Root Pruning Broadleaf Trees Prior to Transplanting Trees in the Nursery – Field Production
Edward F. Gilman
Journal of Proceeding of the Florida State Horticulture Society 100:397. 1987.
Lateral root pruning and root stock undercutting is practiced in field production nurseries. It is intended to produce a compact, fibrous root system, a higher root to shoot ratio and better transplant survival. However, root pruning is not uniformly practiced in Florida. Timing, depth and distance from the trunk vary widely among nurseries which root prune, and not all nurseries and tree movers practice root pruning.
Lateral roots formed in response to pruning reportedly originate close to the pruning cut. Therefore, it is hard to visualize a more compact root system developing unless roots are cut close to the trunk. Rootpruning Quercus virginiana 5 cm inside the root ball 1 year prior to harvest and then again 6 months before harvest at the edge of the harvestable root ball increased dry weight of fibrous roots inside the root ball six-fold compared to non-pruned plants. Root-pruning Picea pungens 20 cm inside the edge of the harvestable root ball 5 years before harvesting resulted in a 4-fold increase in root surface area in the root ball. Apparently, harvesting the ball beyond the point of root pruning can increase root density within the root ball.
Root pruning reduces above-ground plant size and may increase time to harvest a field-grown nursery crop. However, long-term growth is either unaffected or increased.
Number of new roots generated by root pruning varies among species. Platanus occidentalis generated an average of 32 new roots whereas Quercus virginiana and Ulmus parvifolia ‘Drake’ had less than 10. For five of the six species, 69% or more of the new roots originated within 2.5 cm of the cut. However, only 56% of new Acer roots originated within the zone. Acer had 27% of new roots more than 5 cm behind the cut; all other genera had 14% or less. Growth of existing lateral roots was stimulated by root pruning on five of six species tested.