Understanding the regenerative capacity of Japanese knotweed (Reynoutria japonica Houtt.) is essential for developing effective management strategies. This study investigated how rhizome fragment diameter – classified as thin (≤ 1.0 cm), medium (1.1–2.0 cm), and thick (≥ 2.0 cm) –affects early sprouting. The one-way ANOVA and Tukey’s HSD post-hoc test revealed that sprout length increased significantly with rhizome diameter (p < 0.001), accounting for 62.40% of the variation in sprout growth. Specifically, thick fragments produced the longest sprouts (38.29 ± 1.70 cm), significantly outperforming medium (27.38 ± 1.43 cm) and thin (17.67 ± 1.76 cm) fragments. Despite these differences in vigor, resprouting frequency did not differ significantly between rhizome classes (p > 0.05), indicating that even the thinnest fragments retain regenerative potential. The hierarchical cluster analysis (HCA) further identified two functional clusters, separating thin fragments into a “survivalist” cluster characterized by reduced growth compared to medium and thick fragments, which formed a “vigor-driven” cluster with a far superior regenerative potential. Moreover, these findings highlighted a critical management loophole; that is, mechanical control alone may have contributed to rhizome fragmentation, creating the impression of reduced infestations while viable fragments persisted and regenerated following disturbance, a phenomenon termed the “latency period.” Thus, effective management must shift from strategies that merely fragment rhizomes to those that ensure the complete suppression of rhizome viability.