Mechanical Correction and Chemical Avoidance of Circling Roots Differentially Affect Post-transplant Root Regeneration and Field Establishment of Container-grown Shumard Oak

Abstract

Quercus shumardii Buckl. seedlings were grown for 3 or 7 months in 2.3-liter black plastic containers. Containers were either treated or not on interior surfaces with 100 g Cu(OH)2/liter latex carrier. Trees were transplanted in summer or fall to quantify post-transplant responses to mechanical correction or chemical prevention of circling roots. Four treatments were used at each transplant date; nonpruned seedlings from Cu(OH)2-treated or nontreated containers, and seedlings from nontreated containers in which two mechanical root pruning techniques were used, traditional severing of circling roots on the rootball periphery or splitting and splaying the bottom two-thirds of the rootball at transplant (butterfly pruning). Traditional root pruning severed more small-diameter roots (≤0.5 mm), while butterfly pruning severed more large-diameter roots. During the first 21 days following transplant most root regeneration was via elongation of intact root tips. Cu(OH)2-treated seedlings regenerated substantially more roots ≤1.0 mm in diameter and a greater root mass than mechanically root pruned or nonpruned seedlings. Both corrective mechanical pruning techniques resulted in greater predawn water stress during immediate post-transplant (21 days) establishment in October than seedlings chemically treated to prevent circling root development. Treatments that severed more roots and/or removed greater root mass were associated with decreased field performance and increased post-transplant water stress. Increased numbers of small- to medium-diameter new roots were associated with reduced post-transplant water stress and improved post-transplant shoot growth. Nonpruned and traditional root pruned seedlings grew little during the first two post-transplant growing seasons regardless of transplant date. Butterfly pruning resulted in severe dieback of shumard oak seedlings. Cu(OH)2-treated seedlings were the only ones to exhibit a gain in height or stem diameter after 2 years in the field.