There are several non-utilized or under-utilized hardwood trees common to the Midwestern states. Wood flour (WF) derived from fast-growing Midwest trees (Osage orange, black locust, and red mulberry) were evaluated as a source of bio-based fiber reinforcement. Wood-plastic composites (WPCs) of high-density polyethylene (HDPE), 25 wt.% of WF, and either 0% or 5% by weight of maleated polyethylene (MAPE) were produced via twin screw compounding and injection molding. Specimen bars were evaluated for their mechanical and flexural properties. Composite blends that employed the coupling agent MAPE were superior to composites without MAPE or neat HDPE in terms of their mechanical and flexural properties. The Osage orange WPC composed of juvenile WF had mechanical and flexural properties that were the same as the WPC composed of mature WF. The WPC composed of WF from Midwestern trees were comparable with the WPC composed of pine WF in terms of their mechanical and flexural properties. Soaking the bars of the various WPC blends in distilled water for 28 days altered their weights, mechanical properties, and color. Thermal properties of neat HDPE and WPCs were evaluated using differential scanning colorimetry and thermogravimetric analysis.