In reclaimed waste rocks slopes, the soil cover spread for revegetation is prone to erosion. This soil needs to be immediately protected from soil erosion by above and belowground vegetation. The seeding of fast-growing herbaceous species used in agriculture is generally used on waste rock slopes to control erosion, but these compete with planted trees for resources needed for growth. The aim of this study was to assess the ability of an alternative to these herbaceous species (i.e. fast-growing hybrid poplars) to mitigate soil losses in the short-term. However, the growth of poplars could be impaired by competition from weeds’ colonization. Tree planting density was expected to change competition levels among trees and between trees and weeds, so influencing above and belowground vegetation development and hence effectiveness at controlling soil erosion processes. Five treatments were installed in 2013: 1 × 1, 2 × 2, 4 × 4 m spacing of hybrid poplars; 2 × 2 m spacing, with hydroseeding of fast-growing herbaceous plants; and a control plot with no tree planting or hydroseeding. The planting of hybrid poplars did not decrease soil losses compared to the control plot, regardless of the tree planting density. Weed development through natural colonization occurred in all the non-hydroseeded plots and was more effective at soil erosion control in the short-term than planted trees. The selected clone of hybrid poplar coped well with any competition from weeds for water, since 100% of trees survived after 3 years and since the non-hydroseeded plots produced greater length of roots per soil volume (root length density, RLD) for the poplars compared with the weed species. As early as the first year, the hydroseeded plots showed the highest RLD and a complete cover of vegetation which effectively controlled soil erosion compared to the control plots. However, both treatments with increased competition levels (i.e. highest tree planting density and hydroseeding treatment) showed less aboveground tree biomass. On the hydroseeded plots, where interspecific competition with weeds was the highest, a greater increase in root length was seen during the third year after planting. After 3 years, canopy closure was achieved in the 1 × 1 m spacing treatment, which reduced weed cover.