greater root volume increase than those planted 10 and 15 cm deep. Annual shoot new growth did not differ significantly among any species or treatment (data not shown).

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Figure 4.

Mean root volume increase measured by water displacement in milliliters in four tree species planted at four depths in nursery containers. Note: Within species, bars with the same letter are not significantly different at P < 0.05.

While some research has noted how planting deeper in the field can increase stability and tree survival (Parry 1974; Stroempl 1990), this generally results in decreased stem caliper and root growth (Lyons and Yoder 1981; Lyons et al. 1983; Lyons et al. 1987; Arnold et al. 2005; Arnold et al. 2007). The data generated from this study showed a significant reduction in stem caliper increase in ash when trees were planted with any amount of soil over the first main-order root. Bryan et al. (2010) observed a similar, statistically significant reduction in stem growth (dry mass) in container-grown Ulmus parvifolia when root systems were planted 5 cm too deep in 10.8 L nursery containers. In contrast, Gilman and Harchick (2008) found that container-grown Cathedral live oak (Quercus virginiana ‘SLDN’) showed a significant decrease in stem caliper growth at the most shallow planting depth treatment. It is important to note that these works (Bryan et al. 2010; Gilman and Harchick 2008) focused on container-grown nursery stock, whereas the present study was based on finishing field-grown, bare-root stock in a container during a single growing season. The establishment, stability, and growth rate of different trees grown in different production systems may vary greatly.

The significant reduction in root volume increase in birch, with more than 5 cm over the first main-order root, suggests that planting trees deeply in containers may reduce vigor and growth rate of the root system. Furthermore, lower root volume increases may indicate slower establishment and poor anchoring in the container. These trends in data are consistent with data presented from a nine-year study planted at the University of Minnesota (U.S.) examining the effect of deep planting on field-grown nursery stock (Giblin et al. 2005).

The reduction in lean, which was observed in this study in one tree species (birch), does not provide a justification for planting all trees deeply. However, the potential for reduced stem caliper and/or root growth in deeply planted trees, as well as the increased potential for SGRs to develop, does support a shallower planting.

One step to reduce the probability of trees being buried too deeply in the landscape and the associated development of abnormal and/or dysfunctional root systems would be to remove the deeply planted nursery stock from the retail and wholesale market. The only reasonable way to accomplish this involves inclusion of a planting depth component in the ANLA standard for trees produced in containers.

If planting deeply is perceived to reduce or avoid windthrow or lean in the nursery (Arnold et al. 2005) and becomes the common practice, then to follow the example of ANLA standard for preparation of B&B stock, growers of containerized stock would be required to remove all excess soil over the first main-order root in the root ball. Because nursery stock production is dominated by containerized and container-grown stock, the root depth in those finished products must be constructed to be consistent with an existing ANLA standard for field-grown nursery stock.

One of factors currently inhibiting the implementation of planting depth guidelines in production of containerized nursery stock is the ability of nursery growers to identify first main-order roots. In some fields, such as forest ecology, researchers are challenged to define, or even identify, what is commonly called the “root collar” (Gutsell and Johnson 2002). Gutsell and Johnson (2002) and Fahn (1990) both define the root collar as the point at which root tissue transitions to stem tissue. This is, specifically, “the shift in the central pith, from undifferentiated parenchyma cells in the stem, to a vascular cylinder in the root” (Gutsell and Johnson 2002). Precise, laboratory methods of identifying the root collar have been elucidated; however, destruction of the tree is required for positive identification. Further work must continue to better define these structures morphologically, allowing easy and consistent identification outside the lab.

Container size, substrate composition, and the physical characteristics of different tree species may affect windthrow and tree stability in the production of containerized nursery stock. This research, however, focuses on commonly used tree species, container size, and container substrate at Bailey Nurseries, a nationally (U.S.) recognized wholesale production nursery. Additional research examining different tree species grown in different sized containers and with different substrates will add to this growing body of work. This work should further address the issues of tree stability, planting depth, and equitable ANLA nursery stock standards.