Environmental variability and interannual changes in nectar production of common milkweed (Asclepias syriaca L.) in Central Europe

The invasive common milkweed ( Asclepias syriaca L.) is a widespread species in Central Europe that negatively affects native biodiversity and ecosystem processes, particularly soil moisture dynamics, while simultaneously providing a valuable nectar resource for pollinators. This study evaluated the nectar volumes and sugar concentrations of common milkweed across different sites and years in Hungary and a neighboring region of Serbia over three consecutive years (2020–2022), focusing on its ecological role and potential contribution to honey production. Nectar standing crop was measured at three sites – two abandoned vineyards and one dry sandy grassland – alongside assessment of environmental factors and vegetation composition. Nectar volume and sugar concentration exhibited significant spatial and temporal variation, ranging from 0.5 to 2.7 µL per flower and from 27% to 40%, respectively. The two abandoned vineyard sites included in this study were associated with higher nectar volumes and sugar concentrations than the dry sandy grassland site. Significant variability in nectar traits was observed both among individual shoots and among inflorescences within the same plant. Higher nectar volumes were recorded under conditions of higher soil moisture and relative humidity, while extreme heat, with temperatures of 35 to 40 °C and prolonged drought substantially reduced nectar volumes. Estimated honey yields based on standing-crop measurements varied considerably among sites and years, with potential production exceeding 100 kg ha⁻¹ under favorable conditions but declining markedly during periods of drought and heat stress. Our results demonstrated that A. syriaca can serve as a valuable nectar source for honeybees in Central Europe, although its nectar productivity is strongly influenced by site characteristics and climatic conditions. These findings contribute to a better understanding of the ecological and apicultural significance of this invasive species and underscore the importance of considering environmental conditions and climatic variability when assessing its role as a floral resource and its potential contribution to honey production.