Application of natural plant additives allows improving thermal and mechanical properties of concrete composites. Environmentally friendly wood waste is gaining particular popularity. One of the promising filler types for concrete is hogweed chips. In this study, the flexural properties of two types of concrete composites reinforced with plant additive samples, including a large additive of hogweed 50 mm long and a medium additive of hogweed 25 mm long were examined. In addition, a composite sample reinforced with short polypropylene fiber was produced. Each series of concrete composite consists of three samples. A three-point bending test was conducted to determine the reinforcement efficiency of the manufactured composites. Instron 5965 (USA) unit helped determine maximum load and normal stress. The results showed that the flexural strength of composites with long additive pieces is greater than that of the other samples. The increase in flexural strength was 5% and 25% for composite made of short and long pieces, respectively. The interaction mechanism between wood additives and cement matrix in the composite was analyzed by means of optical microscopy. The surface formations were found to significantly affect the bonding properties of the concrete and the hogweed.