Seed size dimorphism and environmental factors govern the phenological variations in invasive plant Hyptis suaveolens

Persistent intra-individual variations, although understudied, may explain the population- and community-level ecological patterns of plant species. However, there is a lack of studies investigating the intra-individual phenological variations of a species under the influence of seed size dimorphism, especially in the case of invasive plant species. Therefore, a study was conducted to assess the phenological changes in plants grown from dimorphic seeds (small and large) produced within the persistent calyces of the invasive plant Hyptis suaveolens. Furthermore, the impact of various environmental factors (temperature, thermal time, photoperiod, and relative humidity) on the phenological events of plants grown from both seed classes was also analyzed. In this study, the duration and performance (%) of three phenological events—leaf emergence, flowering, and seed formation—were documented throughout the plant's life cycle (from July 2019 to February 2020). It was observed that all the phenological events lasted longer in plants of the large seed class compared to those of the small seed class. While leaf emergence began and ended on nearly similar days for plants of both seed classes, an earlier occurrence of flowering and seed formation events was observed for plants of the small seed class. The plants grown from small-sized seeds also exhibited lower thermal time requirements, greater photoperiodic requirements, and higher temperature requirements for flowering. The study concludes that phenology in H. suaveolens is influenced by both seed size dimorphism and varying environmental factors, resulting in phenological adaptability and subsequent spread over space and time, which may contribute to increased invasiveness of the species.