In the development of a street tree planting medium for use as a sidewalk base, we have been testing a series of limestone gravel and soil media with varied amounts of clay loam suspended within the matrix voids. Tilia cordata and Quercus alba seedling roots quickly penetrated and grew in these systems when compacted to densities in excess of 2000 kg·m–3, while they were severely impeded in clay loam soil compacted to 1300 kg·m–3. Limestone mixes of the same design had variable, but consistently acceptable, California Bearing Ratios (>40) when compacted to similar densities; demonstrating their strength as a pavement base. Tilia root growth, based on the volume collected from total root excavations after two growing seasons, increased a minimum of 300% in the limestone mixes over the compacted clay loam control when the treatments were compacted to ≈80% Standard Proctor Optimum Density. Root penetration of Quercus increased >400% in the limestone mixes over compacted loam in a 6-month trial compacted to 95% Standard Proctor Optimum Density.