The oxygen regime of urban tree-growing sites may be sub-optimal for tree growth and the related ornamental value of trees. Problems may especially occur in situations where the substrate is covered by pavement and/or where the water table is very shallow. The stability (oxygen consumption rate) of the organic components of the substrate (peat, green waste compost, etc.) and the design of the growing site also play an important role. The oxygen regime of a number of designs (wide strip of substrate; narrow strip; wide strip with multiple aeration pipes; narrow strip with one aeration pipe) was simulated for a range of substrates with the computer program ¿Matlab, pde toolbox¿. The program computed equilibrium distributions of oxygen in the substrates from gas diffusion coefficients of the different parts of the growing site constructions and from the specific oxygen consumption rate of the substrate. The simulation results were compared with measurements on an experimental substrate strip. The simulation results clearly showed the importance of organic matter stability and growing site design. On wide strips the presence of aeration pipes may be very advantageous, or even necessary.