Replacing glass fibres with flax fibres is a first step in reducing the ecological impact of thermoset composite materials, and employing a biodegradable thermoplastic matrix opens up recycling and composting as end-of-life routes. Here, a range of flax fibre reinforced biodegradable thermoplastics were investigated: poly-(hydroxy alkanoate) (PHA), poly-(butylene-succinate) (PBS) and poly-(lactide) (PLA). Poly-(propylene) (PP) and maleic-anhydride grafted poly-(propylene) (MAPP) were studied as industry benchmarks. This study systematically examines the interface between flax fibres and these matrices at multiple scales, and explores the correlations between the measured interfacial properties and macro-scale composite properties. Micro-droplet tests reveal that the adhesion of flax with biodegradable polymers is at least similar to flax-MAPP, and better than flax-PP. In-plane [±45]s shear tests and tensile tests on unidirectional composites reaffirm the observations at the micro-scale, that biodegradable polymer/flax composites present mechanical properties comparable to or better than MAPP/flax composites. Furthermore, comparison between interfacial and composite tensile properties reveals that fibre-matrix adhesion has a substantial role in biocomposite performance.