Comprehensive Phenological Documentation of Idesia polycarpa Using the BBCH Scale: From Bud Development to Dormancy

Idesia polycarpa Maxim (Salicaceae) is an endemic woody oil tree native to China with multi-purpose economic and ecological applications, including seed oil production, timber utilization, landscape afforestation and biomass exploitation. To date, a unified phenological classification system tailored to this species remains unavailable, restricting standardized cultivation practices and germplasm phenotypic assessment. Here, we developed the first comprehensive Biologische Bundesanstalt, Bundessortenamt und CHemische Industrie (BBCH) scale phenological coding system for I. polycarpa through two consecutive full-year phenological monitoring trials in Guizhou Province, Southwest China. We defined eight principal growth stages spanning the complete annual cycle: bud development, leaf expansion, shoot elongation, inflorescence differentiation, blooming, fruit development, fruit ripening, and leaf senescence coupled with dormancy initiation, together with 40 morphologically described secondary stages supported by field photographic documentation. Synchronous vegetative and reproductive growth was observed, while staminate individuals exhibited earlier and longer flowering periods relative to pistillate trees. Three female genotypes were harvested at BBCH principal stage 89 (full fruit maturity) to verify the applicability of the established scale. Distinct genotypic differences in fruit dimensional traits, 1000-seed weight and fatty acid composition were characterized, confirming that BBCH-standardized harvesting eliminates confounding temporal biases for oil quality comparisons. This standardized BBCH coding system offers a universal phenological descriptor for I. polycarpa and delivers actionable protocols for orchard fertilization, canopy pruning, pollinizer arrangement and uniform fruit collection. Our findings establish a baseline phenotypic framework to facilitate germplasm discrimination, targeted breeding and intensive sustainable production of this commercially valuable indigenous oil tree.