Keep in mind that the 2.5 tonnes is carbon only, the mass of additional plant matter is likely more than double that given the water content and other elements.

Keep in mind that the 2.5 tonnes is carbon only, the mass of additional plant matter is likely more than double that given the water content and other elements. Additionally, it's not evenly distributed over the building, but cantilevered out. That said, I agree with you that it's likely to be trimmed for aesthetic reasons well before it becomes a structural issue; which is why the annual carbon offset claimed is misleading - it assumes the biomass increases annually without intervention.