The ideal mechanical performance of a composite is controlled by the level of interfacial adhesion and to overcome the incompatibility issues between plant fibres and polymeric matrices, the surface of flax yarns has been modified by zinc oxide (ZnO) nanostructures. The ZnO nanorods were synthesized through a lowtemperature hydrothermal treatment, and several parameters have been analysed in order to obtain a uniform and homogeneous ZnO interphase, such as the number of the seeding cycles, the growth reaction times and the replacement of the growth solution. The results showed that it is possible to obtain highly oriented and well aligned ZnO nanostructures (by FE-SEM), with an hexagonal wurtzite structure (by XRD) and a high degree of coverage along the whole yarn (by TGA), reducing the number of the seed cycles, with rather long growth times (5 hours) and substituting the growth solution at least once during the synthesis. The experimental conditions preserved the breaking force of the yarns, while Single Fibre Fragmentation Tests (SFFT) highlighted a better interfacial adhesion of ZnO-modified flax yarns with epoxy matrix, which displayed a 29 % reduction in average debonding length relative to untreated yarns.