Abstract This paper investigates the performances shown during underground exploration by a plant root-inspired soil intruder. Plant roots are efficient soil explorers, moving by growing at their apical extremities and morphing their bodies in response to mechanical constraints. A three-dimensional (3D) discrete element model (DEM) was developed to mimic selected features of plant roots and verify their usefulness in soil penetration operations. Specifically, the model is used to simulate the penetration of an intruder that grows at the tip into both cohesionless granular and cemented soils. In the former case, dense and loose granular media are considered. The model is adopted to compare penetration performances with purely axial growth and a combination of radial and axial growths. Radial growth is hypothesized to be adopted in roots to facilitate soil penetration. Results from our model suggest that implementing a radial growth preliminary to an axial growth is more advantageous in cohesionless dense granular soil, reducing the soil resistance experienced by the intruder for deeper penetration after radial enlargement. When the penetration occurs in cemented soil, the radial expansion results advantageous over a lower penetration depth, and its beneficial effect drops with increasing inter-particle contact adhesion values. The proposed 3D DEM numerical model provides a methodology for evaluating the intruder penetration efficiency and supports the design of artificial robotic systems for the autonomous exploration of soil by allowing the selection of the most performant penetration strategies for their artificial implementation.