Debris flows often increase in size due to bed erosion and entrainment, enhancing their hazardous potential. However, the effects of plant rooting on debris-flow erosion on ubiquitous vegetated slopes remain unknown, which hinders debris-flow hazard assessment. Here, we investigated the effects of roots on debris-flow bed erosion using scaled experiments in a 5 m long, 0.3 m wide laboratory flume with an erodible bed. Roots of fast-growing Sorghum bicolor (Sudan grass) seedlings were used as proxies for tree roots to quantify the effect of varying rooting characteristics on erosion. Our results indicate that erosion decreases non-linearly with increasing Root Length Density (RLD) and Root Area Ratio (RAR). Increases in either parameter enhance root–soil contact, thereby improving soil stability and reducing erosion. Among the two, RLD, and thus the combined effect of root length and root density, appears most influential, as RAR does not capture the three-dimensional structure of the root system. Our experimental results suggest that increasing root-soil contact at the debris-flow bed reduces erosion, decreasing or even preventing debris-flow volume growth. These findings imply that alterations in vegetation characteristics, such as those resulting from forest fires or reforestation, affect debris-flow erosion and open up possibilities for biogeomorphic scale experiments for slope processes.