Abstract The present study aimed to assess the allelopathic potential of four cereals: winter wheat ( Triticum aestivum L.), triticale ( ×Triticosecale Wittm.), spelt wheat ( Triticum spelta L.) and barley ( Hordeum vulgare L.) through a completely randomized (CR) design. The allelopathic effects of water extracts of different parts of the cereal plants (stem, leaf, and spike) at different concentrations (0.5%, 1.0%, 2.0%, and 4.0%) were evaluated on the seed germination and seedling growth. The germination rate, length, and dry weight of the shoot and root of the seedlings were measured. Ferulic acid was detected in all the cereals. The water extracts at 2.0% and 4.0% concentration had an allelopathic effect on the germination rate, shoot and root length of seedlings of spelt wheat, barley, and triticale, and the stem and leaf extracts affected the root and shoot length of winter wheat. The allelopathic effect of the dried powder of the cereals were evaluated in pot experiments. Both spelt wheat and triticale powder treatment at elevated CO ₂ levels increased the dry weight of the root, as well as the length of the shoot and root of winter wheat. Furthermore, treatment with 4.0 g of dry cereal powder combined with an elevated level of CO ₂ increased the shoot length, whereas the root length of winter wheat was unaffected. In summary, the combination of dry cereal powder with elevated CO ₂ stimulates the initial growth of winter wheat.