ROOT OF THE PROBLEM
We all know that sidewalk or street trees have a rough time 1) getting established and 2) thriving once established. Numerous environmental factors play against their success rather than for their success. Research at Cornell University's Urban Horticulture Institute has produced a system that should help alleviate the effects of one environmental factor: poor rooting environment. The usual deadly combination of compaction due to construction and the resulting bathtub that holds water can be overcome. The specifications they have devised — named Cornell Structural Soil — results in a soil mix that is pavement load-bearing and has a proper root environment. This engineered soil utilizes a precise gravel size, along with precise soil texture and hydrogel, resulting in stability, air space and water-holding capacity. One of the studies showed shoot extension twice that of conventional street trees on Acer campestre, 7 times longer on Tilia cordata and nearly double on Malus ‘Adirondack.’ Root extensions were double than in conventional plantings in both A. campestre and T. cordata. Optimum depth of the planting pit should be 36 inches and the soil mix should be compacted to 95 percent or better Proctor density.
Source: Cornell University's Urban Horticulture Institute, http://www.hort.cornell.edu/uhi/outreach/csc/article.html.
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