Aims <p>Electromagnetic sensors are widely used to measure the water content of soil. The aim of the study was to determine whether soil moisture sensors can be used to reliably measure the water content of tree stems and sawdust with the factory-supplied soil calibration equations or require specific calibration.</p> Methods <p>Nine commercially-available soil sensors were calibrated to measure the water content of sawdust of three tree species packed to three bulk densities in the laboratory. Five of the soil moisture sensors were calibrated to measure the water content of stem segments of five tree species.</p> Results <p>The calibration equations for sensors measuring water content of the stem segments and sawdust differed significantly from the factory-supplied soil calibration equations. While one sensor (TLO) gave approximate sawdust water contents and one sensor (TEROS 10) approximate water contents of tree stems with their factory-supplied calibration equations, all sensors required calibration in both sawdust and tree stems to accurately measure their water content; the calibrations derived are reported along with their accuracy. The combined equation for the relationship between measured and estimated water content gave acceptable estimates of the water content for all five species of tree stems and three species of sawdust.</p> Conclusions <p>Factory-supplied soil calibrations of soil moisture sensors are not sufficiently accurate for measuring the water content in tree stems and sawdust. The equations derived in this study provide a more accurate estimate of tree stem and sawdust water contents. The combined equations for the water content of tree stems and of sawdust provide a way to use the soil sensors for the measurement of the water content of tree stems and sawdust without calibration for each tree species.</p>