Planting Depth and Mulch Thickness Affect Establishment of Green Ash (Fraxinus Pennsylvanica) and Bougainvillea Goldenraintree (Koelreuteria Bipinnata)
Michael A. Arnold, Garry V.
McDonald, and Donita L. Bryan
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.
Keywords: Fraxinus pennsylvanica; hypoxia;
Koelreuteria bipinnata; pine bark mulch; planting depth; transplant
establishment; tree planting.