Species and season affect response of container-grown shade trees to pre-plant root modifications

Container-grown shade trees make up an increasing proportion of nursery stock, yet arborists and urban foresters are often concerned about root defects associated with trees grown in smooth-sided containers, such as circling roots, persisting in the landscape post-transplant. The objective of this study was to determine the effect of root modifications (shaving or bare-rooting) at planting on establishment, survival, and growth of container-grown Acer rubrum L. 'October Glory', Liriodendron tulipifera L. 'Fastigiatum', and Platanus x acerifolia (Aiton) Willd. 'Bloodgood'. Trees were planted on two dates (May and July) in 2018 and evaluated for two growing seasons. Modifying roots before planting resulted in increased occurrence of leaf scorch for L. tulipifera and A. rubrum trees. Nearly all trees bare-rooted before planting in July had severe die-back. Survival was excellent (>75 %) for all A. rubrum and P. x acerifolia trees planted in May regardless of treatment. Survival for L. tulipifera trees that were bare-rooted in May was 50 %; all L. tulipifera trees that were bare-rooted in July died. Bare-rooting increased predawn leaf water potential (psi w) immediately after planting. However, psi w did not differ among root treatments for the rest of 2018 and throughout 2019. This suggests that trees with modified root systems achieved a functional equilibrium by adjusting leaf area to reduce whole-tree water loss. Root biomass outside the original root-ball did not differ among root modification treatments two years post-transplant. However, bare-rooting reduced the proportion of circling roots compared to control trees for all species. Shaving root systems reduced circling roots compared to the control for L. tulipifera and P. x acerifolia trees. For practitioners interested in trialing these techniques, we advise performing root modifications in the dormant season and avoiding species known to be difficult to transplant bareroot.