In this study the impact of senescence and harvest time in Miscanthus on the quality of fast pyrolysis liquid (bio-oil) was investigated. Bio-oil was produced using a 1kgh(-1) fast pyrolysis reactor to obtain a quantity of bio-oil comparable with existing industrial reactors. Bio-oil stability was measured using viscosity, water content, pH and heating value changes under specific conditions. Plant developmental characteristics were significantly different (P⩽0.05) between all harvest points. The stage of crop senescence was correlated with nutrient remobilisation (N, P, K; r(2)=0.9043, r(2)=0.9920, r(2)=0.9977 respectively) and affected bio-oil quality. Harvest time and senescence impacted bio-oil quality and stability. For fast pyrolysis processing of Miscanthus, the harvest time of Miscanthus can be extended to cover a wider harvest window whilst still maintaining bio-oil quality but this may impact mineral depletion in, and long term sustainability of, the crop unless these minerals can be recycled.