Effect of Root Pruning Prior to Transplanting on Establishment of Southern Magnolia in the Landscape

Materials and Methods

Southern magnolia trees from #3 (3 gal. -10 liter) containers were planted in rows on 8 ft X 13 ft (2.5 m x 4 m) spacing in the fall of 1984 on a Chandler fine sand near Gainesville, Florida. Irrigation (0.8 gal -3 liters/tree) was applied through microspray stakes generally daily except during rainy periods and winter. Nitrogen was applied to a 10 ft² (1 m²) circular area around each tree as ammonium nitrate at 6 lb N/1000 ft² (482 kg/ha) / yr divided into 5 equal applications. Glyphosate was periodically applied to control weeds in a 3.2 ft (1 m)-wide strip centered on the tree line down each row.

Thirty-two trees were root pruned in 1987 at each of the following times: 1) mid-dormant season - February, 2) just following the first shoot growth flush, after a terminal bud formed - June, and 3) following the second growth flush - October. Root pruning cuts were made with a sharpened hand-spade inserted to a 12-inch (30-cm)-depth at 60° from the horizontal in a circle 14 inches (38 cm) in diameter centered around the trunk. Thirty-seven trees were not root-pruned. Irrigation (2.1 gal. - 8 liters/tree) was applied to all plants, including controls, in the study for 7 days following a root pruning treatment. Treatments were arranged in a randomized complete block design with 4-tree replications/treatment in each of 8 blocks (rows), totaling 32 trees/treatment.

Five trees (1 from each of 5 randomly chosen blocks) from each of the 4 treatments (for a total of 20 trees) were dug on Jan 20,1988 with a 20-inch (50-cm)-diameter root ball according to American Association of Nurseryman standards (1). Soil was washed immediately from the root ball with a stream of water. Roots were separated into 3 diameter classes: 1) 0-2 mm, 2) > 2-5 mm and 3) > 5-10 mm, and dried at 70°C to constant weight. These data were reported earlier (8).

Sixteen trees (2 from each of the 8 blocks) from the nonpruned, and pruned February, June and October treatments (total 64 trees) were transplanted on Feb 17, 1988 with a 20-inch (50-cm)-diameter root ball using a 2-blade mechanical tree spade. Trees were lifted into the air about 3.2 ft (1 m) to assure that all roots were severed, then lowered back into the original hole. A 4-inch (10-cm)-high soil ridge was constructed around each tree to retain water. Sixteen nonpruned trees were not transplanted to serve as a second control. Irrigation (0.8 gal - 3 liters/tree) was applied daily, except when it rained, during 1988. Irrigation was applied every other day during 1989 and 1990. Tree height and stem caliper were measured at transplanting and in February 1989, 1990 and 1991. A randomized complete block design analysis of variance with 8 blocks and 4 treatments was performed for each measured variable each year. Mean separation was accomplished with Duncan’s multiple-range test at the 5% level. Slopes of regression curves were compared among treatments using the GLM procedure in SAS at the 5% level.

Results and Discussion

All 64 transplanted trees survived transplanting. Root-pruned plants were shorter and had less caliper at transplanting than the nonpruned controls but there was no significant difference in height or stem caliper among pruned and nonpruned transplanted trees one growing season following transplanting (Figures 1 & 2). This showed that root-pruned trees grew at a slightly faster, yet significant, rate than trees which were not root pruned. Growth rate during the first year on all transplanted trees was much slower than on trees that were not transplanted, butthere were nodifferences in growth rate during the second and third year after transplanting (Figures 1& 2). Since the growth rate in the second and third year on transplanted trees matched that on trees that were not moved, 3 1/4-cm (1 1/4-inch)-caliper southern magnolia were established within one year after transplanting. Establishment period may be shorter further south in USDA hardiness zones 9 through 11 (4), but can be one year (or longer) per inch (2.54 cm) caliper further north (7, 17). Root pruning prior to transplanting had no effect on rate of establishment.

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Figure 1

Height of southern magnolia trees one year before transplanting, at transplanting and one, two and three years after transplanting. Check trees were not root pruned in the nursery and they were not transplanted.

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Figure 2

Trunk caliper of southern magnolia trees one year before transplanting, at transplanting and one, two and three years after transplanting. Check trees were not root pruned in the nursery and they were not transplanted.

Establishment period has been defined as the time required to completely replace the root system to the same size as it was before transplanting (7) or the time required for shoot growth rate to match that on the tree prior to transplanting (17). Shoot growth rate will probably match pre-transplanting growth rate just as the root system is completely replaced, and further studies are under way to confirm this.

This study was conducted under nearly ideal post-transplanting conditions, with irrigation applied daily the first year following transplanting, and it shows what can be accomplished with adequate irrigation following transplanting. Unfortunately, most trees planted in urban environments do not receive irrigation as frequently as this, and this may significantly increase the time required to establish a tree in the landscape. If resources are directed to deliver frequent irrigation immediately following transplanting, the establishment period in the urban landscape may approach those reported in these studies, and tree survival may increase.

These results question the wisdom of the often discussed, sometimes implemented, practice of root pruning previously unpruned landscape-sized southern magnolia prior to transplanting. Although root pruning increased the ratio of fine roots to coarse roots, trees with higher ratios (more fine roots) grew only slightly more the first year following transplanting (8) compared to trees having a low fine-root:coarse-root ratio which were not pruned. During the second and third year root-pruned trees grew at the same rate as trees that were not root pruned. In addition, there was no difference in tree quality among any treatments du ri ng the th ree years fol lowi ng transpl anti ng (data not shown).

Results from this study may not apply to oaks and other trees with coarse root systems, and they may not apply to trees larger than those used in this study. Root pruning 2-inch (5 -cm)-caliper live oak (Quercus virginiana) increased root density within the root ball 6-fold compared to unpruned trees (9), and 4-fold in blue spruce (Piceapungens) (18). Unfortunately, trees in these two studies were not transplanted so the effect of increased root density on transplanting was not evaluated. The increase in root density in response to root pruning was nowhere near 4- to 6-fold for the magnolias in the current study (8), and there was little effect on post-transplant growth. Further study needs to be conducted on the effect of increased root density within the root ball on transplant survival and growth in the landscape.

Conclusions

1. Root pruning southern magnolia prior to transplanting corresponded to only slightly better post-transplant growth the first year following transplanting. There was no difference in growth rate between root-pruned and nonpruned trees the second and third year after transplanting.

2. Root pruning prior to transplanting did not enhance the rate of establishment of southern magnolia.

3. Southern magnolia with 1 1/4 inch (3 1/2 cm)-caliper trunks were established within one year after transplanting.