In the current quest for the design of advanced complex materials, the functionalization of biological materials having hierarchical structures has been of high interest. In the case of lignocellulosic materials, various modification techniques have allowed one to obtain materials with outstanding properties. However, the control over the spatial distribution of the modification inside the wood scaffold, which is an important parameter to obtain the desired properties, has yet to be understood. In this study, the use of solvents with different wood-swelling capabilities is proposed to control the spatial polymer-modification distribution inside the hierarchical wood structure. Wood cubes were functionalized via SI-AGET-ATRP using solvents with different wood-swelling capabilities. Spectroscopic (Raman and FTIR) and electron microscopy techniques showed that a good wood-swelling solvent as reaction media can transport the polymerization initiator molecule into the cell wall, allowing it to react with all the available -OH groups in the wood structure. Conversely, the use of a bad wood-swelling solvent limits the reaction to the available -OH groups at the lumen/cell wall interface. The subsequently added polymers grow from the available initiator sites and therefore show similar spatial distribution. This diffusion limitation is visible not only at the microscopic level (cellular structure) but also at the macroscopic level (over the length of the sample).