Management of perennial weeds has become increasingly complex with the reduction of herbicide use and tillage. Their capacity to store nutrients into vegetative organs and to regenerate to establish new individuals makes them very persistent and hard to control. A key aspect for managing perennial weeds is to minimise storage reserves to reduce vegetative regeneration. For that prospect, better knowledge of the regrowth capacities from storage organs is needed. Our work focused on the mechanistic understanding of belowground regrowth dynamics from fragments of Elymus repens rhizomes by analysing the effect of source and sink factors. The effect of rhizome fragment weight (i.e., source) and the number of regrowing buds and belowground shoots (i.e., sinks) was assessed for four mechanisms: sprouting, belowground shoot elongation, belowground shoot biomass to length conversion and biomass allocation to belowground shoots versus roots. Heavier fragments contributed to faster sprouting, faster belowground shoot elongation and increased the proportion of remobilised biomass allocated to belowground shoots. Higher belowground shoot density increased the proportion of remobilised biomass allocated to belowground shoots but decreased individual elongation rate. In other words, larger reserves resulted in a better regeneration capacity and a higher number of growing belowground shoots increased the competition for reserves among them. The belowground shoot that sprouted first on a fragment grew faster and dominated the other shoots on the same fragment. Dominant belowground shoots were located toward the apical end of the fragment. These findings support the fact that tillage should aim at cutting storage organs into the smallest fragments possible to reduce regeneration capacity. This study also emphasised that belowground regrowth mechanisms are complex, and further studies should investigate other species with different types of storage organs.