Because windbreaks are planted for sustainable agriculture but may lower crop yields near them due to competition, suitable competition mitigation methods must be applied at the windbreak-crop interface to increase crop yields. Effective underground competition management requires information on important root variables such as preferential root growth direction and root length density (RLD, root length per unit volume of soil). This study examined root isotropy (i.e., uniformity in all directions) in windbreak- grown cadaghi (Corymbia torelliana) trees in south Florida and developed an empirical relationship between RLD and number of roots (N) per unit of surface of soil exiting the trench face. Numbers of roots exiting the frontal face parallel to the windbreak (NX), vertical face perpendicular to the frontal face (NY), and basal horizontal face (NZ) of 10 x 10 x 10 cm soil cubes were counted. Cadaghi roots were anisotropic and had horizontal growth preference. Average root numbers were ranked NX>NY>NZ. Both NX and the average root number exiting X, Y and Z faces of the soil cube (NAVG) were significant variables for estimating RLD. The coefficients of NX and NAVG were 1.1 and 3.1. These results should be helpful to manage underground competition effectively at the windbreak-crop interface to improve crop yields.