Abstract: Plant roots influence the engineering properties of soil, such as erodibility and strength. Plant roots’ contribution to soil shear strength is of particular importance in Arctic and subarctic environments where the shallow subsurface experiences a decrease in shear strength due to permafrost thaw, subsidence, and wildfires. This paper presents the testing method, sample collection and specimen preparation, and tensile strength testing results for laboratory- and greenhouse-grown boreal forest plants to compare root tensile strengths among plant species and functional groups, including deciduous shrubs and trees, evergreen trees, forbs, graminoids, and grasses using a universal testing machine and a modified triaxial device. The results illustrate that root tensile strength increases as root diameter decreases (as a power function). The root diameters successfully tested ranged from 0.063 mm (grasses) to 8.72 mm (deciduous shrubs) across all functional groups. When compared across functional groups and root diameters for each species, grass roots exhibited the highest tensile strength for root diameters less than 0.8 mm, deciduous tree roots displayed the largest tensile strength for root diameters greater than 0.8 mm, and forbs were consistently the weakest, supporting the conclusion that a diverse spread of functional groups is most effective for slope stabilization.