No abstract

Established forest trees planted from small containers are less stable at the point where roots fork, bend or branch as a result of deflection by container wall, but less is known about the post-transplant impact of root deflections resulting from growing trees in large containers. We either root pruned by shaving off the periphery of the #3 container root ball as it was planted into the #15 container or did not root prune on 5 tropical and 2 temperate tree species. Shaving did not affect trunk caliper or tree height on the seven species tested under the conditions of this study. Shaving removed the entire outer and bottom 2.5 cm (1 in) of the root ball and reduced or eliminated culls on five of seven species. The largest diameter roots on trees in #15 containers that were not root pruned when shifted from #3 containers were kinked, descended down the container wall, or circled at the position of the #3 container. These root defects were largely missing on trees with root balls that were shaved of peripheral roots when shifted into #15 containers. The largest roots on shaved trees grew more-or-less straight radially from the trunk. Shaving the root ball periphery and bottom is recommended to improve root ball quality by reducing root ball defects.

After 40 months in air root pruning containers, Quercus virginiana ‘SDLN’ Cathedral Oak® live oak planted 3.8 and 8.9 cm (1.5 and 3.5 in) deep from rooted cuttings had greater caliper than trees planted at 1.3 cm (0.5 in) below substrate surface. Trees in the 1.3 cm (0.5 in) deep treatment grew taller than all other trees except for those in the 3.8 cm (1.5 in) deep treatment. Most (80%) trees were graded as culls according to root evaluations in the Florida Grades and Standards for Nursery Stock. This resulted mostly from roots circling and crossing the top of the root ball in the #3 and/or #15 container sizes. Trees planted 6.4 cm (2.5 in) deep in #3s, then 6.4 cm deep in #15s, and 6.4 cm deep in #45s [19 cm (7.5 in) total depth] had fewer, smaller diameter, and deeper primary roots than trees planted at all other depths. The presence of a trunk flare and surface roots decreased with increasing planting depth indicating that these could be used as an indicator of primary root depth. Cathedral Oak® demonstrated the capacity to generate new roots above the primary flare roots only when rooted cuttings were planted into #3 containers. Trees adjusted their root systems by generating a new set of roots along the buried stem up to the substrate surface. Roots did not grow from the buried portion of the stem when trees in #3 containers were planted 6.4 cm (2.5 in) deep into #15 containers. In other words 75% or more of the primary structural roots were deflected by either the #3 or #15 container wall or both, indicating that most primary roots that emerged from the trunk did so when the tree was in the #3 or #15 container within 22 months of planting from rooted cuttings. Roots often grafted when crossed or laid against other roots, but roots did not graft to trunks.

Significant differences may exist in establishment rate between trees planted from containers and those from field nursery. Containergrown plants have root balls with deflected roots which could impact establishment. Slicing root balls at planting could improve postplanting performance of container-grown trees. Sixty live oak 170 L containers were planted into landscape field soil. Root balls from 30 of these containers were sliced prior to planting. Thirty field-grown trees of slightly larger size, and 30 smaller trees from 57 L containers, were also planted. During dry periods in the first 432 days after planting (DAP), 57 L container trees had the least negative xylem potential. Field-grown trees had the most negative xylem potential when irrigation was withheld 12 DAP. Slicing root balls had little impact on xylem water potential in drought. Defoliation was greater for 170 L container trees than for 57 L containers. Trunk diameter increase of 57 L containers and field-grown trees was greater than for 170 L containers. Field-grown trees grew less in height. Root system radius was similar for 170 L containers and field-grown trees, and greater than 57 L containers. Small trees appear to establish quicker than larger trees.

Height and trunk growth of Quercus virginiana ‘SDLN’ Cathedral Oak® tops was not affected by root pruning that occurred each time trees were potted into a larger container, beginning when rooted cuttings were planted into #3 containers. All trees produced in air root-pruning Accelerator® containers without mechanical root pruning produced enough circling roots to make them culls according to Florida and California standards for nursery stock. Removing root defects by pruning roots when trees are potted to the next larger size reduced culls from 100% to 40% of the crop and is recommended for quality tree production. Root pruning when trees were potted from one container size to the next size had no influence on the number of primary structural roots that grew directly from the trunk base. Root pruning had no impact on the number of roots that were deflected down. Waiting to root prune until #3 containers were potted into #15 containers did not increase the number of straight roots compared to non-pruned controls. Slicing the root ball edges vertically from top to bottom in several places appears to reduce circling roots capable of forming stem girdling roots. But slicing in the manner described in this study did little to reduce the descending root defects.