Plant stems show reversible diurnal fluctuation and irreversible growth, both related to plant water status. The reversible stem diameter shrinkage and swelling are caused by a depletion and refilling of the plant’s internal water storage pools, while irreversible growth occurs when turgor pressure exceeds a certain threshold value. For this reason, stem diameter measurements provide a useful tool to assess plant water status. In this study, the use of continuous stem diameter measurements to detect atmospheric and soil drought stress in Ficus benjamina L. was explored by assessing the deviation between measured and simulated stem diameter variations using a mechanistic stem diameter model with moving window calibration. Sap flow, either directly measured or simulated with measurements of the microclimate, was used as input to the model. Both atmospheric stress by a high vapor pressure deficit and drought stress by a reduced soil water content were detected, with the latter 2 to 3 days earlier than detection based on maximum daily shrinkage and daily growth. Therefore, comparing stem diameter measurements with model simulations shows great potential for irrigation scheduling in horticulture.