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The first phase began with the creation of a sustainable growth environment for the new tree plantings. Research has indicated that, in urban conditions, replacement trees planted in small pits dug slightly larger than the root ball, are less likely to develop healthy root systems than those planted in larger areas of prepared soil. The concept of planting these trees in a continuous trench to maximize root space and a custom soil mix for root health was also pursued. Proposed tree spacing was reviewed and a shift away from curbs to improve traffic safety was recommended. To improve tree growing conditions, continuous trenches measuring six feet in width were excavated to a depth of three and one half feet (center right). These trenches provide an ample root growth zone for the new trees, in contrast to the former, restrictive soil conditions. This work also includes a continuous drainage layer of gravel under the soil. Utility and communication lines are buried in conduits at the edges of the trenches for easy access without damage to trees. Over 280,000 cubic feet of old soil and demolished building rubble was excavated (top right) in this operation and replaced with a gravel drainage layer, filter fabric and 11,000 tons of special soil mix. The soil mixture was developed to improve drainage and available oxygen -- thus maximizing root production and countering compaction. The soil mix contained six parts sandy loam topsoil, one and one half parts sand, one part mushroom soil and two parts large stones. The stones were used to help deter soil compaction by keeping the composition of soil loose, preventing soil from packing together and allowing air exchange. This soil was placed over a ten-inch, filter-fabric wrapped gravel layer to provide a drainage zone under this compaction- resistant soil mix (bottom right). Once the custom soil was installed, tree and turf planting could begin. For this work, two ten-foot planting areas with trees and turf were defined to each side of the ten-foot central walk. The original red oak trees (Quercus rubra) had been planted four feet from each curb and spaced at thirty-foot intervals. In replacing the tree rows the historic thirty-foot spacing was retained while the placement was shifted to seven feet off the curb face, so that basal wounding of trees from vehicle impacts would be less frequent. The details of the rehabilitation approach included the choice of three tree species, red oak, red maple (Acer rubrum) and sweet gums (Liquidambar styraciflua), to replace the monoculture of red oaks. The selection of these mixed plantings resulted from the need to diversify in order to avoid blights and diseases that had in the past destroyed entire formal, urban tree features. As part of a rehabilitation option, the trees chosen were a compatible substitution for the replacement of the original trees. Similarities in stature, form, branch density and leaf texture were sought to reinstate the character of the original red oaks. The trees chosen had comparable upright forms with straight trunks, medium foliage and branch texture, so that while genetic diversity was achieved, similar visual quality resulted, thus reinforcing the parkways historic formality. Replanting favored the original red oak trees (50%) while the red maple and sweet gum provided the other half (25% each). The pattern intermingled types for visual blending with (A) red oak,(B) red maple,(A) red oak,(C) sweet gum in a repeat sequence of ABAC ABAC ABAC along the parkway. In monitoring the work following installation, some trees were lost due to poor storage while waiting for planting. Thereafter, additional losses in the red oaks were sustained. Today, after some replacements, the trees are thriving. A recent inspection has revealed that the 219 trees have settled-in and have begun to increase in size over several growing seasons. It is anticipated that with another few years of growth they will form a continuous canopy.
Upon reviewing the soil conditions, the professionals involved have speculated that the soil mixture was slightly too well drained for the new plantings and that a minor revision in the mix to provide additional humus might solve the problem of overly effective soil drainage. A maintenance program has been developed for the parkway to address the cyclic needs of the new trees plantings. To keep moisture in the ground and prevent mower damage to basal bark, mulch was placed around the trees, as opposed to original lawn surface (photo above). At a detail level this adaptation changes the historic visual character of turf growing up to tree trunks. Pruning is being done to stimulate and direct growth; watering occurs when transpiration exceeds water replacement; and pest management and turf care are underway and are proceeding regularly. An integrated pest management program of properly timed physical and chemical care has been developed to promote tree health, vigor and disease resistance. These maintenance activities are funded through a partnership between the City of Philadelphia and the Logan Circle and Parkway Trust. Maintenance and clean-up related to special events is an additional expense borne by event promoters and the City. Today the two, mixed-species allées convey the original design intent and Philadelphians are experiencing the replanted portions of the parkway today as they appeared in the 1920s (photo below).
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MLO