Journal of Arboriculture 31(4): July 2005 163 PLANTING DEPTH AND MULCH THICKNESS AFFECT ESTABLISHMENT OF GREEN ASH (FRAXINUS PENNSYLVANICA) AND BOUGAINVILLEA GOLDENRAINTREE (KOELREUTERIA BIPINNATA) By Michael A. Arnold1 , Garry V. McDonald2 Abstract. Most available information on the effects of planting depths for trees is anecdotal. Testing of interactions with other important cultural practices implemented during landscape establishment is lacking. Green ash (Fraxinus pennsylvanica H. Marshall), a hypoxia-tolerant species, and bougainvillea goldenraintree (Koelreuteria bipinnata A.R. Franchet), a hypoxia- intolerant species, were grown from seed in 2.3 L (#1) containers, which were later transplanted to 9.3 L (#3) black plastic containers and grown to a marketable size. Root collars of the plants were maintained level with the surface of the substrate. Green ash (1 May 2001) and bougainvillea goldenraintrees (27 April 2001) were transplanted to clay soil field plots with the root collars at 7.6 cm (3 in.) above soil grade, at grade, or 7.6 cm (3 in.) below grade. Planting depths for each species were in factorial combinations with 0, 7.6, 15.2, or 22.9 cm (0, 3, 6, or 9 in.), respectively, of pine bark mulch covering 0.74 m2 (8 ft2 ) of soil beneath each tree. After 2 years, survival of bougainvillea goldenraintrees planted below grade was one-third that of those planted at or above grade. Survival and trunk diameter responses interacted with planting depth and mulch thickness for green ash. Planting below grade in combination with mulching reduced survival of green ash 25% to 50% after 3 years. Mulch applications reduced trunk diameters of green ash trees planted at or above grade. Mean soil water potentials were less negative with 7.6 cm (3 in.) of mulch (–5.8 kPa) compared to bare soil (–9.1 kPa), but increasing the mulch thickness to 22.9 cm (9 in.) inhibited penetration of irrigation water/rainfall (–16.2 kPa). These data suggest that planting with the root collar at or above grade enhances survival and growth potential of green ash and bougainvillea goldenraintree relative to planting below grade and that mulch applications should be only at thin layers sufficient to inhibit weeds. Bougainvillea goldenraintrees were more susceptible to the adverse effects of below-grade installation and excess mulch applications than were green ash trees. Key Words. Fraxinus pennsylvanica; hypoxia; Koelreuteria bipinnata; pine bark mulch; planting depth; transplant establish- ment; tree planting. Planting depth information is mostly anecdotal (Watson and Himelick 1997). One of the more common reasons given for planting trees deeply is to avoid windthrow. Likewise, trees are sometimes intentionally planted below grade to lessen , and Donita L. Bryan3 root conflicts with infrastructure (McPherson et al. 2001). Those studies that have been conducted on planting depth yielded somewhat variable results depending on the soil type, type of planting stock, length of study, and tree species in question (Drilias et al. 1982; Browne and Tilt 1992; Broschat 1995; Broschat and Meerow 2000; Arnold and McDonald 2002; Gilman and Grabosky 2004). In general, the heavier the soil and more susceptible the species to hypoxia (reduced oxygen levels) or anoxia (lack of oxygen), the more likely were plants with root collars below the soil line to grow poorly. Growth and survival of balled-and-burlapped sugar maple (Acer saccharum H. Marshall) and pigmy date palm (Phoenix roebelenii J. O’Brien) were adversely impacted by planting the root collars below grade (Drilias et al. 1982; Broschat 1995). In contrast, Browne and Tilt (1992) reported improved survival and growth during the first year after transplant of red maples (Acer rubrum L.) planted 5.1 to 10.2 cm (2 to 4 in.) below grade in a clay–loam soil and found no adverse effects with root collars as much as 15.3 cm (6 in.) below grade. Browne and Tilt (1992) did however report first-year growth reductions on the same site for Virginia pines (Pinus virginiana) and flowering dogwoods (Cornus florida L.) planted below grade. Planting stock type and size of stock were not reported. Gilman and Grabosky (2004) reported only transitory drought stress on 7.6 cm (3 in.) caliper, container-grown Quercus virginiana planted below grade in a sandy soil, with subsequent reduced drought stress on below-grade trees later during the first growing season. Although this study used larger trees than the other studies, it was of very short duration, including only data for the first 7 months after transplant. Repeated transplanting of trees to successively larger containers or repeated mounding of soil in field nurseries can lead to trees that are produced with root collars significantly below the substrate surface in the marketed container or dug root ball (Ball 1999). In addition, homeowners and landscape installers tend to plant trees with the root collars at a greater depth than the existing soil ©2005 International Society of Arboriculture
July 2005
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