Plant root systems serve as a natural reinforcing material, significantly improving soil stability. Furthermore, the tensile strength of soil is crucial in mitigating the formation of cracks. Consequently, this study aims to investigate the influence of plant roots on the tensile strength of soil. For this investigation, Amorpha fruticose was selected due to its large root diameter and the ease of root extraction. Indoor tensile tests were conducted on individual roots and root-soil complexes under three varying factors. The results indicate a power law relationship between root diameter and tensile strength. Increased root content and dry density notably enhance the tensile strength of the root-soil complex while roots mitigate damage associated with soil brittleness. When root content increases from 0 to 10, the maximum enhancement in tensile strength of the root-soil complex reaches 42.3 kPa. The tensile strength of the root-soil complex at a dry density of 1.7 g/cm3 is four to five times greater than that of the complex at a dry density of 1.4 g/cm3. Moreover, as moisture content increases, the tensile strength of the root-soil complex initially rises before declining, with an increase range of 7.7-35.8 kPa. These findings provide a scientific basis for understanding the role of vegetation roots in soil tensile strength and for guiding slope reinforcement strategies.

The slope erosion in the distribution area of completely weathered granite is often relatively severe, causing serious ecological damage and property loss. Ecological restoration is the most effective means of soil erosion control. Taking completely weathered granite backfill soil as the research object, two types of slope protection plants, Vetiver grass and Pennisetum hydridum, were selected. We analyzed these two herbaceous plants' soil reinforcement and slope protection effects through artificial planting experiments, indoor simulated rainfall experiments, and direct shear tests. The test results showed that the runoff and sediment production rates of the two herbaceous plant slopes were significantly lower than those of the bare slope, with the order of bare slope > Vetiver grass slope > Pennisetum hydridum slope. Compared with the bare slope, the cumulative sediment production of the Vetiver grass slope at 60 min decreased by 56.73-60.09%, and the Pennisetum hydridum slope decreased by 75.97-78.45%. The indoor direct shear test results showed that soil cohesion decreases with increasing water content. As the root content of Vetiver grass roots increases, soil cohesion first increases and then decreases, reaching a maximum value when the root content is 1.44%. As the root content of Pennisetum hydridum increases, soil cohesion increases. The internal friction angle increases slightly with increasing water content, while the root content does not significantly affect the internal friction angle. Therefore, the shear strength of soil decreases when the water content increases. The shear strength of the Vetiver grass root-soil composite reaches a peak at a root content of 1.44%, while the shear strength of the giant king grass root-soil composite increases as the root content increases. At the same root content, the shear strength of the Vetiver grass root-soil composite is slightly higher than that of giant king grass. The reinforcement effect of roots on shallow soil is better than on deep soil. Both herbaceous plants have an excellent soil-fixing and slope-protecting impact on the fully weathered granite backfill slope. Pennisetum hydridum's soil and water conservation effect is significantly better than that of the Vetiver grass. In contrast, Vetiver grass roots slightly outperform Pennisetum hydridum in enhancing the shear strength of the soil. The research results can provide a theoretical basis for the vegetation slope protection treatment of fully weathered granite backfill slopes.