166 Arnold et al.: Planting Depth and Mulch Thickness grade or above grade if they received any mulch application (Table 1). Differential responses increased over time after planting. After three growing seasons in the field, planting a mere 7.6 cm (3 in.) below grade in combination with mulching resulted in as much as 60% greater losses with green ash, a species often thought to be tolerant of urban soils (Arnold 2002). Planting green ash above grade resulted in slight improvements in height and trunk diameter growth but did not affect survival compared to those planted at grade (Table 1). Broschat and Meerow (2000) indicated that deep planting of palms (family Arecaceae) often resulted in reduced survival and root growth, increased water stress symptoms, and iron or manganese deficiency symptoms in the canopy. This finding was in agreement with Browne and Tilt’s (1992) short-term results with Virginia pines (Pinus virginiana) and flowering dogwoods (Cornus florida) but contrasted with their reported improved growth and survival of red maples (Acer rubrum L.) planted 5.1 to 10.2 cm (2 to 4 in.) below grade and a lack of adverse effects with red maples when root collars were as much as 15.3 cm (6 in.) below grade. Pine bark mulch depths of as little as 7.6 cm (3 in.) decreased green ash survival over time (Table 1). Differential responses in green ash survival to mulch thickness appeared to increase over time (Table 1). Mulching accentuated losses associated with planting the root collar below grade. Trees planted above grade had only very slight decreases in survival associated with increasing mulch thickness (Table 1). When planted at grade, green ash survival was similar regardless of mulch thickness, although growth was reduced. It is unclear why survival of trees planted below grade appeared to be poorest at intermediate rather than the more excessive mulch applications. Interestingly, this result is essentially an inverse response pattern to that of soil water potential when averaged over time, which was numerically less negative (moister) with 7.6 cm (3 in.) of pine bark mulch (Table 3) but then became statistically more negative (drier) as mulch thickness increased to 22.9 cm (9 in.). Perhaps greater mulch application was inhibitory to penetration of irrigation water and/or natural precipitation, which would be in agreement with Gilman and Grabosky’s (2004) results when light irrigation was used in combination with hardwood bark applications. Bougainvillea Goldenraintree In general, adverse growth responses to below-grade planting were more accentuated with bougainvillea goldenraintree than with green ash (Tables 1 and 2). Averaged across mulch thicknesses, two-way interactions (P ≤ 0.05) were present with bougainvillea goldenraintree among planting depth and time after transplant for trunk diameter, height, and survival (Table 1). Trunk diameter increased for trees transplanted at grade, and initially for ©2005 International Society of Arboriculture trees transplanted 7.6 cm (3 in.) above grade (Table 1). Bougainvillea goldenraintrees planted 7.6 cm (3 in.) below grade had only a slight increase in trunk diameter during the two growing seasons after transplant. Averaged across mulch thicknesses, height growth of bougainvillea goldenraintree was reduced by planting either above or below grade compared to planting at grade (Table 1). Some dieback, perhaps due to tip dieback in a later winter freeze, occurred in all bougainvillea goldenraintree treatments. Survival of bougainvillea goldenraintree was not as strong as with green ash, across treatments (Table 1). By the end of the second year, survival decreased so severely in bougainvillea goldenraintrees that were planted below grade that the experiment with this species was not continued in the third year. While planting above grade with green ash only slightly improved survival compared to trees planted at grade, transplanting bougainvillea goldenraintrees 7.6 cm (3 in.) above grade improved survival by as much as 40% as long as no mulch was applied (Table 1). This result may be a reflection of the poorer tolerance of bougainvillea goldenraintree to poorly drained soils (Arnold 2002). Although planting slightly above grade appeared to be beneficial in our experiments, Broschat and Meerow (2000) reported that palms planted with root initiation zones as little as 2.5 cm (1 in.) above grade can exhibit problems with root desiccation before they can grow into the soil. Like- wise, Gilman and Grabosky (2004) found few advantages to transplanting live oaks in a sandy soil above grade, suggest- ing that testing among a range of species is needed on a variety of soil types before sweeping recommendations can be provided. In general, pine bark mulch tended to reduce survival (Table 1) and increase symptoms of foliar stress (Table 2) of bougainvillea goldenraintrees, particularly when trees were planted above or below grade, rather than at grade. No termite activity was observed within the mulch during the course of this study. CONCLUSIONS In general, planting either green ash or bougainvillea goldenraintree as little as 7.6 cm (3 in.) below grade was often detrimental to survival. Planting above grade by 7.6 cm (3 in.) improved survival of bougainvillea goldenraintree compared to at-grade planting more so than with green ash as long as mulch levels were not excessive. Advantages to planting above grade were minimal in terms of height or trunk diameter growth for green ash over 3 years and bougainvillea goldenraintrees over 2 years, but planting below grade was very detrimental to survival if done in combination with pine bark mulch applications. Survival of both species in this study was usually best when bare soil was used, compared to applications of pine bark mulch. However, these plots were diligently maintained in a weed-free condition via careful hand weeding (no string
July 2005
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