Flowering in many orchids is determined by the resource status of plants, which in turn is influenced by habitat management. Most European orchids require high light intensities for photosynthesis, failing to flower and fruit at low light levels. Agricultural practices, especially fertilisation and mowing/grazing regime, can therefore influence the fitness and reproductive success of orchids. The studied species, autumn lady's-tresses (Spiranthes spiralis), is a small, long-lived and late-flowering perennial orchid and one of the most sensitive to low light availability. The general species' fitness, measured as a set of robust morphological traits in relation to vegetation height, which directly reflect the time of the last seasonal mowing and so the light availability, was the main scope of this research. A total of 2442 flowering exemplars (of which 427 were morphologically evaluated) were recorded on 26 grassland patches applying the systematic scanning of the potential growing sites in the Goričko Natural Park (NE Slovenia). We revealed that earlier mowing negatively affects the density of flowering individuals and plant fitness in general. The number of the rosette leaves was found to be the most important trait that could be used as a proxy for the general plant fitness of this orchid species because the significant positive correlation with the measured morphological traits was confirmed. The number of the rosette leaves shows a significant negative relation to vegetation height. Late seasonal mowing, which has a significant positive impact on the plant fitness, enhances Spiranthes spiralis population viability and density.