A root-bound index for evaluating planting stock quality of container-grown pines

Abstract

Pot-binding can adversely affect the performance of container-grown stock. One factor that affects potbinding is the size of the root system in relation to the size of the container. The aim of this study was to determine if an objective root-bound index (RBI) would be useful when evaluating the quality of pines grown in various types of containers. A RBIdia was determined by dividing the root-collar diameter (RCD) by the cavity diameter (i.e. the distance from one container wall to the opposite container wall, measured at the top of the container cavity and passing over the centre of the cavity) and a RBIvol was determined by dividing the RCD (mm) by the container volume (ml). Field assessments of seedlings produced in six container types in the United States indicate that both RBI methods produced similar results for survival of Pinus palustris (stock with the highest RBI values exhibited the lowest survival). However, the RBIvol method did a better job of separating the performance of two types of peat pots. Both indices were also related to outplanting performance of P. patula cuttings grown in South Africa. When sorting container stock, P. elliottii x caribaea cuttings with RBIvol values greater than 7 might be culled while P. palustris seedlings might be culled when the RBIvol index exceeds 11. Therefore, the target RBIvol value will vary with species and plant type. When tree planting guides cover a range of container types, listing a maximum RBIvol value would provide some indication of when root-binding might reduce plant quality. In addition to RBI, the age of the roots also appears to affect field performance of pot-bound stock. We rejected the hypothesis that age is not related to post-planting survival of Pinus elliottii x caribaea cuttings.

Keywords: cuttings, pot-bound, root-collar diameter, container volume, seedlings, survival, Pinus palustris, Pinus patula, Pinus elliottii x caribaea

Southern African Forestry Journal Vol. 207 2006: 47-54