Background and aims <p>Organic matter additions in coastal wetlands contribute to blue carbon sequestration and adjustment to sea-level rise through vertical substrate growth, with accurate modelling of these dynamics requiring information of root mass and volume additions across tidal gradients. This study aims to characterise the influence of vegetation zonation and tidal position on root mass and volume dynamics within substrates.</p> Methods <p>The root ingrowth technique was coupled with sediment cores to quantify below-ground root mass and volume production, standing stocks and turnover across two years to 90&#xa0;cm depth at Kooweerup, Victoria, Australia.</p> Results <p>We indicate a complex non-linear relationship between fine root mass production and tidal position, influenced by variable vegetation structures across mangrove (442–3427&#xa0;g&#xa0;m⁻²&#xa0;yr⁻¹), saltmarsh (540–860&#xa0;g&#xa0;m⁻²&#xa0;yr⁻¹) and supratidal forest (599&#xa0;g&#xa0;m⁻²&#xa0;yr⁻¹) zones. Fine root volume additions ranged from 274 to 4055 cm³&#xa0;m⁻²&#xa0;yr⁻¹ across sampling locations. Root production was greatest for older mangroves and tidally defined optimal zones of production were evident for mangrove and saltmarsh. Live roots extended deeper than typically studied, reaching depths of 1.0&#xa0;m in forested zones.</p> Conclusion <p>This information of root mass and volume additions across wetland live rooting zones can be used to improve highly parameterised models accounting for carbon sequestration and substrate vertical adjustment along intertidal gradients. We recommend that future studies measure root production across the entire active rooting zone or to&#xa0;1&#xa0;m depth to align with standard carbon accounting measurement depths.</p>