• A novel 3D limit analysis framework integrates root transpiration and root–soil mechanical interaction to evaluate the stability of geosynthetic-reinforced slopes with vegetation.
• The required dimensionless reinforcement strength is derived via a functional balance equation, providing a design tool for engineers.
• Validation against 2D vegetated and 3D non-vegetated solutions confirms the method's accuracy and reliability.
• Incorporating 3D effects and root reinforcement significantly reduces required reinforcement strength, lowering construction costs and improving slope safety.