Mulch Thickness and Planting Depths Can Interact to Reduce Growth and Survival of Green Ash and Goldenraintree

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  • 1 Texas A&M Univ., Dept. of Horticultural Sciences, College Station, TX 77843-2133
  • | 2 Texas A&M Univ., Dept. of Horticultural Sciences, College Station, TX 77843-2133
  • | 3 Texas A&M Univ., Dept. of Horticultural Sciences, College Station, TX 77843-2133

Most available information on the effects of planting depths for trees necdotal and/or testing of interactions with other important cultural practices implemented during landscape establishment is lacking. Green ash (Fraxinus pensylvanica H. Marshall, hypoxia tolerant species) and bougainvillea goldenraintree (Koelreuteria bipinnata Franchet, hypoxia intolerant species) were grown from seed in 2.3-L containers which were transplanted to 9.3-L black plastic 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 goldenraintree (27 Apr. 2001) were transplanted to clay soil field plots with the root-collars at a 7.6 cm above soil grade, at grade, or 7.6 cm below grade. Planting depths for each species were in factorial combinations with 0, 8, 15, or 23 cm of pine bark mulch covering 2.4 m2 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. Planting below grade reduced survival of green ash by 25% after 3 years. Even the thinnest layer of mulch reduced bougainvillea goldenraintree height and trunk diameters. Height and trunk diameter responses interacted with planting depth for green ash. Mean soil moisture levels were slightly less negative with 8 cm of mulch (-5.8 kPa) compared to bare soil (-9.1 kPa), but increasing mulch thickness to 23 cm (-16.2 kPa) inhibited penetration of irrigation water/rainfall. This data suggests that planting with the root-collar at or above grade greatly enhances survival and growth potential of green ash and bougainvillea goldenraintree and that mulch application should be only at thin layers to inhibit weeds.

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