Expansive soil, characterized by swelling upon water absorption and shrinkage upon drying, is widely distributed. Plants are often used to enhance the engineering properties of soil due to their environmental friendliness. Investigating the effects of vegetation on the compressive and shear strength characteristics of weakly expansive soil is fundamental to the application of plants in treating expansive soil. The effects of Bermuda grass on the strength properties of weakly expansive soil were systematically investigated. Specifically, unconfined compressive strength tests and unsaturated direct shear tests were conducted to explore the effects of factors such as planting density, vertical pressure, and suction on the strength of weakly expansive soil. Bermuda grass was planted at densities of 25 g/m², 40 g/m², and 50 g/m², corresponding to groups P25, P40, and P50, respectively. The test results show as follows. Plants enhance the unconfined compressive strength of weakly expansive soil, with this strengthening effect becoming more pronounced as soil water content increases. For instance, compared with the bare soil, the P50 group exhibits compressive strength increases of 11.5%, 17.3%, and 25.2% at water contents of 27.5%, 32.5%, and 37.5%, respectively. Plants significantly increase the effective cohesion (c') and the unsaturated friction angle (ϕ_b) of weakly expansive soil. The effective cohesion values of groups P25 (root content: 0.98 kg/m³), P40 (root content: 1.25 kg/m³), and P50 (root content: 1.49 kg/m³) are 1.15, 1.42, and 1.54 times that of the bare soil group, respectively. With increasing suction, the ϕ_b values of the planted groups stabilize at approximately 1.5 times that of the bare soil. Plants mitigate the volume changes in weakly expansive soil during shearing. As suction increases, the soil behavior transitions from shear contraction to dilation, with higher planting densities resulting in progressively smaller volumetric change.