Si-containing organic-inorganic epoxyurethane biocomposites were obtained. BPA type (Bisphenol A based) epoxy resin, used for synthesis of polymer matrix, was partially or completely replaced with epoxidized soybean oil (ESO). High amount (60 and 70 wt%) of hemp wood core (HWC) that is natural filler was used. Such materials may find potential application as plates or other pressed materials for building industry. To predict the behaviour of biocomposites under outdoor conditions during their operation period and after its expiration the samples were subjected to accelerated weathering and soil burial test respectively. The results of visual observation, measurements of weight, thickness, and flexural strength as well as thermogravimetric analysis (TGA) before and after exposure in a climate chamber evidenced that investigated biocomposites are stable to weathering. Particularly, values of gain in weight and thickness of the weathered samples were up to 2.9% and 1.9% respectively (with the lowest of them for the ones with maximum ESO content). The values of flexural strength of weathered ESO-modified composites were even by 13.4–32.0% higher than those of the unweathered ones. Such a phenomenon is mostly attributed to the reactions of post-curing promoted by high temperature and humidity applied during accelerated weathering. The results of soil burial test showed that the degree of biodegradation was directly proportional to the amount of ESO in biocomposites. Notably, after 1 year of soil exposure the weight loss was the lowest for biocomposite without ESO (30.1%), while it was the highest for biocomposite with maximum amount of ESO (44.5%).