Continuous measurement for stem diameter variations allows to monitor the phenology, growth dynamic and related-physiology of plants. During a drought event, the decrease in stem diameter is due to a loss of water balance, mainly detected in the non-rigid bark. We have used a dedicated device (PepiPIAF system, Hydrasol) to monitor the response to extreme drought of lavender (Lavandula angustifolia). Lavender is a crop living under very dry conditions (Mediterranean biome) and barely studied. During a dehydration-rehydration experiment, physiological parameters such as water potential, stomatal and hydraulic conductances, cell damages and ultrasonic acoustic activity, were continuously monitored in relation with stem diameter. Our results highlight the dehydration threshold (as a ratio of diameter loss) that significantly affect the resilience to drought and related mortality. We therefore developed a mortality index based on stem diameter monitoring that could help to anticipate lavander death. Based on the common mechanisms affecting plant, it should apply to many different species.