Planting Depth Affects Root Form of Three Shade Tree Cultivars in Containers

Study was designed to evaluate impact of planting depth on root morphology inside nursery containers. Trees were planted shallow (13 mm) or deep (64 mm) into #3 Air-Pot™ containers, then shallow (0 mm) or deep (64 mm) into #15 containers prior to shifting them to their final #45 container size at the same depth. Trunk diameter (caliper) was significantly larger for both magnolia and maple planted shallow (13 mm) into #3, and then shallow into #15 containers when compared to planting deeper. However, differences were small and may not be relevant to a grower. No caliper or height differences among planting depths were found for elm. Presence of stem girdling roots in elm and magnolia growing in #45 containers increased with planting depth into # 3 containers. Downward re-orientation of main roots comprising the flare by #3 container wall, likely contributed to amount of roots growing over root flare. Maple root systems were not impacted by planting depth into #3 primarily due to adventitious root emergence from the buried portion of stem. Distance between substrate surface and top of root flare in finished #45 containers was not impacted by planting depth into #3 containers for any species. Planting elm and maple deeply into #15 led to more trunk-girdling by roots, a deeper root flare, and more roots growing over flare compared to planting shallow. Most root defects in all species were hidden from view because they were found below substrate surface. Presence of a visible root flare was not related to occurrence of root defects. Root balls on elm and maple were packed with roots which made it time consuming to remove substrate and roots above the root flare. Planting depth appears most crucial when shifting into #15 containers.