Background Landslides caused by soil erosion are common natural geological disasters all over the world. Planting plants on hillsides is an effective project to conserve soil and water during urban construction and mountain development. The mechanical properties of plants are affected by many factors, such as root diameter, root length and rate of water content, as well as the loading ways, loading speed, logging time, etc. Effect of root length and diameter on root tensile properties of herbaceous plants in earth-rocky mountain area is still scarce.

Methods This study aims at three slope protection plants:Lolium perenne L., Vetiveria zizanioides L. and Paspalum notatum Flugge in earth-rocky mountain areas in Shanxi Province, northern China. Three different root lengths:60, 80 and 100 mm were selected in this study. Root tensile tests were conducted with a WDW-5 universal electronic testing system. In the tests, maximum tensile resistance, ultimate tensile strength and elastic modulus with changing root diameter and root length were measured and analyzed.

Results The root diameter of V. zizanioides L. and L. perenne L. mainly varied in the range of 0.2-1.0 mm and P. notatum Flugge varied in the range of 0.1-0.4 mm, and the average values of the diameters were 0.639, 0.557 and 0.230 mm respectively. The average maximum tensile resistance of the three species was in the order:V. zizanioides L. (16.258 N) > L. perenne L. (11.734 N) > P. notatum Flugge (4.891 N). The average tensile strength ranked as P. notatum Flugge (116.226 MPa) > L. perenne L. (50.839 MPa) > V. zizanioides L. (49.650 MPa). The average elastic modulus was in order as P. notatum Flugge (20.392 MPa/mm) > V. zizanioides L. (3.257 MPa/mm) > L. perenne L. (3.245 MPa/mm). The elastic modulus of P. notatum Flugge was about six times larger than the others. With certain root length, the maximum root tensile strength of the three plants decreased with root diameter increasing. Within certain root diameter, the maximum root tensile strength decreased with root length increasing. In addition, the elastic modulus decreased with both root diameter and length increasing.

Conclusions According to the results, P. notatum Flugge has the best ability of soil fixation and slope protection among the studied three species. It can be a suitable species to maintain soil and water. The results provide a data basis for slope protection and soil and water conservation with vegetation in earth-rocky mountain areas.