Green roofs provide beneficial ecosystem services in urban environments, including mitigating heat island effects, reducing stormwater runoff, and enhancing biodiversity. However, these benefits largely depend on plant species that are well adapted to the local climatic conditions. This study evaluated the performance of six native grassland species —Imperata cylindrica, Patrinia scabiosifolia, Dianthus superbus var. longicalycinus, Sanguisorba officinalis, Prunella vulgaris subsp. asiatica, Adenophora triphylla var. japonica —on extensive green roofs by assessing their growth characteristics (height, coverage, flowering rate, and survival rate) during the first year after planting. We used three different kinds of substrate that differed mainly in nutrition level: (i) low-nutrient commercial perlite-only substrate (P), (ii) nutrient-rich reused green roof substrate (R), and (iii) a mixture of P and R in a 1:1 volume ratio (PR). The results indicated that Patrinia scabiosifolia exhibited high adaptability regardless of substrate nutrient levels. Imperata cylindrica showed high survival rates, though its size varied significantly depending on nutrient availability. Dianthus superbus var. longicalycinus, Sanguisorba officinalis, and Prunella vulgaris subsp. asiatica appeared to require additional nutrients when grown in the low-nutrient P substrate. For Adenophora triphylla var. japonica, further investigation was needed to determine the optimal nutrient conditions. Our findings suggest that several Japanese grassland species can perform well on extensive green roofs when provided with adequate nutrients, contributing to both green roof sustainability and the conservation of declining grassland species.