The Effects of Planting Depth on Windthrow, Stability, and Growth for Four Tree Species in Containers

Establishing the effects of planting depth on tree stability and growth is critical in understanding the role nursery production plays in planting depth issues at the landscape level. In this study, bare root Whitespire birch (Betula platyphylla var. japonica ‘Whitespire’), green ash (Fraxinus pennsylvanica), Snowdrift crabapple (Malus × ‘Snowdrift’), and bicolor oak (Quercus bicolor) were grown for 17 weeks in a container production setting with four levels of substrate over the first main-order root: 0, 5, 10, and 15 cm. Birch demonstrated the greatest instability of all species, leaning significantly more when planted at 0 cm than at 15 cm. In ash and crabapple, there were no significant differences in either the number of trees leaning or the amount of lean in all treatments throughout the study. Oak stems bent excessively, invalidating lean measurements. Stem caliper increase was significantly greater in ash planted 0 and 5 cm deep than 10 and 15 cm deep. There was no significant difference in stem caliper increase between planting depths in other species. Birch planted 0 and 5 cm deep had greater root volume increase than those planted 10 and 15 cm deep. Root volume increase in ash, crabapple, and oak did not differ significantly between treatments. Infrequent windthrow events were observed, but appeared random and apparently unrelated to planting depth. The perceived benefit of planting trees deep in containers to improve stability was observed in only one species (birch) at one depth (15 cm) and was at the expense of significantly reduced root volume increase.