Traffic-related air pollution (TRAP) is associated with a range of health issues which are causing adverse effects to communities that are living or working in near-road environments. Roadside vegetation can mitigate TRAP and improve the local air quality. For current and new roadside barriers, the vegetation will continue to grow over time and its properties (dimensions and density) will change thus affecting the barriers’ pollutant reduction capabilities. While urban planners and local communities usually prune their barriers to control their growth, there is little guidance on how to maintain a barrier’s size to achieve optimal pollutant reduction. In this project, we investigated the growth patterns of coniferous vegetation to determine how its properties, such as height and leaf area density, change with time. Then, we conducted 75 Large Eddy Simulations (LES) to explore how the growth of roadside coniferous vegetation barriers affects pollutant reduction under different urban conditions. Our results indicated that as vegetation grows, its pollutant reduction capabilities change, and that there is an ideal maturity stage at a height of 4 m, at which it achieves the most pollutant reduction. It is important that after planting, local communities and urban planners actively prune and maintain the vegetation barrier at its optimal height to achieve its ideal pollutant reduction capacity.