Planting Depth and Mulch Thickness Affect Establishment of Green Ash (Fraxinus Pennsylvanica) and Bougainvillea Goldenraintree (Koelreuteria Bipinnata)

Green Ash

Green ash did not exhibit any significant (P ≤ 0.05) interactions between planting depth and mulch thickness involving time after planting for height or trunk diameter (Table 1). Height growth responses for green ash were significant (P ≤ 0.05) only for main effects of time, mulch, and planting depth, with no two- or three-way interactions significant for this variable (Table 1*). Across planting depths and mulch treatments, mean green ash trunk diameter increased for all 3 years after transplanting, while height growth increased slowly during the first 2 years and more rapidly in the third (Table 1).

The equal or greater increase in height and trunk diameter after cessation of irrigation in the third year suggests that the trees were fully established in the landscape. This finding would be consistent with Gilman’s (1997) model of establishment in USDA plant hardiness zone 8b. With the exception of the 7.6 cm (3 in.) below-grade planting depth treatment, mulching had a consistent negative effect on green ash height growth when pine bark mulch depths were maintained at greater than a 7.6 cm (3 in.) depth (Table 1). The apparent inconsistency of no reduction in height or trunk diameter due to mulch thickness when green ash were planted below grade may have been an artifact of the much lower survival of trees that were planted below grade and then mulched. Perhaps only the more vigorous trees survived, biasing the growth response to this treatment because the smaller, less vigorous trees in the mulch treatments died in the second and third years of the study.

Mean height after three growing seasons for green ash was reduced for trees transplanted 7.6 cm (3 in.) below grade (156 cm [61 in.]) compared to those planted at grade (166 cm [65 in.]), and planting 7.6 cm (3 in.) above grade slightly increased height growth (172 cm [68 in.]).

Likewise, the proportion of the canopy exhibiting transplant stress symptoms during the first year following transplanting (Table 2) increased by 12% to 26% for plants with mulch depths of 7.6 cm (3 in.) or greater. Gilman and Grabosky (2004) reported that 7.6 cm (3 in.) thick hardwood mulch treatments impeded light irrigation or rainfall events from penetrating to the planted root balls, but they found no effect of planting depth on container-grown live oak trunk diameter at 7 months after transplant. This apparent lack of response during the first growing season for live oaks is not inconsistent with the results of green ash in the current study because differential growth and survival responses to planting depth were minimal for green ash during the first growing season, but they were manifested more strongly the second and third years after transplant (Table 1).

Significant (P ≤ 0.05) two-way interactions among planting depth and time after transplant, mulch thickness and time after transplant, and planting depth and mulch thickness were present for green ash survival (Table 1). Green ash survival declined much more rapidly over time after transplant for trees planted below grade than those at 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 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. Likewise, Gilman and Grabosky (2004) found few advantages to transplanting live oaks in a sandy soil above grade, suggesting 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.