Backgrounds and aims – Origanum vulgare L. is a successfully recovering gynodioecious species in the ecological restoration project along the Common Meuse (Belgium). Strong contrasts exist between spreading – mostly newly established – populations on the one hand, and remnant, declining populations on the other hand. The flowering strategy of the species was assumed to be related to these differences in population status. Two alternative hypotheses for small, isolated populations were postulated: either they contain more sterile individuals due to their presence in marginal conditions, or they show more hermaphrodites for reproductive assurance. Additionally, we aimed at relating neutral genetic variation of the populations with their status and sex ratio. Methods – An analysis at two levels was performed to get a consistent image of the differences in flowering strategy among and within populations. For the population-level survey, 25 populations were sampled for flowering strategy and for genetic analysis. For the within-population analysis, the survey focused on the expanding populations. Five expanding populations on the riverbanks were fully surveyed for the sex ratio of all individuals. Statistical analysis focussed on relationships between the populations' sex ratios and population genetics, size and dynamics, and stand conditions related to management intensity and isolation. Key results – Both hypotheses were supported, but at a different level. At the population level, the smaller populations showed more sterility. But within the expanding populations, isolated parts showed less sterility. For the observed sexual expression at population level, no significant correlation was present between sexual expression and population dynamics, isolation or management intensity. For the within-population differentiation, a relationship with reproductive assurance in expanding populations was shown. For the observed differentiation in sex expression, indications are present for the flower strategy being a developmental plasticity. Conclusion – For O. vulgare, the gynodioecy proves a successful plastic strategy for expanding and new establishing populations to cope with pollen limitation and inbreeding depression.

ADAPTATION; COLONISATION; PLASTICITY; REPRODUCTIVE ASSURANCE; RIPARIAN CORRIDOR