While nanoparticle uptake by plants has primarily been studied for environmental remediation, its purposeful use to enhance plant structural and mechanical properties remains unexplored. We hypothesized that cellulose nanofibers (CNFs), introduced into the growth medium, could be absorbed by flax stems, reinforcing their cell walls formation and, as a consequence, improve the mechanical performance. Thus, in this paper it is proved that flax plants treated with a 0.2% w/v CNF solution after root excision showed increased stem diameter, reduced pith size, and significantly accelerated root regeneration (~ 7 cm in 20 days) compared to controls treated with autoclaved deionized water. Analyses of the CNF-treated stems showed an increase in the mechanical performance of the stems revealing up to 50% increase in energy to fracture, 22% increase in Young's modulus, and approximate 33% improvement in stiffness at a reduced density compared to the non-treated stems. To explain these effects, morphology analysis of stems was conducted. Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) revealed enhanced hydrogen bonding, cellulose crystallinity, and thermal stability. This study developed a novel in vivo approach by incorporating CNFs into plants during early growth to trigger structural reinforcement, which in turn improves mechanical performance.