Non-woven composites reinforced with plant fibers are widely used in the automotive and construction sectors. The vast majority is composed of petroleum-based, non-compostable polyolefins, which are no longer a viable solution in an environmental context where the end-of-life management of industrial products is becoming a major societal issue. Here, fully green composites are produced by reinforcing three bio-based and biodegradable matrices – poly-(hydroxyalkanoate) (PHA), poly-(butylene-succinate) (PBS) and poly-(lactide) (PLA) – with non-woven flax fiber preforms. Notably, their mechanical performance was observed to be at least equivalent to the industry reference – poly-(propylene) (PP) reinforced non-woven flax. These composites were then buried in an instrumented garden compost, and the evolution in microstructure and mechanical properties was studied over a period of six months. Microtomography studies revealed that evolution in composite microstructure principally depended on the polymer matrix: surface degradation was predominant for PBS and PHA biocomposites, whereas rapid fiber-matrix interface degradation in the core was observed for PLA biocomposites. Interestingly, even after six months in the compost, all composites exhibit tensile strengths of at least 50% of their initial value. Moreover, the strength reduction in biodegradable composites was of the same magnitude as the industry reference, flax/PP composite. These results demonstrate the potential of biocomposites in resolving the ‘biodegradation paradox’: flax composites with biopolymers like PLA, PHA and PBS can be designed to have adequate mechanical performance for industrial products, even after ageing in harsh conditions, and yet offer an alternative end-of-life route to the typical incineration (with or without energy recovery).