The establishment of urban forests on landfills is an untapped opportunity for urban greening, with the potential to: 1) achieve safe hydraulic containment of wastes through phytocapping; 2) enhance biodiversity and ecological connectivity; and 3) open new possibilities for public amenity. To achieve these multiple benefits requires phytocap designers to be aware of the living nature of these systems, and to create soil conditions that allow the long-term flourishing of phytocap plant communities, while also addressing the critical civil design constraints of waste containment. Ecological engineering expertise crucially needs to be sought throughout the design phase, especially during soil and plant specification. Soil is the foundation of good phytocap design and economics demand that the soil is readily available at the site or within a practical haulage distance to make the project feasible (Michael et al., 2004). This often means using stockpiled soil, spoils, clean fill, or sourcing soil within a local government area opportunistically from a construction project. Although the use of these ‘non-native’ soil products appears challenging, many Australian native plants are adapted to nutrient impoverished,skeletal soils, sometimes even affording a competitive advantage under these circumstances ((Prober and Thiele, 2005)Successful soil selection is therefore not a quest for the perfect horticultural medium or topsoil. Rather, it requires a comprehensive understanding of the physical and chemical characteristics of soils at hand, and careful soil-plant matching, to ensure the establishment of the intended plant community. Once a suitable soil is selected and found to be non-limiting for plant growth, there are two remaining design considerations: the thickness of the soil, and the density at which it is placed. The impact of soil density was investigated in a one year glasshouse study, where a variety of native trees and grasses were grown at relative compaction levels (%RC) ranging from 67% (very soft) to 87% (very hard). This enabled relationships to be derived: between soil density and plant water use; and between soil density and plant root growth; transferable for any soil type. Correlations between the two variables were also considered.