Poplar catkins are an environmental pollutant because they can trigger sneezing, shortness of breath, skin allergy and even cause forest fire. It is very attractive to discover ways for reducing the threats of poplar catkins to the environment and human health and even to convert poplar catkins into useful materials. Herein we report on a facile and cost-efficient strategy for the synthesis of hierarchical porous carbon microtubes using poplar catkins as the carbon source. The synthesis involves pre-carbonization and subsequent ZnCl2 activation. The resultant materials not only inherit the natural tubular morphology of poplar catkins, but also develop a hierarchical porous structure, with nitrogen from the biomass being self-doped in the resultant carbon skeleton. The hierarchical porosity, especially the microporosity with the pore size from 0.5 to 1 nm, surface structure, tube wall thickness and nitrogen content can be readily controlled by adjusting the activation temperature and the ZnCl2 to-precursor mass ratio. The produced materials exhibit an excellent CO2 capture capacity. The optimal sample activated at 800 degrees C with a ZnCl2-to-precursor mass ratio of 4:1 shows an extraordinarily high CO2 adsorption capacity of 6.22 and 4.05 mmol g(-1) at 273 and 298 K, respectively, at 1 bar of CO2. Furthermore, the material also displays an excellent recyclability and CO2/N-2 selectivity. Our approach not only relieves the environmental pollution of poplar catkins but also leads to the production of a high-performance CO2 adsorbent, which is promising for industrial CO2 capture and separation. It therefore demonstrates an example of trash-to-treasure transformation.