Plastic pollution, especially pollution by micro- and nanoplastics, is now considered a potential threat to all ecosystems, including terrestrial ecosystems such as grassland habitats. This study investigated the impacts of micro- and nano-sized plastics on Bromus hordeaceus, a common grass species in European grasslands. The micro and nanoparticles were fossil-based polyethylene (PE) or plant-based polybutylene adipate terephthalate (PBAT), and these two plastics were used at two different concentrations. Here, we report data on plant development and plastic-plant interactions from two different experiments, (1) an in vitro experiment to test seed germination and establishment and (2) a soil experiment to test plant development and plastic-plant interactions specifically investigated as a form of perforation. Results from the in vitro experiment indicate that while seed germination success was unaffected by plastic type, the presence of all plastic particle types acted as a stimulant, increasing the total length of radicles and sprouts of germinated seeds. Conversely, results from the soil experiment showed that the growth of Bromus hordeaceus was negatively affected by the presence of microPBAT in the soil during the pot assay. Microscopic analysis confirmed that seed and plant structures interacted with all plastic particles via adsorption or perforation. This study demonstrates for the first time the ability of roots to penetrate plastics, especially microPBAT particles. Overall, our study concludes that both fossil-based and plant-based micro- and nano-plastics can influence plant growth, with effects varying based on plastic type, concentration, and plant growth phase. Further research is crucial to fully understand the intricate interactions between microplastics, soil properties, and plant development.