Vegetation clearly affects soil strength, but how to take these effects into account in conventional calculations of slope stability is still an unsolved problem. To quantify the important influence of plant roots on shear strength of a moraine, we performed isotropic, consolidated-undrained triaxial compression tests with different confining pressures (σ3' = 50, 75, 100 kPa). Three different types of samples were tested: A) planted soil and B) pure soil at low dry unit weight (γ ≈ 15.5 kN/m3) as well as C) pure compacted soil at γ ≈ 19 kN/m3. The planted samples were prepared with alder seeds (Alnus incana). For each sample type, failure lines (kf lines) were calculated using the peak shear strength points of the corresponding p'-q paths. Robust statistics were performed to fit the failure lines and to test for significance. No differences were found in the cohesion (c') of the different soils. However, there was a significant difference in the angle of internal friction (φ') of about 5° between the samples of pure soil at low dry unit weight and those of both compacted and planted soil. The vegetation effect is thus apparent as an increase in the angle of internal friction φ' in planted soil compared to pure soil at the same dry unit weight. This finding can also be considered as a virtual increase in soil density (from γ ≈ 15.5 to α ≈ 19 kN/m3).