Survival of a newly transplanted tree or shrub is dependent on the development of a root system that extends into surrounding native soil. Until this occurs, the plant must rely on the water and nutrients in the transplanted container substrate, which may be extremely limited as a result of the tendency of texture and moisture gradients to pull moisture and nutrients from the root ball into the surrounding soils (Heiskanen and Rikala, 2000).
Newly transplanted shrubs may also struggle with the specific environment into which they are planted. Normal undisturbed soils have established horizons, ample pore space and organic matter, beneficial organisms, and soil aggregates that allow air, water, and nutrients to penetrate (Perry, 1982). Urban areas, where most shrubs are planted as part of a larger landscape installation, often lack the natural topsoil, and what remains is often alkaline with high clay content. These areas may also have restricted space for root growth, poor aeration, and inadequate drainage, making them less-than-optimal environments for root growth (Craul, 1985). Once a landscape is installed, regular maintenance procedures often remove leaves and plant waste, depriving the soil of a natural litter layer and incorporation of degrading organic matter. Application of mineral fertilizers may compensate chemically, but there may still be a lack of physical benefits and microbial life from organic matter.
A common practice intended to improve transplant success is amending the backfill of the planting hole with organic matter, but studies show inconsistent results with this technique (Day et al., 1995; Ferrini et al., 2005; Smalley and Wood, 1995; Watson et al., 1993). Planting above soil grade (above-grade) has proven to be a very successful technique both with trees (Arnold et al., 2005) and shrubs (Wright et al., 2007). Wright et al. (2007) described a modified above-grade planting technique in which the upper 7.6 cm of the root ball remained above the soil surface and pine bark was mounded around the above-grade portion of the root ball, tapering from the top of the root ball down to the surrounding soil grade. More root growth occurred in shrubs planted with this technique than in those planted at grade with pine bark-amended backfill.
Use of native plants in the landscape is often promoted because these species may be better suited to the soils and climate of their native region than nonnative species. Although native species may not perform well in the poor transplanting situations described here, their climatic adaptations may make them more successful than nonnatives while requiring fewer inputs of fertilizer and water (Southeast Exotic Pest Plant Council, 2008). This study intends to expand on the successful specialized above-grade planting practice of Wright et al. (2007) by experimenting with three native woody shrubs chosen for their varied native habitats and transplanting success (Dirr, 1998) and other types of organic matter chosen based on current practices and availability. Therefore, the objective of this study was to determine the effect of organic matter type on posttransplant root growth of three native landscape shrubs in simulated above-grade planting conditions.
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