Impact of paclobutrazol on root-pruned live oak

Abstract

This study evaluated the impact of paclobutrazol (PBZ) on the overall growth and vitality of root-pruned, field-grown live oak (Quercus virginiana). Live oak trees with 10-cm trunk diameter (measured 30 cm aboveground) were treated with full rate (0.8 g·cm-1 trunk diameter) of PBZ as basal drenches, full or half rate (0.8 or 0.4 g·cm-1 trunk diameter) of PBZ and trenching at 45 cm from trunk, full or half rate of PBZ and trenching at 60 cm from trunk, trenching alone at 45 or 60 cm from trunk, and only water. Trunk diameter and canopy growth was significantly reduced (P < 0.001) and new root growth was also reduced by applications of PBZ, root pruning, or both. Starch content in twigs decreased and glucose content increased on treatment by full-label rates of PBZ and root pruning at 60 cm. PBZ and/or root pruning caused slight improvements in chlorophyll fluorescence (Fv/Fm). Results of this research indicate that PBZ (full rate) in combination with root pruning (45 cm) reduces tree growth and exhibits an overregulation effect for at least 16 months after treatment. Therefore, PBZ applications on root-pruned trees can temporarily decrease root and tree growth and improve foliage chlorophyll fluorescence.