Urban landscapes have far-reaching impacts on a wide range of factors relevant to climate, ecosystem functioning, biodiversity and human health. Impervious surfaces typical of urban areas generate heat islands and alter patterns of runoff, while contaminants associated with commercial, residential and vehicle use present challenges for stormwater management. Urban green spaces mitigate these impacts by providing vegetated surface cover, slowing water passage and absorbing nutrients and contaminants via phyto-remediation. Our trial project, undertaken in partnership with ACT Government, Canberra, Australia assesses the performance of purpose-built garden beds in treating stormwater and mitigating urban heat island effects through enhanced tree growth. Underground plastic cells were installed to store stormwater, increasing the residence time in contact with the root zone of trees planted in the overlying bed. A ‘control-impact’ study design, comparing the treatment garden bed to traditional designs was implemented monitoring water quality, microclimate, soil characterisation and tree condition. Decreased metal concentrations in outflows suggested phyto-remediation was occurring. No effects on microclimate were observed but were also recorded over an atypically wet climatic period. Trees in the treatment bed exhibited higher leaf chlorophyll content and carbon accrual, indicating better condition than the equivalent trees in the traditional garden beds. By understanding the effects of water sensitive urban design (WSUD) infrastructure we can provide recommendations for future WSUD projects and facilitate the modification of existing infrastructure in aging cities for ecological and social benefit.