Trees play a crucial role in outdoor heat mitigation, offering a natural solution to the escalating problem of urban warming. Their shade-casting canopies significantly reduce surface and air temperatures, creating cooler, more comfortable environments. Additionally, the transpiration of trees, where they release water vapor into the air, contributes to cooling through evaporative cooling. While these benefits are generally well-regarded, some less-discussed facts, such as the time required for the growth of trees, the nocturnal warming due to the blocking of longwave radiation, the wind blocking and reduction of heat removal by urban ventilation, and the background climate-dependent efficacy, warrant further discussion. To provide insight on these less-discussed factors and support long-term and tailored strategic urban heat mitigation planning, we explored some of these facets using our open source urban microclimate model, urbanMicroclimateFoam, implemented in OpenFOAM (Kubilay et al., 2020; Carmeliet & Derome, 2023). This model comprehensively simulates all the dominant physical processes, including solar radiation, building-resolved flow, heat and mass transfer fields, aerodynamic, shadowing, transpirative effects of trees, evaporative cooling, and heat and water storage in urban surface materials. Specifically, we modelled an elementary urban setting – a street canyon with two rows of trees, to examine how the growth of trees over time influences both pedestrian-level thermal comfort and ground surface temperature. While the growth rate of trees varies by species and is affected by multiple factors, our findings indicate that it typically takes about 30 years for trees to reach maturity and to be able to provide significant shade and transpirative cooling benefits. In the race against climate change, simply planting more trees in cities from this point may not be an adequate response. Prompt and innovative cooling interventions are essential to complement tree planting for effective urban climate adaptation.