In the present field study, the capability of Canada thistle to developshoots from intact roots and root fragments at different soil depths wasstudied. The experiments were performed on four sites with high-densityCanada thistle, with three or four replications per treatment. At each site,the soil in the plots was removed layer by layer (to 30 or 40 cm, dependingon the site), within a 1 by 1-m quadrat, and spread out on a plastic sheet.All roots and other plant parts were removed, and the soil was eitherreplaced without any root material (two sites), or the roots of the thistleswere cut into 10-cm-long fragments and replaced into the source holes (twosites). The measured variables were shoot number and biomass. The number ofshoots of Canada thistle decreased with increasing depth (P < 0.001) andincreased with time. Additionally, the two factors interacted (P < 0.001)such that shoot development was slower from greater depths. Roots from ≤ 20cm depth produced higher biomasses than did roots from below 20 cm depth.Replacement of root fragments did not affect the amount of biomass produced.It was concluded that the intact root system contributed considerably moreto the total biomass produced by Canada thistle than did the root fragmentsin the upper soil layers.