Tree pits require a sufficient quantity of soil media to support trees through maturity.
Trees can intercept additional rainfall in their canopy, direct rainfall to tree pits via stemflow, and percolate stormwater runoff through soil layers and root pores.
Tree pits often have bypass systems to avoid localised ponding from surface runoff. Tree pits may also require increased drainage such as perforated coiled pipes to avoid continuous saturation of root zones and to aerate soils. These pipes can also be used for watering the trees during dry periods by adding water through the cleanout. Where the area around the tree roots is likely to be trafficked, additional structural support may be required, such as a concrete wall, structural soils or specialised root cells.
A research paper by the University of Melbourne outlined the relationship between stormwater treatment and horticultural requirements for street trees (Kerrie et al., 2008). Results indicated that stormwater provided for faster growth rates than tap water, possibly due to increased nutrients. The study also showed that it is feasible to use under-pavement tree pits as a stormwater treatment method and that tree growth was satisfactory in soils with a range of infiltration characteristics. Even at a very young age, trees appeared to modify the hydraulic conductivity of tree pit systems.
Trees are important green infrastructure in urban environments, providing oxygen, passive cooling and heating, intercepting dust, and acting as an ecological corridor for avifauna, lizards and insects. Trees are also an important landscape element, providing seasonal interest, defining landscape corridors, and establishing human-scale spaces under their canopy.