Poor water retention capacity and the interior porous structure of vegetal concrete are the main factors for the limited lifespan of plants. In this study, bio-based miscanthus is applied to enhance water absorption-release behaviour and plant growth adaptability by filling the pore structures of porous vegetal concrete. The effects of varying miscanthus powder content (0%, 5% and 10% by volume) on the physical properties (including density, porosity and water absorption-release behaviour), mechanical strengths (compressive and flexural strengths) and plant growth performance indicators (plant height, number of leaves, rooting behaviours and plant cover) of concrete are investigated. Furthermore, the influences of different cement-to-miscanthus ratios (1:0.25, 1:0.5, 1:0.75, 1:1) and planting methods (mixing seeds with mortar matrix vs sowing seeds on mortar surface) on the performance of miscanthus mortar are analysed. The addition of miscanthus not only strongly improves the water absorption-release behaviour of porous vegetal concrete but also helps lower its density and high alkalinity, both of which enhance plant growth. The miscanthus increases the 24-hour water absorption of bio-based vegetal concretes by 119-246%, compared to the reference concrete, rapidly reaching a saturation status when soaked in water. Moreover, adding miscanthus at a 10% volume enhances the plant cover percentage by 36.5% and slows down early plant degradation. It also shows that sowing seeds on the mortar surface is better than mixing them with mortar matrix because it makes plants much taller and covers more ground. It can be concluded that the cement-to-miscanthus ratio of 0.5-0.75 is recommended for bio-based vegetal mortar to foster lush plant growth considering the mechanical strengths and plant growth adaptability.