Planting Depth at Onset of Container Production and Subsequent Root Ball Remediation at Transplanting Affects Pin Oak and Littleleaf Linden

Abstract

Root flares of landscape trees are increasingly found to be much deeper than their forest counterparts, indicating that their root systems have been situated deeper in the soil. Planting deeply in production containers contributes to this phenomenon, yet the consequences of deep planting in production containers or the consequences of any adjustments made to planting depth at the time of transplant on growth in the landscape have not been reported for many species. Container-grown (11.4 L) liners of Tilia cordata Mill. (littleleaf linden) and Quercus palustris Münchh. (pin oak) were planted in 50-L containers with the first main lateral roots (structural roots) at substrate-surface grade or 10 cm or 20 cm below grade (deep planting). Trees were grown in the 50-L containers for two growing seasons and in a simulated landscape for three additional seasons after transplanting with the top of the container substrate at soil level or with some roots and substrate removed such that the original structural roots were just below the soil surface (remediated). Deep planting pin oak, but not littleleaf linden, slowed growth during container production; however, the effect did not persist after transplanting. Remediation of the 20-cm-deep pin oaks slowed growth during all three post-transplant years. Littleleaf linden remediation slowed growth for the first season after transplanting to a simulated landscape for 10-cm-deep trees and for the first two seasons for 20-cm-deep trees. Evaluation of pin oak root systems 3 years after transplanting revealed vigorous growth of non-deflected adventitious roots that had formed on the trunks of deep trees, and these roots appeared to be developing into main structural roots. No adventitious roots were present on littleleaf linden; instead, deflected roots grew and produced deformed root systems. Deep planting of linden reduced suckering; however, we conclude that remediation of deep-planted littleleaf linden is warranted as a result of potential hazards from trunk-girdling roots. In some species such as pin oak, non-deflected, strong adventitious root systems may assume the role of structural roots and diminish the effect of deflected and circling roots systems formed during container production. Remediation of these trees is likely not as critical as for species without abundant adventitious roots.