This study optimizes flexural properties of composites made from waste materials, corn stalk, seed flax and Agave americana fibres along with waste linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) matrices. The surface morphology of fibres and composites was characterized with light and scanning electron microscopes. Thermal behaviour of the corn stalk outer rings and pith parenchyma was shown using thermogravimetric analysis. There was no significant difference between the LLDPE composites made with corn stalk outer rings versus whole stalks, possibly due to an insignificant role played by parenchymatous part of the pith. Water retted seed flax fibres in LLDPE matrix optimized the composite flexural strengths at 6-8 days of retting, above which the properties declined. Field retted seed flax varieties in LLDPE matrix showed no significant differences in their flexural strengths. With additional fibre loading, flexural strength of A. americana HDPE composites first dropped before improving after extrusion, and only improved when using the layer method.