The Japanese knotweeds (Reynoutria japonica complex; Polygonaceae) are characterised by a highly efficient vegetative propagation. Their resprouting capacity is associated with rhizomes, whose nodal structure distinguishes them from non-regenerative roots. Knowing the distribution of both organ types is therefore a key prerequisite for eradication measures. The present study aimed to assess (i) the distribution patterns of rhizomes and roots, (ii) the resprouting capacity of rhizomes, and (iii) the overall resprouting potential of Reynoutria japonica and Reynoutria ×bohemica. For this purpose, we characterised the underground organs of six established and undisturbed populations and carried out resprouting tests with rhizomes in a greenhouse. Our results highlighted that the rhizome biomass of both taxa is mainly concentrated in the top 40 cm of soil, although some outlying rhizome organs may be found deeper down depending on soil compaction. On average, the observed density of nodes per square metre was 2646, ranging from 1825 in compact soil to 3825 in loose soil. Conversely, roots mostly occur in deeper soil layers. The rhizome resprouting capacity was significantly explained by pith brightness, organ diameter, and taxon. Based on these findings, we estimated the overall potential of resprouting nodes by linking the number and characteristics of visible rhizomes along the soil profile to their resprouting capacity. We obtained expected densities of 1981 ± 198 and 1935 ± 305 nodes/m² for R. japonica and for R. ×bohemica, respectively. We conclude with practical recommendations on incorporating preliminary rhizome depth and resprouting potential assessments into management practices to optimise resource allocation.