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Introduction Irrigation management in modern filbert ( Corylus avellana L.) orchards is becoming increasingly important due to climate change, with rising temperatures and more frequent arid periods occurring across many hazelnut growing regions. While irrigation is essential to sustain productivity, water scarcity in several areas highlights the need for efficient and sustainable irrigation strategies. Stem water potential (SWP) is one of the most reliable indicators for assessing plant water status, yet no species-specific baseline has been defined for filbert under non-limiting soil moisture conditions. This study aimed to establish an SWP baseline for filbert, following approaches previously developed for olive and Prunus species. Material and Methods Vapor pressure deficit (VPD) was used as the environmental driver defining SWP variability under well-watered conditions. A large dataset of SWP–VPD pairs was collected during the 2024 growing season in two major hazelnut-producing countries, Italy (Piacenza) and Oregon (USA), and supplemented with measurements from previous trials. Results and discussion The resulting baseline, derived from the upper 0.07 fraction of SWP values within VPD classes, revealed a stable and linear relationship between SWP and VPD. This baseline functions as an operational reference: SWP values close to the baseline indicate non-limiting soil water conditions, negative deviations reflect increasing water stress, whereas values exceeding the baseline may signal excessive soil moisture. Overall, the proposed SWP baseline provides a plant-based threshold that can be directly applied in field irrigation management to optimize timing and amount of water application and can be integrated with other plant-based indicators, such as baseline deviations or the Crop Water Stress Index (CWSI), within threshold-based or advanced irrigation decision-support frameworks, supporting more sustainable water use under increasingly variable climatic conditions.