Gilman, E.F. and F.J. Masters

Effect of tree size, root pruning and production method on root growth and lateral stability of Quercus virginiana

Arboriculture & Urban Forestry: 36 (6): 281-291

The objectives of this research were to evaluate impact of slicing the outer edge of container root balls, initial tree size at planting, and root ball composition on post-planting tree stability in a simulated wind and rain storm. One-hundred twenty Quercus virginiana Mill. ‘SNDL’, PP#12015, Cathedral Oak® live oak were planted March 2005 into sandy landscape soil in USDA hardiness zone 8b in Florida. Thirty previously root pruned field grown trees were transplanted with a 91 cm diameter tree spade, and 60 trees of similar size were planted from 170 L (47 cm tall x 75 cm top diameter) containers. Root ball sides on 30 containers were sliced from top to bottom about 3 to 5 cm deep in six places just prior to planting into landscape soil. Thirty smaller trees from 57 L containers were planted without slicing. Trees were pulled with a 3,629 kg capacity electric winch and blown with a hurricane simulator three growing seasons later from estimated center of gravity. To simulate wind loading, trunks were pulled to 25 degrees tilt in 5 degree increments. Slicing the root ball had no impact on root growth, bending moment, or trunk bending stress required to pull trees to a set angle. More bending stress was required to pull field grown trees transplanted B&B than trees planted from containers of either size. Growing trees in containers for three years prior to landscape planting changed root morphology inside and outside of the root ball compared to field grow trees, and this corresponded to reduced stability three growing seasons after landscape planting. Decomposing organic substrate in original container root ball may also have contributed to some instability. Trees planted from small containers were as stable as, and had greater root diameter/cm2 trunk cross sectional area than those from larger containers. Root cross sectional area windward was most correlated with bending stress required to tilt trees with a winch and cable. Bending moment scaled to the 3.4 power of trunk diameter.
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